e D A Q Am p User Ma nua l

Bridge & GP Amp

Bridge Amp

GP Ampe-corder®

www.eDAQ.com

This document was, as far as possible, accurate at
the time of printing. Changes may have been madeto the software and hardware it describes sincethen: eDAQ Pty Ltd reserves the right to alterspecifications as required. Late-breaking information
may be supplied separately. Latest information andinformation and software updates can be obtained
from our web site.
Trademarks of eDAQ
e-corder and PowerChrom are registered trademarksof eDAQ Pty Ltd. Specific model names of data
recording units, such as e-corder 201, andPowerChrom 280, are trademarks of eDAQ Pty Ltd.
Chart and Scope are trademarks of ADInstrumets Pty
Ltd and are used under license by eDAQ. EChem isa trademarke of eDAQ Pty Ltd.

Products: Bridge Amp (EA110); GP Amp (EA142)
Document Number: U-EA110/EA142-1003
Copyright © October 2003
eDAQ Pty Ltd
6 Doig Avenue
Denistone East, NSW 2112
AUSTRALIA
http://www.eDAQ.com
email: info@eDAQ.com
All rights reserved. No part of this document may be
reproduced by any means without the prior written
permission of eDAQ Pty Ltd.

Other Trademarks
Mac OS, and Macintosh, are registered trademarksof Apple Computer, Inc. Windows 98, Windows
Me, Windows 2000, and Windows XP aretrademarks of Microsoft Corporation.
PostScript, and Acrobat are registered trademarks ofAdobe Systems, Incorporated.
All other trademarks are the properties of theirrespective owners.

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Bridge & GP Amp

Bridge Amp

Contents

1 Overview 1
How to Use this Manual 2eDAQ Amps 2
Checking the Bridge or GP Amp 3

2 The Bridge Amp 5
The Front Panel 6
Input Connector 6
The Online Indicator 6
The Back Panel 7
Output Connector 7
I2C Connector 7
Connecting to the e-corder 8
Using Chart & Scope Software 8
Previewing the signal 10
Connecting Transducers 14
Wiring of the Transducer Plug 15
Setting Excitation Voltage 16
Setting the Offset Range 18
Connecting Signal Leads 20
Connecting the Excitation Leads 23
Testing the Transducer 23

3 The GP Amp 25
The Front Panel 26
Input Connector 26
The Online Indicator 26
The Back Panel 27
Output Connector 27
I2C Connector 27
Bridge & GP Amp

Connecting to the e-corder 28
Using Chart & Scope Software 28
Previewing the signal 30
Connecting Transducers 35
Wiring of Transducer Plug 35
Setting Excitation Voltage 37
Setting the Offset Range 39
Connecting Signal Leads 41
Connecting the Excitation Leads 42
Testing the Transducer 42

A Technical Aspects 45
Overview 45
Bridge Amp Construction 46
GP Amp Construction 48

B Troubleshooting 51
C Specifications 55
Bridge Amp 55
GP Amp 57

Index 61
Licensing & Warranty 63

iii

iv

Bridge & GP Amp

C H A P T E R

1 Overview

O N E

The Bridge Amp is designed for use with most full and half–bridgetransducers that require a differential DC amplifier providing high gain.
Typically these include force transducers, load cells, pressuretransducers, and similar devices.

The GP Amp is designed for use with powered transducers requiring
either a single–ended or differential high impedance (100 MΩ)amplifier including piezoelectric devices, light meters, displacementtransducers and similar devices.

Bridge & GP Amp

1

How to Use this ManualThis Manual describes how to set up and begin using your Bridge Amp(Chapter 2) or GP Amp (Chapter 3). Their use with Chart and Scope
software is also described. The appendices provide technicalinformation, and cover troubleshooting should you encounter aproblem.

eDAQ Amps
The Bridge and GP Amps are part of a family of preamplifiers known
as eDAQ Amps designed for use with your e-corder system.
The Bridge and GP Amps are designed to be operated under fullsoftware control and are automatically recognised by Chart or Scope
software which control gain range, offset and filter settings.
The range of eDAQ Amps includes the:• pH/mV Amp, suitable for connection of pH, ion selective, andpotentiometric (ORP) electrodes• Potentiostat, a three–electrode potentiostat that can be used forvoltammetric and amperometric experiments. Gain ranges of 20 nA
to 100 mA in 1:2:5 steps.
• Picostat, a high sensitivity three–electrode potentiostat suitable foruse with carbon fibre and other microelectrodes. Current gain
ranges of 10 pA to 100 nA in 1:2:5 steps.
• Bridge Amp, suitable for sensors requiring a low drift, high gain
differential amplifier. Also provides DC excitation• GP Amp, suitable for high output sensors requiring a highimpedance single ended or differential amplifier. Also provides DC
excitation.
See our web site at www.eDAQ.com for more information.

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Bridge & GP Amp

Checking the Bridge or GP Amp
Before you begin working with the Bridge or GP Amp, you should
check:• the contents of the box in which you received your eDAQ Amp
against the packing list
• carefully for any sign of physical damage that might haveoccurred during transit.
If you find a problem, please contact your eDAQ representative
immediately.
You should also become familiar with the basic features of youre-corder system, which are discussed in the Chart and Scope Software
Manuals on the Installer CD.

Chapter 1 — Overview

3

4

Bridge & GP Amp

C H A P T E R

T W O

2 The Bridge Amp

This chapter describes the Bridge Amp, how to connect it to youre-corder, and how to ensure that it is working properly. Configuring
your system for one or more eDAQ Amps is also discussed, along withhow to use the Bridge Amp with Chart and Scope software. Details on
how to wire a suitable connector plug for the attachment of atransducer, or other sensor, to the Bridge Amp are also covered.

Bridge & GP Amp

5

The Front Panel
The front panel of the Bridge Amp is shown in Figure 2–1.

Input Connector
The Input connector of the Bridge Amp provides connection pins for thetransducer signal, as well as excitation and signal offsetting. Pin
assignments of are shown in Figure 2–2.
See Chapter 4 for instructions on wiring a transducer for correct
operation with the Bridge Amp.

The Online Indicator
Located at the bottom right of the front panel is the Online indicator,
Figure 2–1. When lit, it indicates that the Chart or Scope software has
located and initialised the Bridge Amp. If the light does not go on when
the software is run, check that the Bridge Amp is properly connected. If
there is still a problem, please refer to Appendix B.

Figure 2–1
The Bridge Amp front
panel

Transducer input connector
DIN socket

Online indicator light

Bridge Amp

5. Excitation setting resistor
2. Signal input –Figure 2–2
Input connector pin
assignments, as seen
when looking at the frontpanel

4. Excitation –3. Signal input +7. Common (ground)
8. Excitation setting resistor

1. Excitation +Chassis ground
6. Offset range resistor

6

Bridge & GP Amp

The Back Panel
The back panel of the Bridge Amp is shown in Figure 2–3.

Output Connector
The Bridge Amp back panel, Figure 2–3, has a BNC connector
labelled Analog Out. This is connected to an e-corder input channel. A
suitable cable is included with the Bridge Amp.

I2C Connector
The Bridge Amp back panel, Figure 2–3, has two DB-9 pin ‘I2C bus’
connectors labelled Input and Output. The Input connector enables

Signal Output

Figure 2–3
The Bridge Amp backpanel

Signal output,
BNC connector

2

I C
Bus

Input

Output

1

5
6

9
Input

Chapter 2 — The Bridge Amp

INT

DSD

SDA

DSC

I2C control signals

SCL

Regulated +17 V DC

Regulated +8 V DC

Regulated –17 V DC

Digital Ground

INT
DSD
SDA
DSC
SCL

Figure 2–4
I2C connector pin
assignments

Power lines

Regulated –17 V DC
Digital GroundI2C control signals

Regulated +17 V DC
Regulated +8 V DC

I2C bus, DB-9 pin connectors

5

1

9

6
Output7connection to the e-corder (or to the output of other eDAQ Amps). A
cable is provided with the Bridge Amp for this purpose. This connection
provides power to the Bridge Amp and carries the various control
signals (for gain range and filter selection) to and from the e-corder.
The pin assignments are shown in Figure 2–4.
The Output connector can be used to attach other eDAQ Amps.
More information about the I2C connector can be found in youre-corder Manual.

Connecting to the e-corder
To connect an eDAQ Amp, such as your Bridge Amp, to the e-corder,
first make sure that the e-corder is turned off. Failure to do this may
damage the e-corder, the Bridge Amp, or both.
Connect the I2C output of the e-corder to the I2C input of the Bridge
Amp, using the cable provided, as shown in Figure 2–5. Check that theplugs for the cable are screwed in firmly. Connect the back panel
Analog output of the Bridge Amp to one of the front panel input
channels on the e-corder.
Check that all connections are firm. Loose connectors can cause theeDAQ Amp to fail to be recognised by the software, erratic behaviour,or loss of signal.
Multiple eDAQ Amps can be connected to a e-corder. The number thatcan be connected depends on the number of input channels on thee-corder. The initial eDAQ Amp should be connected as shown in
Figure 2–5. The remainder are linked via I2C cables, connecting the
I2C output of one eDAQ Amp to the I2C input of the next, as in Figure
2–6. The analog output of each eDAQ Amp is connected to one of the
input channels of the e-corder.

Using Chart & Scope Software
When using the Chart or Scope data recording software with the
Bridge Amp, the Input Amplifier dialog box that normally controls the
e-corder input channel settings is replaced with the Bridge Amplifier
dialog box. In this dialog box the separate amplification and filtering
8

Bridge & GP Amp

I2C input

Figure 2–5
Connecting a Bridge
Amp to the e-corder

Signal output

e-corderI2C output

I2C connector cableI2C output

I2C input

Figure 2–6
Connecting multiple
Bridges or other eDAQ
Ampssettings for the Bridge Amp and e-corder are combined — that is you
see only one menu for amplification, and another menu for filter
settings. The Chart and Scope Software Manuals (on the Installer CD)
provide details the Input Amplifier dialog box.

The Bridge Amp Self-Test
Once the Bridge Amp is properly connected to the e-corder, and when
the Chart and Scope software is installed on the computer, a quick
check can be performed on the Bridge Amp:
• Turn on the e-corder and check that it is working properly, as
described in the owner’s guide that was supplied with it.
Chapter 2 — The Bridge Amp

9• Once the e-corder is ready, open the Chart or Scope software. As
the software opens, you should see the Bridge Amp indicator light,
Figure 2–1, glow green, flash briefly, and then remain lit.
If the indicator glows green, the Bridge Amp is working properly.
Otherwise it is not connected properly (re-check the connections) or that
there is a software or hardware problem.
In addition when a Bridge Amp is properly connected to a channel, theusual e-corder Input Amplifier dialog box (see the e-corder Manual on
the Installer CD) is replaced by the Bridge Amp dialog box, Figure 2–7
and Figure 2–8.

Previewing the signal
The Bridge Amplifier dialog box allows you to preview a signal so thatyou can select the amplification, filtering and other settings. The dialog
boxes for Chart software are shown in Figure 2–7 and Figure 2–8. The
Scope software controls are similar.
The incoming signal is displayed in real time, but is not recorded tohard disk (once the signal moves across the display area it is lost). Clickthe OK button to apply the selected settings. You are now ready to
begin recording.

Signal Display
The input signal is previewed in real time — no data is being written to
hard disk at this stage. Slowly changing waveforms will be represented
quite accurately, whereas quickly changing signals will be displayed asa solid dark area showing only the envelope (shape) of the signal
formed by the minimum and maximum recorded values. The average
signal value is shown at the top left of the display area.
You can stop the signal scrolling by clicking the Pause button,.
Click the Scroll button,, to start scrolling again. You can shift andstretch the vertical Amplitude axis to make the best use of the available
display area (see the Input Amplifier dialog in the Chart Software
Manual for further details). Changes made here update settings in themain window of the program.

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Bridge & GP Amp

Select input
channelFigure 2–7
The Bridge Amplifier
dialog box, forpreviewing a signal.
(Chart software on a
Windows computer)e-corder input
channel number

Pause/Scroll buttons

Gain range(sensitivity)

Average signal
amplitude

Filtering
options

Amplitude axis

Signal invert
control
Offset display

Incoming signal

Offset controls

External offset
adjustment display

Select input
channelFigure 2–8
The Bridge Amplifier
dialog box, forpreviewing a signal.
(Chart software on a
Macintosh)

Units calibration

Click the OK button
to apply settingse-corder input
channel number

Pause/Scroll button

Gain
range
(sensitivity)

Average signal amplitude

Amplitude axis

Filtering
options
Offsetdisplay

Incoming signal

Offsetcontrols
Signalinvertcontrol

Units calibration

Chapter 2 — The Bridge Amp

External offset
adjustment display

Click the OK button
to apply settings

11Setting the Range
The Range pop-up menu lets you select the input range or sensitivity of
the channel — this is the total combined range of the e-corder andBridge Amp. The default setting is 200 mV (the least sensitive range),
but you can select from 12 ranges down to 50 µV (the most sensitive
range). Within each range the resolution is 16 bits (Chart software) or
12 bits (Scope software).

Filtering
The High Pass and Low Pass pop-up menus provide signal filtering
options appropriate to the type of transducer and quality of the signal
recorded with the Bridge Amp.
High Pass. There are only two options: DC and 0.1 Hz. If DC is chosenthen the high pass filter is turned off. When 0.1 Hz is chosen, high-passfilter (AC coupling) before the first amplification stage removes any DC
and very low frequency components from the input. The 0.1 Hz option
is sometimes useful to observe oscillating signals as it removes thebaseline (DC) signal.
Low Pass. The Low Pass pop–up menu gives a choice of low-pass filters(1, 2, 10, 20, 100, 200 Hz, and 1 and 2 kHz) which can be used toremove high frequency (noise) components from the signal.

Inverting the Signal
Ticking the Invert checkbox inverts the direction of the signal. Positive
signals are shown as negative values and vice versa.
For example, you might be recording from a force transducer where an
increase in force downwards gives a negative signal, but you want tohave a downwards force shown as a positive signal on the screen.
Checking the Invert checkbox will change the display to do this.

Offset Adjustment
Even in the resting state most transducers still produce a non–zerosignal. Zeroing, or offsetting, is the process by which this signal is set toa zero value. The offset controls in the Bridge Amplifier dialog box can

12Bridge & GP Amp

be used to zero the reading manually or automatically. The total range
of offset and the resolution of the offset controls are governed by theplacement of a offset resistor across pins 6 and 7 in the transducer
connector — see ‘Setting the Offset Range’, page 48, for more details.
Note that the zeroing features below are unavailable when the 0.1 Hz
high pass filter (AC coupling) is selected, as this setting should remove
DC offset component in the signal.
Manual Zeroing. The up and down arrow buttons next to the Zerobutton allow manual adjustment of signal zeroing. Click the up arrow toshift the signal positively, the down arrow to shift it negatively. The
offset added by each click of the arrow buttons depends on the range
setting.
NOTE: Make sure thatthe transducer isrecording a steadysignal during the zeroingprocedure. Changes inthe transducer signal
during the procedure
could cause the autozeroing to fail.

Automatic Zeroing. To perform automatic zeroing, click the Zero button.
Auto-zeroing may take 20 seconds or so to work out the best zeroingvalue at all ranges, but is a great deal quicker than manually going
through each range. If there is still some offset after auto-zeroing, thenControl+click (Option–click on Macintosh) the up and down arrow
buttons to adjust the zeroing by small increments.
The offset display, a small numeric indicator above the Zero button,shows the offset used as a percentage value of the maximum offset
available. When the Bridge Amp is first powered up, the software sets
the offset circuit to no applied offset (its default position) and the offset
display has a value of zero. When either the auto-zeroing function is
selected or one the manual offset controls is used, this number willchange to indicate the amount of offset applied.
On Windows computers you can manually type in an offset value. A
value of zero causes no offset to be applied.
On Macintosh click the
button to restore the offset circuit to remove
any applied offset (the offset display will return to zero).

Display Offset
Clicking the Display Offset… button causes the Input Voltage dialog
box to appear (Figure 2–9). If your transducer has its own offset
adjustment capabilities, you can use this widow to help while zeroing
the signal. This feature is unavailable when the 0.1 Hz high pass filter

Chapter 2 — The Bridge Amp

13

Figure 2–9
The Display Offset
window is used with atransducer that has its
own offset adjustment.

Macintosh

Windows computeris selected (AC coupling) as this setting should remove signal offset inany event.

Units
Clicking the Units… button brings up the Units Conversion dialog box,
allowing you calibrate and specify the units for a channel. The
waveform in the data display area of the dialog box is transferred to
the Units Conversion dialog box. (Use the Pause button to capture a
specific signal.) However, it is more usual to use the Units Conversion
command from the channel menu in the main Chart window (see theChart Software Manual on the Installer CD)

Connecting Transducers
The Bridge Amp is designed to work with a wide variety of transducers
and sensors that are configured as a DC Wheatstone bridge. These
configurations require high gain, differential, low drift amplifiers.
Differential signals up to 200 mV can be measured. Please refer toAppendix A (and especially Figure A–1 on page 47) for a technicaldescription of this amplifier. This will assist you in the correct operation
and wiring of the transducers used with the system.
The Bridge Amp is fitted with a female eight–pin DIN–style input socket
to connect the transducer. A corresponding eight–pin DIN plug (male)

14

Bridge & GP Amp

is also included. Additional eight–pin DIN plugs can be purchased fromelectronics suppliers, or from your eDAQ representative.

Wiring of the Transducer PlugIf you have purchased your transducer from eDAQ it will already havea suitable plug fitted. Otherwise, if you intend to use a transducer
already fitted with a different connector you will have two options:• Remove the existing connector and replace it with a compatibleplug.
• assemble an adapter lead which allows the existing transducer toretain its original connector.
To wire a suitable plug for connection to the Bridge Amp you will needto perform the following actions:
1. Determine the required transducer excitation supply, select anappropriately sized Excitation Setting Resistor and fit this resistor
between pins 5 and 8 of the DIN plug.
2. If a half–bridge transducer is being used, install appropriate
half–bridge compensating resistors in the DIN plug.
3. Determine the required signal offset range, select an appropriate
Offset Range Resistor and fit this resistor between pins 6 and 7of the
DIN plug.
4. Identify the transducer signal leads and connect them to theappropriate pins of the DIN plug.
5. Identify the transducer excitation (power) leads and connect themto the appropriate excitation supply on pins 1 and 4 of the DIN
plug.
6. The main insulation sheath of the transducer cable should beclamped with the strain-relief device within the plug. Any unused
wires from the transducer should be insulated to prevent shorting of
signals or damage to the equipment.
The order of this procedure is important because any resistors usedshould be fitted before the signal and excitation leads are attached.
Chapter 2 — The Bridge Amp

15

You will also need the following:• a soldering iron and resin-cored solder (use only resin-coredsolder) and some basic small electronic tools.
• a eight-pin DIN style male plug with 45° pin spacing (one issupplied with your Bridge or GP Amp)
• the connection diagram and specifications of the transducer you
intend to use.
• a five–core shielded cable, if a cable is not already fitted to yourtransducer• If you are using a half–bridge configuration, you will also need
two 1000 Ω, 1%, 0.125 W high stability resistors. These resistors
need to fit inside the plug body so small miniature types should be
used.
While not difficult, it is recommended that this work should be done bya suitably qualified electronics technician with good soldering skills as
incorrect wiring could damage the transducer and/or Bridge Amp.
Such damage is not covered under the terms of your warranty.
There are several things to note when wiring connectors:• Make sure that the transducer wiring is passed through theconnector shell before soldering the wires to the plug.
• Wires should be cut, stripped, and tinned prior to soldering, toensure a good connection.
• Use a small vice to hold the plug while soldering.
• The pin numbers shown in the diagrams are the numbers marked
on standard DIN plugs. If the plug has no numbers or different ones,
go by the layout shown in the Figures below.

Setting Excitation Voltage
Pins 1 and 4 of the input socket on the front panel of the Bridge Amp(and GP Amp) provide a DC excitation voltage from 0 V to
20 V (±10 V) to power the transducer. Pin 1 supplies up to +10 V, andpin 4 supplies down to –10 V with respect to ground, pin 7.

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Bridge & GP Amp

Pin 5Figure 2–10
Placement of the
Excitation Resistor in thetransducer plug

Excitation resistor, Rs

2

5

4
3
1

8
6

7

Pin 8

The excitation voltage is set by connecting an Excitation Set Resistor
between pins 5 and 8 on the DIN plug of the transducer. Correct
mounting of the resistor is shown in Figure 2–10.
The relationship between the excitation voltage applied and the size ofthe resistor is given by:
150
V ex = -------------------- × 20
150 + R s

…equation 2.1

where,
Rs = Excitation setting resistor in kΩVex = Excitation voltage produced between pins 1 and 4 in volts
For example, the excitation voltage produced by a 330 kΩ resistor
would be 6.25 V, or ±3.125 V, with respect to ground (pin 7).
Using this method will ensure that your transducer is automatically set tothe correct excitation voltage when plugged into a Bridge or GP Amp.

Special Cases
1. If there is no resistor across pins 5 and 8 (Rs = ∞) the value of Vex
will be zero and your transducer will effectively be unpowered. Thisis a fail safe condition: if no excitation resistor is fitted then no
excitation is generated.
2. Many transducers require ±5 V excitation. This is provided byusing a 150 kΩ resistor.

Chapter 2 — The Bridge Amp

17

3. If a direct link is placed across pins 5 and 8 (Rs = 0) the value of
Vex will be 20 V, a value which could damage some transducers. In
this case, please take care that your transducer is able to accept theexcitation level safely.
Table 2–1 shows the results of using some common values of resistors.
We recommend that the excitation setting resistor be 0.125 W, 1%
metal film (for stability). Physically larger resistors may not fit in theconfined space of the transducer’s DIN plug.
Excitation, V
Table 2–1
Excitation produced bydifferent sizes of resistorRs, kΩ

Excitation, V

Rs, kΩ

20±10

Short circuit

4.8

±2.4

470

18.5

±9.3

12

4.2

±2.1

560

15.2

±7.6

47

3.6

±1.8

680

12

±6

100

3.1

±1.5

820

10

±5

150

2.6

±1.3

1000

8.1

±4.1

220

2.2

±1.1

1200

6.25

±3.1

330

0

0

Open circuit

In many cases, the output signal from the transducer will be directly
proportional to the excitation voltage placed across it. Thus the greater
the excitation voltage, the more sensitive the transducer becomes.
However, the excitation voltage should always be set within thetransducer manufacturer’s specifications. Transducers can be damaged
if an excitation voltage in excess of their rating is applied. Even if thesevoltages do not actually cause damage, the transducer will haveresistive elements that can heat up, causing its resistance to change anddegrading the accuracy of the reading.

Setting the Offset Range
Many transducers produce a non–zero ‘offset’ or ‘background’ signal
even in their resting state. This offset can be significantly larger than thesignal to be measured thus preventing sufficient gain to be applied.
The Bridge Amp provides facilities to zero the background signal thus
enabling additional gain to be applied so that subsequent signal
variations can be measured accurately. Zeroing is sometimes called‘zero suppression’, or ‘offsetting’: the terms can be usedinterchangeably.
18

Bridge & GP Amp

Figure 2–11
Placement of the Offset
Range Resistor in thetransducer plug (withexcitation range resistor
present)

Excitation resistor, Rs

2

Pin 7

5

4
3
1

8
6

7

Offset range resistor, Roff

Pin 6

You can zero the signal within the Chart or Scope software, Figure 2–7
and Figure 2–8. This is done by a circuit that uses a 12-bit DAC(digital–to–analog converter) to produce a voltage to zero the signal.
The size of this voltage, Voff, is set by a Offset Range Resistor, the valueof which, RB, is determined by procedures defined below.
The DAC can produce ±2048 steps within the offset range thus allowing
very fine control of zeroing.
The maximum signal that you expect to obtain from your transducer isoften a suitable offset range to select. For example a 1000 kg range
force transducer (load cell) with a gain of 5 µV/V/kg, at an excitationof ±5 V, would be expected to output a 50 mV signal for a 1000 kg
load. Thus if you need a 1000 kg offset you would need to set an offset
range of 50 mV.
If you don’t know the gain of your transducer you first need todetermine it by direct measurement of known values (for example for aload cell you would use one or more calibrated weights).
The maximum offset available depends on the impedance of the bridge
element of the transducer that you are using and the size of the offset
range resistor is determined by:
10 + R off- × RB± V off = -------------------------110 + R off…equation 2.2

where
Voff is the offset range available (V),Roff is the size of the offset range resistor (kΩ), andRB is the impedance of the transducer bridge element (kΩ)
Chapter 2 — The Bridge Amp

19

Special cases
1. For RB = 300 Ω (a commonly encountered value) and Roff = 0(short circuit across pins 6 and 7) the maximum offset is ±0.027 V,
or 13 µV per step. This is very effective setting for many bridgetransducers.
2. For RB =1000 ohms and Roff = ∞ (open circuit across pins 6 and7) the maximum offset is ±0.91 V, or 444 µV per step.
By installing a resistor between pins 6 and 7 you can reduce the size of
the DAC voltage steps and get increased resolution. However, themaximum offset available will be decreased correspondingly.
Decreasing values of resistance increase the resolution with which thesignal can be zeroed, but decrease the maximum offset range
available. Maximum resolution is obtained by short-circuiting theterminals (equivalent to using a zero ohm resistor).
In most cases an exact step size is not required and you should be able
to use a resistor close to a value determined by the formula. It is
recommend that you use a 1% metal film resistor rated at 0.125 W.
Larger resistors may not fit in the confined space of the transducer’s
DIN plug.
Once the required Offset Range resistance is calculated and a resistor
is selected, it can be soldered between pins 6 and 7 of the DIN plug.

Connecting Signal Leads
After the excitation and offset setting resistors have been fitted, thesignal leads can now be connected. The general procedure is
described below for both full and half–bridge type transducers.
The transducer cable will normally have a cable shield, which should
be connected to pin 7 of the DIN plug. It is good practice to ensure thatthe shield is only connected to one ground point, either at thetransducer or at the amplifier (but not both) otherwise ground loops will
be set up leading to excessive noise.
If the casing of the plug is metal, it is good practice to ensure that thecasing will also be connected to the shield.

20

Bridge & GP Amp

Ground or centre tap (shield)
3

Figure 2–12
Plug connections for atypical full–bridgetransducer. The Offset
Range resistor has been
omitted.

7

5

Signal positive (+)

2

8Signal negative (–)

Excitation positive (+)

Wire fromtransducer

1

4

6

Excitation negative (–)

Excitation
voltageprogramming
resistor

Transducer ground(earth) wire

The main insulation sheath of the transducer cable should be clampedwith the strain-relief device within the plug. Any unused wires from thetransducer should be insulated to prevent shorting of signals or damage
to the equipment.

Full–Bridge transducers
Full-bridge transducers will have two signal leads ‘signal +’ and‘signal –’ and often a ‘ground’ or ‘shield’ lead. These leads should beconnected to pins 3, 2 and 7 respectively as shown in Figure 2–12.

Half–Bridge Transducers
A full–bridge arrangement is first made from the half–bridge transducer
by adding two ‘half–bridge compensating’ resistors to the connector
plug. One of these resistors links pin 1 and pin 2, while the second
resistor links pin 4 and pin 2. Solder the resistors as shown in Figure
2–13. Notice that the top ends of the resistors are joined together andthen soldered to pin 2.

Chapter 2 — The Bridge Amp

21Ground or centre tap (shield)
3

Excitation
voltageprogramming
resistor

8

7

5

Signal positive (+)

2

Figure 2–13
Plug connections
showing placement ofhalf–bridgecompensating resistors.

Half–bridgecompensating
resistors

Excitation positive (+)

1

4

6

Excitation negative (–)

Half–bridgecompensating
resistors

Pin 2

Pin 4
Pin 1

Half–bridge compensating resistors should have the following specifications:Power rating:0.25 or 0.125 W
Temperature coefficient:<10 ppm per °C
Matched temperature coefficient:
1 ppm per °C
Resistor matching:<0.05%
Resistor tolerance:≤1%

The value of these resistors should be equal to the nominal resistance of
the active arms of the bridge, or 1 kΩ, whichever is the higher value.
A change of 1 ppm in the relative values of the resistors will result in achange of 10 µV in the output for a 10 V excitation voltage. To avoid
self-heating effects, keep resistance high and excitation low.
A half–bridge configured plug can require up to four resistors (anexcitation setting resistor, offset range resistor, and the two half–bridgecompensating resistors) to be fitted in the DIN plug. This requires
careful placement to avoid overcrowding.

22

Bridge & GP Amp

Connecting the Excitation Leads
Finally, the transducer excitation leads can now be connected. Bridgetransducers will normally use balanced positive and negative excitationin order to produce signals that vary around zero volts. The positive
excitation lead is connected to pin 1 and the negative excitation lead isconnected to pin 4.

Testing the Transducer
After connecting the excitation voltage programming resistor,
connecting the offset voltage range resistor and wiring up thetransducer to the DIN plug, the transducer should now be fully
configured for your purposes. The excitation voltage and offset range
values will be set automatically when the transducer is plugged into theBridge Amp.
Looking at the Bridge Amplifier dialog box, Figure 2–7 or Figure 2–8,
you can see the output from the transducer as you change its operating
conditions. You may have to adjust the input range to get a good
response. If there appears to be no response from the transducer,
recheck the wiring against the diagrams for the appropriate transducer
and the manufacturer’s instructions.

Chapter 2 — The Bridge Amp

23

24

Bridge & GP Amp

C H A P T E R

3 The GP Amp

T H R E E

This chapter describes the GP Amp, how to connect it to your e-corder,and how to ensure that it is working properly. Configuring your systemfor one or more eDAQ Amps is also discussed, along with how to use
the GP Amp with Chart and Scope software. Details on how to wire a
suitable connector plug for the attachment of a transducer, or othersensor, to the GP Amp are also covered.

Bridge & GP Amp

25

The Front Panel
The front panel of the GP Amp is shown in Figure 3–1.

Input Connector
The Input connector of the GP Amp provides connection pins for thetransducer signal, as well as excitation and signal offsetting. Pin
assignments of are shown in Figure 3–6.
See Chapter 4 for instructions on wiring a transducer for correct
operation with the GP Amp.

The Online Indicator
Located at the bottom right of the front panel is the Online indicator,
Figure 3–1. When lit, it indicates that the Chart or Scope software has
located and initialised the GP Amp. If the light does not go on when thesoftware is run, check that the GP Amp is properly connected. If there is
still a problem, please refer to Appendix B.

Figure 3–1
The GP Amp front panel

Transducer input connector
DIN socket

Online indicator light

GP Amp5. Excitation setting resistor

Figure 3–2
Input connector pin
assignments, as seen
when looking at the front
panel.

2. Signal input –4. Excitation –3. Signal input +7. Common (ground)
8. Excitation setting resistor

1. Excitation +Chassis ground
6. Offset range resistor

26

Bridge & GP Amp

The Back Panel
The back panel of the GP Amp is shown in Figure 3–3.

Output Connector
The GP Amp back panel, Figure 3–3, has a BNC connector labelled
Analog Out. This is connected to an e-corder input channel. A suitable
cable is included with the GP Amp.

I2C Connector
The GP Amp back panel, Figure 3–3, has two DB-9 pin ‘I2C bus’
connectors labelled Input and Output. The Input connector enables
connection to the e-corder (or to the output of other eDAQ Amps). A

Signal Output

Figure 3–3
The GP Amp back panel

Signal output,
BNC connector

2

Input

I C
Bus

Output

1

5
6

9
Input

Chapter 3 — The GP Amp

INT

DSD

SDA

DSC

I2C control signals

SCL

Regulated +17 V DC

Regulated +8 V DC

Regulated –17 V DC

Digital Ground

INT
DSD
SDA
DSC
SCL

Figure 3–4
I2C connector pin
assignments

Power lines

Regulated –17 V DC
Digital GroundI2C control signals

Regulated +17 V DC
Regulated +8 V DC

I2C bus, DB-9 pin connectors

5

1

9

6
Output27

cable is provided with the GP Amp for this purpose. This connection
provides power to the GP Amp and carries the various control signals(for gain range and filter selection) to and from the e-corder. The pin
assignments are shown in Figure 3–4.
The Output connector can be used to attach other eDAQ Amps.
More information about the I2C connector can be found in youre-corder Manual.

Connecting to the e-corder
To connect an eDAQ Amp, such as the GP Amp, to the e-corder, firstensure that the e-corder is turned off. Failure to do this may damage thee-corder, the eDAQ Amp, or both.
Connect the I2C output of the e-corder to the I2C input of the GP Amp,
using the cable provided, as shown in Figure 3–5. Check that the plugs
for the cable are screwed in firmly. Connect the back panel Analog
output of the GP Amp to one of the front panel input channels on thee-corder.
Check that all connections are firm. Loose connectors can cause theeDAQ Amp to fail to be recognised by the software, erratic behaviour,or loss of signal.
Multiple eDAQ Amps can be connected to a e-corder. The number thatcan be connected depends on the number of input channels on thee-corder. The initial eDAQ Amp should be connected as shown in
Figure 3–5. The remainder are linked via I2C cables, connecting the
I2C output of one eDAQ Amp to the I2C input of the next, as shown in
Figure 3–6. The analog output of each eDAQ Amp is connected to oneof the input channels of the e-corder.

Using Chart & Scope Software
When using the Chart or Scope data recording software with the
GP Amp, the Input Amplifier dialog box that normally controls the
e-corder input channel settings is replaced with the GP Amplifier dialog
box. In this dialog box the separate amplification and filtering settings

28

Bridge & GP Amp

I2C input

Figure 3–5
Connecting a GP Amp to
the e-corder:

Signal output

e-corderI2C output

I2C connector cableI2C output

I2C input

Figure 3–6
Connecting multiple
GP Amps, or othereDAQ Amps

for the GP Amp and e-corder are combined — that is you see only one
menu for amplification, and another menu for filter settings. The Chartand Scope Software Manuals (on the Installer CD) provide details the
Input Amplifier dialog box.

The GP Amp Self–Test
Once the GP Amp is properly connected to the e-corder, and when theChart and Scope software is installed on the computer, you canperform a check on the GP Amp:
• Turn on the e-corder and check that it is working properly, as
described in the owner’s guide that was supplied with it.

Chapter 3 — The GP Amp

29• Once the e-corder is ready, open the Chart or Scope software. As
the software opens, you should see the GP Amp indicator light,
Figure 3–1, glow green, flash briefly, and then remain lit.
If the indicator glows green, the GP Amp is working properly.
Otherwise it is not connected properly (re-check the connections) or that
there is a software or hardware problem.
In addition when a GP Amp is properly connected to an e-corder input
channel, the usual e-corder Input Amplifier dialog box (see the e-corder
Manual on the Installer CD) is replaced by the replaced by GP Amp
dialog box, Figure 3–7 and Figure 3–8.

Previewing the signal
The GP Amplifier dialog box allows you to preview a signal so that you
can select the amplification, filtering and other settings. The dialog
boxes for Chart software are shown in Figure 3–7 and Figure 3–8. The
Scope software controls are similar.
The incoming signal is displayed in real time, but is not recorded tohard disk (once the signal moves across the display area it is lost). Clickthe OK button to apply the selected settings. You are now ready to
begin recording.

Signal Display
The input signal is previewed in real time — no data is being written to
hard disk at this stage. Slowly changing waveforms will be represented
quite accurately, whereas quickly changing signals will be displayed asa solid dark area showing only the envelope (shape) of the signal
formed by the minimum and maximum recorded values. The average
signal value is shown at the top left of the display area.
You can stop the signal scrolling by clicking the Pause button,.
Click the Scroll button,, to start scrolling again. You can shift andstretch the vertical Amplitude axis to make the best use of the available
display area (see the Input Amplifier dialog in the Chart Software
Manual for further details). Changes made here update settings in themain window of the program.

30

Bridge & GP Amp

Select input
channelFigure 3–7
The GP Amplifier dialog
box, for previewing asignal. (Chart software
on a Windows computer)e-corder input
channel number

Pause/Scroll buttons

Gain
range
(sensitivity)

Average signal
amplitude

Filtering
options

Amplitude axis

Offsetdisplay
Offsetcontrols

Incoming signal

Signalinvertcontrol

External offset
adjustment display

Select input
channelFigure 3–8
The GP Amplifier dialog
box, for previewing asignal. (Chart software
on a Macintosh)

Units calibration

Click the OK button
to apply settingse-corder input
channel number

Average signal amplitude

Amplitude axis

Pause/Scroll button

Gain
range
(sensitivity)
Filtering
options
Offsetdisplay

Incoming signal

Offsetcontrols
Signalinvertcontrol

Units calibration

Chapter 3 — The GP Amp

External offset
adjustment display

Click the OK button
to apply settings

31

Setting the Range
The Range pop-up menu lets you select the input range or sensitivity of
the channel — this is the total combined range of the e-corder and
GP Amp. The default setting is 10 V (the least sensitive range), but you
can select from 12 ranges down to 2 mV (the most sensitive range).
Within each range the resolution is 16 bits (Chart software) or 12 bits
(Scope software).

Filtering
The High Pass and Low Pass pop-up menus provide signal filtering
options appropriate to the type of transducer and quality of the signal
recorded with the GP Amp.
High Pass. There are only two options: DC and 0.3 Hz. If DC is chosenthen the high pass filter is turned off. When 0.3 Hz is chosen, a highpass filter (AC coupling) before the first amplification stage removes
any DC and very low frequency components from the input. The 0.3 Hz
option is sometimes useful to observe oscillating signals as it removes
the baseline (DC) signal.
Low Pass. The Low Pass pop-up menu gives a choice of low-pass filters(1, 2, 10, 20, 100, 200 Hz, and 1 and 2 kHz) which can be used toremove high frequency (noise) components from the signal.

Inverting the Signal
Ticking the Invert checkbox inverts the direction of the signal. Positive
signals are shown as negative values and vice versa.
For example, you might be recording from a force transducer where an
increase in force downwards gives a negative signal, but you want tohave a downwards force shown as a positive signal on the screen.
Checking the Invert checkbox will change the display to do this.

Offset Adjustment
Even in the resting state most transducers still produce a non–zerosignal. Zeroing, or offsetting, is the process by which this signal is set toa zero value. The offset controls in the GP Amplifier dialog box can be

32

Bridge & GP Amp

used to zero the reading manually or automatically. The total range ofoffset and the resolution of the offset controls are governed by theplacement of a offset resistor across pins 6 and 7 in the transducer
connector — see Setting the Offset Range, page 48, for more details.
Note that the zeroing features below are unavailable when the 0.1 Hz
high pass filter (AC coupling) is selected, as this setting will remove any
DC offset component in the signal.
Manual Zeroing. The up and down arrow buttons next to the Zerobutton allow manual adjustment of signal zeroing. Click the up arrow toshift the signal positively, the down arrow to shift it negatively. The
offset added by each click of the arrow buttons depends on the range
setting.
NOTE: Make sure thatthe transducer isrecording a steadysignal during the zeroingprocedure. Changes inthe transducer signal
during the procedure
could cause the autozeroing to fail.

Automatic Zeroing. To perform automatic zeroing, click the Zero button.
Auto-zeroing may take 20 seconds or so to work out the best zeroingvalue at all ranges, but is a great deal quicker than manually going
through each range. If there is still some offset after auto-zeroing, thenControl+click (Option–click on Macintosh) the up and down arrow
buttons to adjust the zeroing by small increments.
The offset display, a small numeric indicator above the Zero button,shows the offset used as a percentage value of the maximum offset
available. When the GP Amp is first powered up, the software sets theoffset circuit to no applied offset (its default position) and the offset
display has a value of zero. When either the auto-zeroing function is
selected or one the manual offset controls is used, this number willchange to indicate the amount of offset applied.
On Windows computers you can manually type in an offset value. A
value of zero causes no offset to be applied.
On Macintosh click the
button to restore the offset circuit to remove
any applied offset (the offset display will return to zero).

Display Offset
Clicking the Display Offset… button causes the Input Voltage dialog
box to appear (Figure 3–10). If your transducer has its own offset
adjustment capabilities, you can use this widow to help while zeroing
the signal. This feature is unavailable when the 0.1 Hz high pass filter

Chapter 3 — The GP Amp

33

Figure 3–9
The Display Offset
window is used with atransducer that has its
own offset adjustment.

Macintosh

Windows computeris selected (AC coupling) as this setting should remove signal offset inany event.

Units
Clicking the Units… button brings up the Units Conversion dialog box,
allowing you calibrate and specify the units for a channel. The
waveform in the data display area of the dialog box is transferred to
the Units Conversion dialog box. (Use the Pause button to capture a
specific signal.) However, it is more usual to use the Units Conversion
command from the channel menu in the main Chart window (see theChart Software Manual on the Installer CD)

34

Bridge & GP Amp

Connecting Transducers
The GP Amp is designed to work with a wide variety of poweredtransducers and sensors that require single–ended or differential input
configurations. Signals can be handled up to ±10 V. Please refer toAppendix A (and especially Figure A–2 on page 48) for a technicaldescription of this amplifier. This will assist you in the correct operation
and wiring of the system.
A DIN–style socket (female) used to connect the transducer to the
GP Amp. A corresponding eight–pin DIN plug (male) is also included.
Additional eight–pin DIN plugs can be purchased from electronics
suppliers, or from your eDAQ representative.
If you have purchased your transducer from eDAQ it will already havea suitable plug fitted. Otherwise, if your transducer is already fitted witha connector you will have two options:• Remove the existing connector and replace it with a compatibleplug.
• assemble an adapter lead which allows the existing transducer toretain its original connector.

Wiring of Transducer PlugTo connect a transducer to the GP Amp you will need to perform thefollowing actions:
1. Determine whether the transducer requires power. If so,
determine the required excitation, select an appropriately sized
Excitation Setting Resistor and fit this resistor between pins 5 and 8of the DIN plug.
2. Determine the signal offset range (if required), select anappropriate Offset Range Resistor and fit this resistor between pins
6 and 7of the DIN plug.
3. Identify the transducer signal leads and connect them to theappropriate amplifier inputs.

Chapter 3 — The GP Amp

35

4. Identify the transducer excitation (power) leads and connect themto the appropriate excitation supply — use pins 1 and 4 of the DIN
plug for a ± power supply, or pins 1 and 7 for + supply only.
5. The main insulation sheath of the transducer cable should beclamped with the strain-relief device within the plug. Any unused
wires from the transducer should be insulated to prevent shorting of
signals or damage to the equipment.
The order of this procedure is important because any resistors usedshould be fitted before the signal and excitation leads are attached.
You will also need the following:• a soldering iron and resin-cored solder (use only resin-coredsolder) and some basic small electronic tools.
• a eight-pin DIN style male plug with 45° pin spacing (one issupplied with your Bridge or GP Amp)
• the connection diagram and specifications of the transducer you
intend to use.
• a five–core shielded cable, if a cable is not already fitted to yourtransducer• If you are using a half–bridge configuration, you will also need
two 1000 Ω, 1%, 0.125 W high stability resistors. These resistors
need to fit inside the plug body so small miniature types should be
used.
While not difficult, it is recommended that this work should be done bya suitably qualified electronics technician with good soldering skills as
incorrect wiring could damage the transducer and/or GP Amp. Suchdamage is not covered under the terms of your warranty.
There are several things to note when wiring connectors:• Make sure that the transducer wiring is passed through theconnector shell before soldering the wires to the plug itself.
• Wires should be cut, stripped, and tinned prior to soldering, toensure a good connection.
• Use a small vice to hold the plug while soldering.

36

Bridge & GP Amp

Pin 5Figure 3–10
Placement of the
Excitation Resistor in thetransducer plug.

Excitation resistor, Rs

2

5

4
3
1

8
6

7

Pin 8

• The pin numbers shown in the diagrams are the numbers marked
on standard DIN plugs. If the plug has no numbers or different ones,
go by the layout shown in Figure 3–10.

Setting Excitation Voltage
Pins 1 and 4 of the input socket on the front panel of the Bridge Amp(and GP Amp) provide a DC excitation voltage from 0 V to
20 V (±10 V) to power the transducer. Pin 1 supplies up to +10 V, andpin 4 supplies down to –10 V with respect to ground, pin 7.
The excitation voltage is set by connecting an Excitation Resistor
between pins 5 and 8 on the DIN plug of the transducer. Correct
mounting of the resistor is shown in Figure 3–10.
The relationship between the excitation voltage applied and the size ofthe resistor is given by:
150
V ex = -------------------- × 20
150 + R s

…equation 3.1

where,
Rs = Excitation setting resistor in kΩVex = Excitation voltage produced between pins 1 and 4 in volts
For example, the excitation voltage produced by a 330 kΩ resistor
would be 6.25 V, or ±3.125 V, with respect to ground (pin 7).
Using this method will ensure that your transducer is automatically set tothe correct excitation voltage when plugged into a Bridge or GP Amp.

Chapter 3 — The GP Amp

37

Special Cases
1. If there is no resistor across pins 5 and 8 (Rs = ∞) the value of Vex
will be zero and your transducer will effectively be unpowered. Thisis a fail safe condition: if no excitation resistor is fitted then no
excitation is generated.
2. Many transducers require ±5 V or +5 V excitation. This isprovided by using a 150 kΩ resistor.
3. If a direct link is placed across pins 5 and 8 (Rs = 0) the value of
Vex will be 20 V, a value which could damage some transducers. In
this case, please take care that your transducer is able to accept theexcitation level safely.
Table 3–1 shows the results of using some common values of resistors.
We recommend that the excitation setting resistor be 0.125 W, 1%
metal film (for stability). Physically larger resistors may not fit in theconfined space of the transducer’s DIN plug.
Excitation, V
Table 3–1
Excitation produced bydifferent sizes ofresistorRs, kΩ

Excitation, V

Rs, kΩ

20±10

Short circuit

4.8

±2.4

470

18.5

±9.3

12

4.2

±2.1

560

15.2

±7.6

47

3.6

±1.8

680

12

±6

100

3.1

±1.5

820

10

±5

150

2.6

±1.3

1000

8.1

±4.1

220

2.2

±1.1

1200

6.25

±3.1

330

0

0

Open circuit

The excitation voltage should always be set within the transducer
manufacturer’s specifications. Transducers can be damaged if an
excitation voltage in excess of their rating is applied. Even if thesevoltages do not actually cause damage, the transducer will haveresistive elements that can heat up, causing its resistance to change anddegrading the accuracy of the reading.

38

Bridge & GP Amp

Figure 3–11
Placement of the Offset
Range Resistor in thetransducer plug (withexcitation range resistor
present).

Pin 7

Excitation resistor, Rs

2

5

4
3
1

8
6

7

Offset range resistor, Roff

Pin 6

Setting the Offset Range
Many transducers produce a non–zero ‘offset’ or ‘background’ signal
even in their resting state. This offset can be significantly larger than thesignal to be measured thus preventing sufficient gain to be applied.
The GP Amp provides facilities to zero the background signal thus
enabling additional gain to be applied so that subsequent signal
variations can be measured accurately. Zeroing is sometimes called‘zero suppression’, or ‘offsetting’: the terms can be usedinterchangeably.
You can zero the signal with the Zero button in GP Amplifier dialog
box in the Chart or Scope software, Figure 3–7 or Figure 3–8. This isdone by a circuit in the GP Amp that uses a 12-bit DAC (digital-toanalog converter) to produce a voltage to zero the signal. The size of
this voltage, Voff, is set by a Offset Range Resistor, positioned between
pins 6 and 7 in the transducer plug, Figure 3–11. The value of thisresistor, Roff, is determined by procedures defined below.
The DAC can produce ±2048 steps within the offset range thus
allowing very fine control of zeroing.
The first step in this procedure is to determine or estimate the amount ofoffset present in your transducer under typical conditions.
The maximum signal that you expect to obtain from your transducer isoften a suitable offset range to select. For example a 1000 kg range
force transducer (load cell) with a gain of 5 µV/V/kg, at an excitation

Chapter 3 — The GP Amp

39

of ±5V, would be expected to output a 50 mV signal for a 1000 kg
load. Thus if you need a 1000 kg offset you would need to set an offset
range of 50 mV.
If you don’t know the gain of your transducer, you can measure thesignal produced by samples of known values. For example, a load cell
could be used to record a signal from one or more calibrated weights.
The maximum offset available is determined by:
10 + R off- × 10V off = -------------------------110 + R off…equation 3.2

where
Voff is the offset range available (V), andRoff is the size of the offset range resistor used (kΩ).

Special cases
1. If Roff = 0 (short circuit across pins 6 and 7) the offset range is±0.91 V, or 444 µV per step.
2. If Roff = ∞ (open circuit across pins 6 and 7) the offset range is±10 V, or 4.88 mV per step, Figure 3–12.
3. If pin 2 is connected to pin 7, to implement a single–ended
configuration, then offset range will be reduced by up to 50%.
The Offset feature will be disabled when an internal link, LK3, Figure
A–2 on page 48, is removed in order to configure the system for a highimpedance differential input. Note that this link is normally enabled
unless you specify for it to be removed when ordering.

40

Bridge & GP Amp

Connecting Signal Leads
After the excitation and offset setting resistors have been fitted, thesignal leads can now be connected. Several configurations arecommonly encountered:• High (or low) impedance single-ended configuration
The ‘signal’ lead is connected to pin 3 and the ‘common’ isconnected to pin 7. Offset range is configured by a resistor acrosspins 6 and 7 as described above. If pins 6 and 7 are left
unconnected then the offset will have a range of ±10 V. It is
recommended that pin 2 also be linked to pin 7 to provide a direct
ground reference to the amplifier, however this will halve theamount of offset available (Equation 3.2, page 40).
• Pseudo–differential configuration (with offset)
Differential transducers will have two signal leads ‘signal +’ and‘signal –’ and often a ‘ground’ or ‘common’ lead. These leads
should be connected to pins 2, 3 and 7 respectively, Figure 3–12.
The offset circuitry reduces the effective impedance on pin 2 (if an

7

5

Signal positive (+)
Excitation
voltageprogramming
resistor

Wire fromtransducer

1

4

6

Excitation negative (–)
Excitation positive (+)

Chapter 3 — The GP Amp

8Signal negative (–)
2

Figure 3–12
Typical connection for adifferential transducer.
Offset Range Resistor has
been omitted, Roff = ∞,for maximum offset
range.

3

Ground or centre tap (shield)

Transducer ground(earth) wire41

Offset Range Resistor is used). This provides a differential(unbalanced) configuration with offset capability, which is suitable
for many differential transducers.
• High impedance, true differential configuration
Occasionally, a high impedance, balanced differential configurationis required. In this case the internal link LK3 must to be removed (theGP Amp can be specially ordered in this configuration), and theoffset range resistor must not be fitted (consequently there is no offset
capability available in this configuration). Differential transducers
will have two signal leads ‘signal +’ and ‘signal –’ and often a‘ground’ or ‘common’ lead. These leads should be connected topins 2, 3 and 7 respectively.
The transducer cable will normally have a cable shield, which should
be connected to pin 7 (ground) of the DIN plug. It is good practice toensure that the shield is only connected to one ground point, either at
the transducer or at the amplifier (but not both) otherwise ground loops
will be set up leading to excessive noise. If the casing of the plug ismetal, it is good practice to ensure that the casing will also beconnected to the shield.

Connecting the Excitation Leads
If balanced positive and negative excitation is required (for example±5 V). The positive excitation lead is connected to pin 1 and thenegative excitation lead is connected to pin 4, Figure 3–12.
If the transducer requires a single supply voltage and a common. The
positive supply is connected to pin 1 and the common is connected topin 7. In these cases the negative excitation is not used and pin 4 is left
unconnected.

Testing the Transducer
After connecting the excitation voltage programming resistor,
connecting the offset voltage range resistor and wiring up thetransducer to the DIN plug, the transducer should now be fully
configured for your purposes. The excitation voltage and offset range
values will be set automatically when the transducer is plugged into the
GP Amp.
42

Bridge & GP Amp

Looking at the GP Amplifier dialog box, Figure 3–7 or Figure 3–8, you
can see the output from the transducer as you change its operating
conditions. You may have to adjust the input range to get a good
response. If there appears to be no response from the transducer,
recheck the wiring against the diagrams for the appropriate transducer
and the manufacturer’s instructions.

Chapter 3 — The GP Amp

43

44

Bridge & GP Amp

A

P

P

E

N

D

I

X

A Technical Aspects

A

This appendix describes technical aspects of the Bridge and
GP Amplifiers. You do not need to understand the material here to use
these amplifiers. It is likely to be of especial interest to the technicallyminded, providing insights into what these amplifiers can and cannot
do, and their suitability for particular purposes. You should not use thismaterial as a service manual: user modification of the Bridge or
GP Amp, or the e-corder, voids your rights under warranty.

Overview
The Bridge Amp, GP Amp, and other eDAQ Amps have been designed
to integrate fully with the e-corder system so that the software–controlled
gain ranges and filter settings you see in the Chart and Scope software
are the combination of both pieces of hardware. The Bridge Amp andGP provide:• additional low drift amplification (chopper stabilized) necessary todeal with the very low signal outputs of most bridge transducers(Bridge Amp only)• additional programmable filtering, to remove unwanted signal
and noise frequencies• a stable and protected DC excitation voltage supply for powering
the transducer where necessary.
• digitally-controlled transducer zeroing or offset circuitry.
The digital control interface used to control filter settings, gain,
coupling, and zeroing circuits uses a modified I2C interface system

Bridge & GP Amp

45

(Phillips), which provides a 4-wire serial communication bus between
the e-corder and Amp: all control of the Bridge and GP Amp functions
and settings are through this bus. Also present on the I2C connector is aset of regulated power supply rails derived from the e-corder (+17 V,–17 V and +8 V). The eDAQ Amp has its own on–board regulators togenerate a stable power supply for the internal circuitry.
The amplified and filtered transducer signal is sent from the eDAQ Amp
to the analog input channels of the e-corder via a BNC–to–BNC cable.
The status On–line indicator on an eDAQ Amp is turned on only after
the software ‘finds’ or locates the eDAQ amplifier and carries out atwo–way dialog with the unit. This requires that the software, drivers,
electronics and power supplies are all performing correctly. It does notcheck the transducer side of the circuit — that is, there will be no
warning of a missing or badly connected transducer.

Bridge Amp Construction
The overall operation of the Bridge Amps can be better understood byreferring to Figure A–1.
The input stage consists of a chopper–stabilized amplifier with
(software–controlled) programmable gain of up to ×1000. Secondary
amplification is provided by the e-corder to give a total system gain of
up to ×200000 (±50 µV full scale). From the input amplifier, the signal
is passed to a fifth order, low–pass, switched capacitance filter, thecutoff frequency of which is selected under software control.
AC coupling is provided by an integrator feedback loop. This loop
forms an effective high–pass filter circuit with a time constant of 1 s. The
output of the integrator is fed back into the input terminals of the
chopper–stabilised amplifier, thus removing the DC content of thesignal. When AC coupling is selected, the automatic zeroing function isdisabled.
The excitation voltage is generated by a complementary output stage,
derived from a stable internal voltage reference, capable of giving up
to ±10 V (20 V DC) excitation at up to 50 mA. The transducer
excitation voltage can be adjusted by connecting a resistor between
pins 5 and 8 on the connector that plugs into the Bridge Amp’s input

46

Bridge & GP Amp

socket, see Figure 4–1 on page 46. This resistor is usually placed
inside the transducer’s DIN connector so that the transducer will becorrectly powered when it is attached to the Bridge Amp.
To remove any offsets in a transducer or signal baseline, the Bridge
Amp uses a 12 bit DAC (digital–to–analog convertor) to generate anoffset voltage. This is internally connected to the input stage when DC
coupling is used. Transducer offsets are zeroed by applying acorrective DC voltage to the input stage via the DAC, under software
control. In fact current is actually supplied in maximum offset steps of±0.05 µA, giving a resolution dependent on the transducer impedance.
For example, steps would be about 50 µV for a transducer withimpedance of 1 kΩ, less for a transducer with lower impedance.
The DAC is only capable of producing voltages in discrete steps. The
offset range can be adjusted downwards by a factor of about ten to
decrease the size of these steps and make the zeroing circuit moresensitive, especially at the lower ranges. The range can be adjusted byconnecting a resistor between pins 5 and 8 of the transducer connector
to Bridge Amp input socket, Figure 2–2 on page 6. The correct offset
range for the transducer will always be set when it is attached to theBridge Amp.

Figure A–1
Block diagram of theBridge Amp

Back panel

Front panel
3Signal input –2

Analogoutput10 kΩ

Common (ground)

7

User supplied
offset range resistor, Roff

10 kΩ
10 kΩ

6
Excitation +1

Excitation –4AC
integrator

12 bitDAC
150 kΩ

8

100 kΩ
150 kΩ

5
User supplied
excitation resistor, Rs

Chopperstabilised
amplifierVRef ±10 V

Low
pass
filters

I2C output
I2C control
interface

Signal input +I2C input

On-line indicator

Appendix A — Technical Aspects

47

GP Amp Construction
The block diagram of the GP Amp is shown in Figure A–2.
The input stage consists of a differential amplifier with (software–controlled) programmable gain up to ×1000. Secondary amplification
is provided by the e-corder to give a total system gain of up to ×5000(±2 mV full scale). From the input amplifier, the signal is passed to aselection of three 4–pole linear filters. The filter allows a range of cutoff
frequencies to be selected under software control. A ‘Filter off’ position
allows the full bandwidth of the amplifier to be employed.
The excitation voltage is generated by a complementary output stage,
derived from a stable internal voltage reference, capable of giving up
to ±10 V (20 V DC) excitation at up to 50 mA. The transducer
excitation voltage can be adjusted by connecting a resistor between
pins 5 and 8 on the connector that plugs into the GP Amp’s input
socket, see Figure 3–2 on page 26. This resistor is usually placed
inside the transducer’s DIN connector so that the transducer will becorrectly powered when it is attached to the GP Amp.
Figure A–2
Block diagram of the
GP AmpTo remove any offsets in a transducer or signal baseline, the GP Amp
uses a 12 bit DAC (digital–to–analog convertor) to generate an offset
Front panel

Signal input +Signal input –Common (ground)

Back panel

10 kΩ

3

2

Analog output
10 kΩ

7

User supplied
offset range resistor, Roff

Differential
Amplifier
10 kΩ

LK3

6

Low
pass
filters

150 kΩ
Excitation +1

Excitation –4

100 kΩ

User supplied
excitation resistor, Rs

12 bitDAC
150 kΩ

8

VRef ±10 V

I2C output
I2C control
interface5

I2C input

On-line indicator

48

Bridge & GP Amp

voltage. Signal offsets are zeroed by applying a corrective DC voltage
to the negative input stage, under software control. The input amplifier
under these circumstances functions as a high impedance single–ended
amplifier.
The DAC is only capable of producing voltages in discrete steps. The
offset range (and the size of the steps) can be decreased by a factor of
about ten to make the zeroing circuit more sensitive. The range can be
adjusted by connecting a resistor between pins 6 and 7 of thetransducer plug, Figure 3–11 on page 39. The correct offset range forthe transducer will then always be set when it is attached to the
GP Amp.
If you wish to use the GP Amp to provide a high impedance fully
differential input then the internal link marked LK3 must be removed,
and the offset range resistor, Rs, between pins 6 and 7 must not be
fitted to the transducer plug. This also renders the DC offset inoperative.
AC coupling (high pass filtering with a time constant of about 0.3 s)
can be selected in software. This is provided by an RC network at the
input. This setting is useful if small fast signal oscillations need to beobserved superimposed on a larger DC background. DC offset is notprovided in this mode.

Appendix A — Technical Aspects

49

50

Bridge & GP Amp

A

P

P

E

N

D

I

X

B Troubleshooting

B

This appendix describes some of the more common problems that canoccur, how they are caused, and what you can do to fix them. If you
continue to have difficulties then please your eDAQ representative ore–mail support@edaq.com.
Most of the problems that you are likely to encounter are caused byloose or missing connections. In the even of a problem always check allconnections and re–start the hardware and software. It is also
imperative that the transducer plug be wired correctly as outlined in
Chapters 3 and 4. Only rarely will there be an actual problem with the
Bridge or GP Amp or the e-corder itself.
The eDAQ Amp is not recognised by the Chart or Scope software.
• In this case the Status indicator on front panel of the Bridge or
GP Amp will fail to light when the software is started. Also thesoftware will indicate ‘Input Amplifier…’ rather than ‘Bridge Amp…’
or ‘GP Amp…’ on the channel to which the eDAQ Amp isconnected. The Status indicator is turned on at the end of abidirectional dialog between the e-corder unit and the eDAQ Amp
after the software checks most of the functions of the eDAQ Amp.
This dialog will fail if the I2C cable or the BNC–to–BNC cables fromthe eDAQ Amp to the e-corder are not be connected, or are loose.
Check to see that all cables are firmly attached. The BNC cables
must be connected to an e-corder input channel (not the e-corder
output!). Make sure you are looking at the software channel that
corresponds to the input channel to which the eDAQ Amp isconnected. You will need to re–start the software after changing or
adjusting cables.

Bridge & GP Amp

51

• The BNC or I2C cable may be faulty. Replace the cable (ifpossible) and try again. Spare cables are available from manyelectronics suppliers or can be obtained from your eDAQ
representative. You will need to re–start the software after changing
or adjusting cables.
• The eDAQ Amp or e-corder may have developed a fault. This is
the less likely than problems discussed above. Try using the eDAQ
amp on another e-corder (if available), or try using a different
eDAQ Amp (if available) on the e-corder. If you feel that you have afaulty unit please contact your eDAQ representative.
On starting up the software, an alert indicates that there is a problem
with the eDAQ Amp or driver. This will also prevent the Status Indicatorfrom being turned ON.
• The correct eDAQ Amp driver is not installed or is missing on yourcomputer. Reinstall the Chart and Scope software.
• The BNC or I2C cable is faulty, or there may be a faults with theeDAQ Amp or the e-corder (proceed as in the previous section).
The trace will not zero properly when using the automatic or manual
zeroing controls.
• Variations in the transducer signal during auto–zeroing procedure
can cause the software to fail to zero properly. Make sure that thetransducer is providing a steady signal during auto–zeroing.
• If the transducer is defective or subject to excessive load, it could
cause the offset range of the eDAQ Amp’s zeroing circuitry to be
exceeded.
• Check the transducer with another Bridge (or GP) Amp if possible
and try again. If you have bought the transducer from a supplierother than eDAQ then you will have wired the transducer connector
yourself. Please check your wiring and values of the excitation andoffset resistors. Excessive excitation could be causing heating of thetransducer and a drifting signal. An incorrectly selected offset
resistor will provide insufficient (or too much) offset.

52

Bridge & GP Amp

The signal from the transducer is noisy at high gain ranges• No transducer is completely noise free, and with sufficient
amplification noise will become apparent. Noise can be reduced by
the use of low pass filtering.
• If you suspect mains hum or signal interference from other sources(your PC and monitor can be a noise source) record the noise data
at high speed for a few seconds with the Chart software. Then
perform a Spectrum on the recorded data. The frequencies of mainshum (50 or 60 Hz and harmonics thereof) and other noise sources
will be visible as peaks in the power spectrum. This information issometimes useful in identifying the source of the noise.
There is no, or only a small, signal from the transducer, even at highgain ranges• Check that you have selected a sufficiently sensitive gain range on
the Bridge or GP Amp.
• Try to identify if the problem lies with the transducer or with the
Bridge (or GP) Amp by checking how the transducer performs withanother Bridge (or GP) Amp if possible. Also try a similar transducer
with the Bridge (or GP) Amp if possible. If the problem locatedwithin the transducer, then check the transducer plug wiring as
below. If you feel the problem resides in the Bridge (or GP) Amp
then please contact your eDAQ representative.
• The wiring in the transducer plug could be loose or incorrect. If
you have wired your own transducer plug, check the wiring andsoldering within the plug.
• Excitation (power) to the transducer could be insufficient or
missing. Check the wiring in the transducer plug and confirm that an
excitation resistor of the correct size has been fitted.

Appendix B — Troubleshooting

53

54

Bridge & GP Amp

A

P

P

E

N

D

I

C Specifications

X

C

Bridge Amp
Input
Connector:

8–pin DIN socket

Input configuration:

Differential or single–ended

Amplification range:±50 µV to ±200 mV full scale in 12 steps(combined e-corder and Bridge Amp)
±200, 100, 50, 20, 10, 5, 2, 1mV±500, 200, 100, 50 µV

Amplification accuracy:±0.5% (combined e-corder andBridge Amp)

Maximum input voltage:±5 V

Input impedance:

2 × 10 kΩ

Low-pass filtering:

1 Hz to 2 kHz in eight steps (software
selectable), using fifth order, switchedcapacitance filter typeHigh-pass filtering:

DC (off) or 0.1 Hz (software–selectable)

Frequency response (–3 dB): 2 kHz maximum at all gains with the
2 kHz filter selected

Bridge & GP Amp

CMRR (differential):

100 dB @ 50 Hz (typical)

Input noise:<2 µVrms referred to input at highest gain

55

Excitation and Zeroing
DIN Excitation voltage range:0 – 20 V DC (±10 V referred to ground),adjusted by external resistor
Transducer drive current:±50 mA maximum

Zeroing circuitry:

Software-controlled, either manual or
automatic

Internal offset resolution:

12–bit (internal DAC) 0 V ± 2048 steps.
Maximum offset steps of 5 µA, giving a
resolution of about 0.15 mV for atransducer impedance of 300 Ω.

Control PortI2C input and output:

Male and female DB–9 pin connectors.
Provides control and power.

Power requirements:±17 V DC
+8 V DC
3.2 W (without transducer)

Physical Configuration
Dimensions (h × w × d):

50 × 76 × 260 mm
1.96 × 3.0 × 10.2 inches

Weight:0.8 kg (1.8 lb)

Operating conditions:0 – 35˚C
0 – 90% humidity (non–condensing)eDAQ reserves the right to alter these specifications at any time.

56

Bridge & GP Amp

GP Amp
Input
Connector:

8–pin DIN socket

Input configuration:

Differential or single–ended

Amplification ranges:±2 mV to ±10 V full scale in 12 steps(combined e-corder and GP Amp)
±10, 5, 2, 1V±500, 200, 100, 50, 20, 10, 5, 2 mV

Amplification accuracy:±0.5% (combined e-corder and GP Amp)

Maximum input voltage:±15 V

Input impedance:

100 MΩ

Frequency response (–3 dB): 5 kHz maximum at all gains with filters off
CMRR (differential):

100 dB @ 50 Hz (typical)

Input noise:<2 µVrms referred to input at highest gain

Excitation and Zeroing
Excitation voltage range:

0 – 20 V DC (±10 V referred to ground),adjusted by external resistor

Transducer drive current:±50 mA maximum

Zeroing circuitry:

Software–controlled, either manual or
automatic

Internal offset resolution:

12–bit (internal DAC) 0 V ± 2048 steps.
Designed to offset a maximum of ±5 V,
giving a resolution of about 2.5 mV.

Filters
Bandwidth:

5 kHz (Low pass filters set to Off)Low pass filtering:

Fourth order Bessel
1, 10, or 100 Hz (software–selectable)

High pass filtering:

DC (off) or 0.3 Hz (software–selectable)

Appendix C — Specifications

57

Control PortI2C input and output:

Male and female DB–9 pin connectors.
Provides control and power.

Power requirements:±17 V DC
+8 V DC
3.2 W (without transducer)

Physical Configuration
Dimensions (h × w × d):

50 × 76 × 260 mm
1.96 × 3.0 × 10.2 inches

Weight:0.8 kg (1.8 lb)

Operating conditions:0 – 35˚C
0 – 90% humidity (non–condensing)eDAQ reserves the right to alter these specifications at any time.

58

Bridge & GP Amp

Appendix C — Specifications

59

60

Bridge & GP Amp

GP Amp

Index

A

E

AC coupling
Bridge Amp 46
GP Amp 49
automatic zeroing
Bridge Amp 13
GP Amp 33

eDAQ Amps 2
Bridge Amp 2
GP Amp 2
pH/mV Amp 2
Picostat 2
Potentiostat 2
eDAQ web site 2
excitation voltage
Bridge Amp 46
GP Amp 48

Bback panel
Bridge Amp 7
GP Amp 27
block diagram
Bridge Amp 47
GP Amp 48
Bridge Amp 2

Cchecking the eDAQ Amp 3
connecting multiple eDAQ Amps 8, 28

D
DAC 19, 39
differential transducer
offset required 41
digital-to-analog converter 19, 39, 47, 48
display offset
Bridge Amp 13
GP Amp 33

Bridge & GP Amp

Ffiltering
Bridge Amp 12
GP Amp 32
front panel
Bridge Amp 6
GP Amp 26
full–bridge transducers 21

G
GP Amp 2

H
half–bridge transducers 21
high impedance transducers
differential 42
single–ended 41
high–pass filter
Bridge Amp 46
GP Amp 49

61

I

R

I2C bus 45
I2C connector
Bridge Amp 7
GP Amp 27
I2C interface system 45
input connector
Bridge Amp 6

rangesBridge Amp 12
GP Amp 32

pin assignments 6
GP Amp 26

pin assignments 26
input ranges
Bridge Amp 12
GP Amp 32
inverting the signal
Bridge Amp 12
GP Amp 32

Mmanual zeroing
Bridge Amp 13
GP Amp 33

O
offsetBridge Amp 47
GP Amp 48
offset display
GP Amp 13, 33
offsetting the signal 19, 39
Online indicator
Bridge Amp 6
GP Amp 26
output connector
Bridge Amp 7
GP Amp 27

P
pH/mV Amp 2
Picostat 2
Potentiostat 2
power supply 46

62

S
self-test
Bridge Amp 9
GP Amp 29
sensitivity
Bridge Amp 12
GP Amp 32
signal filtering
Bridge Amp 12
GP Amp 32
single–ended transducers 41
specifications
Bridge Amp 55
GP Amp 57

Ttransducers
differential, offset required 41
full–bridge 21
half–bridge 21
high impedance true differential 42
single–ended 41
troubleshooting 51

Uuser modification 45

Z
Zero button
Bridge Amp 13
GP Amp 33
zero offset 19, 39
zeroing
Bridge Amp 13, 18
GP Amp 33, 39
zeroing the signal 19, 39

Bridge & GP Amp

Bridge Amp

Licensing & Warranty

Trademarks
e-corder and PowerChrom are registeredtrademarks of eDAQ Pty Ltd. All eDAQ software,
hardware, and documentation is protected bycopyright. eDAQ retains the exclusive ownership of
the trademarks and registered trademarks
represented by its company name, logo, andproduct names.

Responsibilities
You and any others using any eDAQ product agree
to use it in a sensible manner for purposes for which
it is suited, and agree to take responsibility for theiractions and the results of their actions.
If problems arise with an eDAQ product, eDAQ will
make all reasonable efforts to fix them. This service
may incur a charge, depending on the nature of the
problems, and is subject to the other conditions inthis Agreement.

Hardware Warranty
eDAQ Pty Ltd warrants the Bridge or GP Amp, to be
free of defects in material and workmanship for one
year from the date of purchase. eDAQ Pty Ltd will
repair or replace defective equipment as
appropriate.
To obtain a warranty repair/replacement you mustfirst notify us before return of the instrument and wewill issue you with a RAN (return authorization
Bridge & GP Amp

number). You must ship the defective product at yourexpense. We will pay return shipping. The productshould be packed safely (preferably in its original
packaging) and have the RAN on the shippinglabel. Returns sent without a RAN may be refuseddelivery.
This warranty does not cover hardware that has:• been modified by the user in any way;
• been subjected to unusual physical, electrical,
or environmental stress. This includes damage
due to faulty power sockets, inadequate
earthing, or power spikes or surges;• been damaged because of incorrect wiring toancillary equipment, or because of substandard,connectors or cables; or• had the original identification marks removed
or altered.

Software License
You have the non-exclusive right to use the suppliedeDAQ software (Chart, Scope etc). Your employees
or students, for example, are entitled to use it,provided they adhere to this agreement. Eachseparate purchase of the eDAQ software licenses it
to be used on two computers at any given time (onone computer for data acquisition with a e-corder
hardware unit, and on a second computer for the
analysis of existing data files). Although multiple
copies of a program may exist on several
computers, more than two copies must not be used63

simultaneously. Departmental/company licences areavailable if you wish to run more than two copies
simultaneously.

Technical Support
Please register your unit to receive technical support.
Technical assistance is available via email. Please
describe the problem with as much detail as
possible. Include a small example data file, if
appropriate. Please also state:• the model and serial number of your e-corderunit and Bridge or GP Amp.
• the type of computer and operating systembeing used (for example Windows XP, or
Macintosh OS 10.2)• the software version you are using (forexample Chart v5.1.2)
We endeavor to answer all your questions, but insome cases, for example where the problem relates
to the other equipment that you are using, a nominal
fee may be charged.

Jurisdiction
eDAQ Pty Ltd is bound by the laws of New South
Wales in Australia, and any proceedings shall beheard by the Supreme Court of New South Wales inAustralia.

Disclaimer
eDAQ reserves the right to alter specifications ofhardware and software without notice.
No liability can be accepted for consequential
damages resulting from use of eDAQ products.

64

Bridge & GP Amp