DBK4

2-Channel Dynamic Signal Input Card
Overview …… 1
DBK4 Power Notice …… 2
Hardware Setup …… 3
Card Configuration …… 3
Card Connections …… 6
CE Compliance …… 6
DaqBook/100 Series & /200 Series and DaqBoard [ISA type] Configuration …… 7
DaqBook/2000 Series & DaqBoard/2000 Series …… 7

Software Setup …… 7
Hardware Description …… 7
Current Source (Transducer Biasing) …… 7
Signal Coupling ……8
Amplifier …… 8
Low-Pass Filter …… 8
Sample and Hold …… 8Power Management …… 8
Software Controls …… 9

DBK4 – Specifications …… 9
Accelerometer Tutorial …… 11
What is a Piezoelectric Accelerometer? …… 11
Accelerometer Specification Parameters …… 11
Physical Setup …… 13
Electrical Grounding …… 14
Practical Limitations ……15
Cable-Connector Handling Precautions ……15
Cable Driving ……16

Reference Notes:
o Chapter 2 includes pinouts for P1, P2, P3, and P4. Refer to pinouts applicable to yoursystem, as needed.
o In regard to calculating system power requirements refer to the DBK4 Power Notice on page 2of this section and to DBK Basics located near the front of this manual.

Overview
DBK4 is a 2-channel signal conditioner for dynamic signals. The unit provides a means of interfacing thedata acquisition device with piezoelectric transducers that include, but are not limited to: accelerometers,
microphones, and force/pressure transducers. The DBK4 interfaces with the LogBook’s or Daq Device P1
connector.

DBK Option Cards and Modules

887793

DBK4, pg. 1

DBK4 Power Notice
DaqBook/100 cannot provide adequate power for a DBK4. For information regarding the use of powercards and power modules see Power Requirements in the DBK Basics section near the front of thismanual.
Power-Requirement Worktable

CAUTION

Quantity x milli-Watts = power required

Excessive power consumption can cause equipment damage.
Calculate system power requirements before adding a DBK4
to the system.

Note: It is important to carefully read this DBK4 power notice and othersections of the manual that apply to your LogBook or Daq device
system.
In comparison to other DBK cards, the DBK4 demands significant power.
It is important that you calculate your system’s power demand, as you may
need to add auxiliary power supplies. This document provides a worktablefor calculating system power demand.
To use the worktable: list the quantities of the applicable DBK options, andmultiply these by the mW value. Place the results in the Power column andadd them. The result is the total power required.
If the power required exceeds, or is close to the available
power, you must add a power module or card (for example
a DBK30A or DBK32A).

The following two charts provide information regarding available power.
A/D Available Power Chart
Product
LogBook/300, /360 (Note 2)
DaqBook/100, /112, /120
DaqBook/200, /216, /260
DaqBook/2000A, E, X
DaqBook/2001, /2005
DaqOEM/2001, /2005
DaqBook/2020
DaqBoard/100A, /112A
DaqBoard/200A, /216A
DaqBoard/2000 Series
DaqLab/2000 Series
DaqScan/2000 Series
Daq PC-Cards

Available

mW
5000
2100
4000
15000
10000
10000
7500
3300
3000
2250
10000
10000
0

Option
DBK1
DBK2
DBK4
DBK5
DBK7
DBK8
DBK9
DBK11A
DBK12
DBK13
DBK15
DBK16
DBK17
DBK18
DBK19
DBK20
DBK21
DBK23**
DBK24**
DBK25
DBK40
DBK41
DBK42**

Qty

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

mW
0
565
2750

x
x
x
x
x
x
x
x

135
420
455
560
0
455
455
485
1040
905
1085
200
50
50
10
10
10
0
0
35

DBK43A**

=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=

35

Auxiliary Power Supplies

DBK44

x

330

=

Product

Available mW

DBK45

x

1565

=

DBK32A

15000 (Note 1)

DBK46
DBK50**DBK51**
DBK52
DBK53
DBK54
DBK55
DBK65
DBK70**
DBK80
DBK81
DBK82
DBK83
DBK84
DBK85

x
x
x
x
x
x
x
x
x
x
x
x
x
x
x

2600
35
35
200
455
455
950
35
35
755
1060
1810
1810
1810
755

=
=
=
=
=
=
=
=
=
=
=
=
=
=
=

DBK90
DBK100

x

1400
300

=
=

DBK101x2700=

For information regarding
Battery and UPS Modules,
refer to the following sectionsof the DBK Options Manual:
DBK30ADBK34A

power

Total Power Required ⇒(Column Total)
Note 1: DBK32A provides an additional 15000 mW of power for Daq and LogBook
data acquisition systems. Its output is ±15 VDC. DBK32A is not to be
used for Daq PC-Cards.
Note 2: DBK2 and DBK5 are not to be used with LogBooks.
** Two asterisks indicate that the module has an internal power supply that is powered separately.
DBK4, pg. 2887793

DBK Option Cards and Modules

Hardware Setup
If using a DaqBook/200, DaqBook/216, DaqBoard/200A, or DaqBoard/216A that has a
serial number of 103350 or earlier, you must contact the factory for a hardware
upgrade (EO-1911) before using the DBK4.

Software calibration for each DBK4 requires a set of unique constants supplied on adiskette identified by a serial number matching the DBK4 card. To install these
constants, follow instructions on the “readme” file included on the calibration disk.

Card Configuration
Factory Defaults•

Current Level – 4ma•

Coupling – 10 Hz•

Current Source – On•

40Khz Filter – In-Line

Both hardware and software are used in configuration. This section focuses on the hardware. Jumpers andswitches are used to set: current source (current level and source enable/disable), filter bypass, signal
coupling, and channel addressing. It is best to remove power from the card to perform these selections.
The figure reveals the jumper and switch locations.
Note: The hardware jumper settings referenced in the following subsections are per-channel selections
unless otherwise specified.

DBK4, Board Layout (not to scale)

DBK Option Cards and Modules

887793

DBK4, pg. 3

Signal Coupling (JP2 & JP3)
The figure shows jumper settings for selecting AC coupling, 10 Hz or 0.1 Hz High Pass Filter (HPF), orDC coupling.
Signal coupling is application specific. 10 Hz HPF suits most applications for acceleration measurements
on “light” structures. When performing seismic measurements (or measurements on “massive" structures)the 0.1 Hz HPF rejects the DC bias component while preserving the low-frequency signals.

JP2 Default: 10 Hz

JP3 Default: 10 Hz

DC coupling may be used when making a voltage mode measurement.

CAUTION
When the input signal is DC-coupled, the current source must be deselected (Jumper JP4
and/or JP5 removed). Failure to do so could result in damage to the transducer.

Current Source for Transducer Biasing (JP4 & JP5)
The DBK4 provides a current source for transducer biasing. Bias current is selected (enabled or disabled)
via jumpers JP4 (channel 0) and JP5 (channel 1) as shown in the figure.

CAUTION
When the input signal is DC coupled, the current source must be “off-line” (jumper
removed) to protect the transducer. While DC coupled, the output voltage of the current
source can approach 30 V depending on the output impedance of the transducer.

DBK4, pg. 4887793

DBK Option Cards and Modules

Current Level (Transducer Biasing) (JP6)
Current level is not channel-specific. The level selected applies toboth channels. The figure shows the JP6 jumper settings for
current level. Most transducers operate with either 2 or 4 mA ofbias current. However, biasing at 4 mA allows the transducer todrive longer cables.
Reference Note:
For more information, refer to the Cable Driving section of the DBK4 Accelerometer Tutorial.

Filter Bypass (JP7 & JP8)
Jumpers JP7 and JP8 are used to enable or disable programmablefiltering for channel 0 and 1 respectively. This selection determines
the bandwidth of the signal to be processed. When disabled(bypassed) the DBK4 can process a signal with a bandwidth up to 40
kHz. The gain in bypass mode is greater than in the filter mode by afactor of 1.583. For example, When the PGA gain is set to 10, thefilter-mode gain is ×10 and the bypass-mode gain is ×15.83.

Main Channel and Sub Channel Selection
Up to 8 DBK4 cards can use 1 of 16 main channels.
Each DBK4 has 2 inputs; thus the input capacity is
256 channels (2 inputs × 8 cards × 16 main channels).
The DBK4 uses JP1 and SW1 for channel and subchannel selection. (Position s8 on SW1 is not used.)
The DBK4 card or a group of DBK4 cards (0-7) musthave a dedicated main channel. Do not intermix
DBK4 cards with other types of DBK cards in achannel. LogBook or Daq device will not function
properly if channel settings are in conflict.

DBK4 Channel Hierarchy
Record all channel settings and label the transducers for future reference.

JP1 on the DBK4 is a 2×16 header located on the far left side of the card. A shunt jumper setting on JP1
allows a card to reside in 1 of 16 possible main channels. Any main channel in the range 0-15 is valid.
The figure below shows that sub-switches s1 to s4 of SW1 can be set by binary values to equal the main
channel selection. This setting must match the jumper on JP1 so the onboard microcontroller knows theJP1 selection. The card then interprets and responds to only the correct card-related commands via P1.

DBK Option Cards and Modules

887793

DBK4, pg. 5

The DBK4’s sub-channel address is selected using switches s5 - s7 of DIP switch SW1 (located above andto the right of JP1). As a 2-channel card, 3 switches are used to select 8 sub-channel addresses. Therefore,
it is possible for each main channel to use up to 8 DBK4s. The figure below shows 16 switch settings forthe main channel and 8 settings for the sub-channel (card) selection.

Card Connections
The DBK4 connects to the LogBook’s or Daq device’s P1 port or a P1 port on a DBK Expansion Module.
For signal inputs, a BNC connector interface is provided for each channel. The DBK4 PC Board has a
microdot connector footprint that is wired in parallel with each BNC connector. The user is can customize
the input connector as needed. As a factory default, the DBK4 is not populated with microdot connectors.

CE Compliance
Special Cables and Connectors for CE Compliance
To insulate BNC connectors from high-voltage sources such as static charges that can enter the circuitry
via an exposed connector and cause ESD damage. Specific cables and connectors that must be used (asstated in the Declaration of Conformity), PVC dust caps (p/n CN-96) must cover all unused BNC
connectors. With dust caps removed, special coaxial cables with insulated end-connectors and rubber Orings(p/n 418-0800) must be used (see figure). Properly installed connectors and dust caps ensure that no
metallic surfaces are exposed.

DBK4, pg. 6887793

DBK Option Cards and Modules

DaqBook/100 Series & /200 Series and DaqBoard [ISA type] Configuration
Several setup steps of DaqBooks /100
Series & /200 Series devices and
DaqBoards [ISA type] are required to use
DBK4 cards in a system. Note that the
Daq PC-Card is configured in software; itcannot supply power to DBKs.
1.

If not using auxiliary power, place
the JP1 jumper in the expanded
analog mode (Analog Option Card
Use). In this mode jumpers are at
JP1’s –15 V and +15 V positions.

Note: This default position is necessary
to power the interface circuitry of
the DBK4 via the internal ±15
VDC power supply. If using
auxiliary power DBK32A or
DBK33, you must remove both
JP1 jumpers.

DaqBook/100 & /200 & DaqBoard [ISA]
Settings for the DBK4The DBK4 is power intensive; refer to the DBK4 Power Notice (DBK4 page 2) and toPower Requirements in the DBK Basics section near the front of this manual.
2.

Place the JP2 jumper in the SSH position.

CAUTION
When using the SSH output, do not use an external voltage reference for DAC1.
Applying an external voltage reference for DAC1, when using the SSH output, will
result in equipment damage due to a conflict on P1, pin #26.
3.

For DaqBook/100, DaqBook/112 and DaqBook/120 only, place the JP3 jumpers in bipolar mode.

4.

For DaqBook/100, DaqBook/112 and DaqBook/120 only, place the JP4 jumpers in single-ended
mode.

DaqBook/2000 Series & DaqBoard/2000 Series
No jumper configurations are required for these /2000 series devices.

Software Setup
Reference Notes:
o DaqView users - Refer to the chapter 2, DBK Setup in DaqView.
o LogView users - Refer to the chapter 3, DBK Setup in LogView.

Hardware Description
Current Source (Transducer Biasing)
Each channel has a constant-current source that can be jumper-set to 2 mA or 4 mA. The bias current issourced through the center conductor of a coaxial lead and returns to the DBK4 from the transducer’s case
via the outer conductor. The current source provides a dynamic load of more than 1.4 MΩ and has anegligible loading effect on the transducer’s output. If bias is not required, removing a jumper andcoupling the signals directly to the input preamplifier can disable the current source.
Note: Use the 4 mA for long cables, high peak voltages, and high signal frequencies.

DBK Option Cards and Modules

887793

DBK4, pg. 7

Signal Coupling
The input can be configured via jumpers to maximize the usable signal spectrum. The DBK4 provides
two, 1-pole High-Pass Filters (HPF) and one DC path. The HPF can be set to 0.1 Hz, 3 dB cut-off
frequency. In this case, the system frequency response is limited by the source characteristics. The HPF
can also be set to 10 Hz, 3-dB cut off frequency, for high-frequency measurements. The 10 Hz setting isuseful in attenuating setup-induced low-frequency signals that can reduce the dynamic range (e.g., whenusing tape recorders as signal sources).
Note: When using 0.1 Hz coupling, you should set the baseline (via software) to one-shot mode.
The DC path allows the DBK4 to be used in voltage-mode measurements that convey DC as well as AC
information. For example, when motion is measured with a DC LVDT, the transducer is biased with anindependent power supply and its output is referenced to ground.

CAUTION
JP4 or JP5, as applicable, must be removed to disable the current source whenever the
input is DC-coupled. Failure to remove JP4 or JP5, when the input is DC-coupled, canresult in transducer damage.

Amplifier
The DBK 4 includes a Programmable Gain Amplifier (PGA) with three gain settings (×1, ×10, and ×100).
The gain settings are set for each channel via software. These gain settings are performed at configuration
and remain unchanged during a measurement sequence. The PGA has a flat response up to 100 kHz for
in-band and out-of-band frequencies.

Low-Pass Filter
To protect the in-band components from distortion caused by aliases of the out-of-band spectrum, theDBK4 features a 12-pole programmable Butterworth Low-Pass Filter (BLPF). There are eight 3 dB cut-off
frequency (Fc) settings per channel, the lowest frequency is 141.6 Hz and the highest is 18 kHz. In
general, high order BLPF introduce amplitude and phase distortion near the 3 dB cut off frequency.
Limiting the measurement range can reduce this error. A typical measurement range is 70% to 80% of the
3-dB cut-off frequency.
For narrowband measurements, the anti-alias filter can be bypassed providing an extended bandwidth of
40 kHz. In this configuration, the channel has lower distortion and a gain factor of 1.583. This setup may
be useful when calibrating accelerometers with sinusoidal motion. However, in this case, special attention
should be given to external noise since high-frequency noise will be aliased into the signal band.
The switched capacitor clock must be turned on in software for the filter to work.

Sample and HoldThe card’s sample and hold amplifier samples all channels within a system (up to 256) within 50 ns of one
another and preserves their phase information. Measurement of transmissibility can be made between any
two of the 256 possible channels.
The Simultaneous Sample and Hold (SSH) is triggered on the first channel of a scan sequence; after which,
the A/D converter sequentially measures the output of each of the SSH amplifiers. In a scan sequence, thefirst channel should be a non-SSH channel. However, if the scan sequence contains only SSH channels thefirst channel will contain data taken in the sample mode. This data is inaccurate and therefore should bediscarded; however, the second sample (valid) can be a repeat of the first.

Power Management
The DBK4 has circuits for a variety of measurement environments. When not needed, the filters and/or
current source can be powered down via software. This feature can be useful in field units to reduceoverall power consumption.

DBK4, pg. 8887793

DBK Option Cards and Modules

Software Controls
Power management, the PGA, and the low-pass filter’s cut-off frequency are all software controllable.
These parameters are sent via the external address bus in the P1 connector to the DBK4. The microcontroller decodes the configuration message, sets the PGA and filter controls, and illuminates the onboard LED. However, if transmission errors are detected, the micro-controller flashes the LED and ignoresthe message. The error is cleared when an error-free message is received. At power up, the current source
and filters are powered off and the LED is off. The LED will remain off until the DBK4 is successfully
addressed for the first time.

DBK4 – Specifications
Name/Function: 2-Channel Dynamic Signal Input Card
Physical Specifications
Channels: 2
Signal Connection: 1 BNC connector per channel
Dimensions: 8.28" W × 3.25" H
Operating Specifications
Power Consumption (typical): 20 mA @ +5 V, 95 mA @ +15 V, 80 mA @ -15 V
Current Source:
Output Impedance: >1.4 MΩ
Compliance: 27 VDC
Current Levels: 2 & 4 mA
Broadband Noise: 0.3 µA RMS (Bw = 100 kHz)
Input Impedance: 150 kΩ
Gain:
Bypass Mode:
×1.583×15.83 ×158.3
Filter Mode:
×1
×10
×100
Input Ranges: ±5 V
±500 mV ±50 mV FS
Coupling: AC, DC
AC:
1-pole, 10 Hz HPF
1-pole, 0.1 Hz HPF
Input Signal/Noise: >96 dB (Bw = 100 kHz)
Filter(Typical):Distortion (typical)
Bypass Mode: The jumpers JP7 and JP8 in the 1-2 position disabling
the programmable low pass filter.
@ 100 Hz - 81 dB@ 1 kHz - 82 dB@ 5 kHz - 81 dB
Filter Mode: The jumpers JP7 and JP8 in the 2-3 position enabling the
programmable low pass filter. With the following cutoff
frequencies selected via software.
4.5 kHz cutoff frequency:@ 100 Hz - 79 dB@ 1 KHz - 75 dB
18 kHz cutoff frequency:@ 100 Hz - 68 dB@ 1 KHz - 68 dB@ 5 kHz - 58 dB*Note the filter mode measurements were taken at the +5 V range
with the software enabled switched capacitor clock and excitation.

DBK Option Cards and Modules

887793

DBK4, pg. 9

Cut-off(Fc):
18 kHz 9 kHz
4.5 kHz 2.25 kHz
1125 Hz 562.5 Hz 281.2 Hz 141.6 Hz
Flatness DC - 80% Fc: ±0.2 dB
Channel Matching DC - 80% Fc: Phase: ±6°
Accuracy Passband Center: ± 0.5 dB

DBK4, pg. 10887793

DBK Option Cards and Modules

Accelerometer Tutorial
This accelerometer tutorial covers the following topics. Page numbers refer to DBK4 document module
pages.
What is a Piezoelectric Accelerometer?......11
Accelerometer Specification Parameters......11
Physical Setup......13
Electrical Grounding......14
Practical Limitations...... 15
Cable-Connector Handling Precautions......15
Cable Driving......16

What is a Piezoelectric Accelerometer?
A low-impedance piezoelectric accelerometer consists of a piezoelectric crystal and an electronic amplifier.
When stretched or compressed, the two crystal surfaces develop a charge variation that is related to theamount of stress, shock, or vibration affecting the crystal. The amplifier outputs a corresponding signal
and transforms the sensor’s high impedance to a lower output impedance of a few hundred ohms. Besides
acceleration, such sensors can measure pressure and force.
The circuit requires only 2 wires (coaxial or twisted pair) to transmit both power and signal. At low
impedance, the system is insensitive to external or “triboelectric” cable noise. Cable length does not affectsensitivity.

The figure shows a simple sensor-DBK4 connection. The MOSFET is powered from a constant-current
source of 2 or 4 mA at 27 volts. In the quiet state, the MOSFET circuit will bias off at about 12 V. As thesystem is excited, a voltage develops across R and the crystal and is applied to the gate of the MOSFET.
This voltage will cause linear variation in the impedance of the MOSFET and a proportional change in bias
voltage. This voltage change will be coupled to the DBK4 input amplifier through the capacitor C. The
value of R and the internal capacitance of the crystal control the low-frequency corner. Units weighing
only a few grams can provide high-level outputs up to 1 V/g with response to frequencies below 1 Hz.

Accelerometer Specification Parameters
Noise in Accelerometers
The noise floor or resolution specifies the lowest discernible amplitude (minimum “g”) that can bemeasured. There are two main sources of noise:• Noise from the crystal and circuit inside the accelerometer. Some types of crystals, such as
quartz, are inherently more noisy than others. A good noise floor is 10 to 20 µV.
• Noise from electrical activity on the mounting surface. Since the signal is a voltage, 60 Hz orother voltages (induced or ground loop, etc) may interfere. The best protection is toelectrically isolate the accelerometer.

DBK Option Cards and Modules

887793

DBK4, pg. 11

Sensitivity
The sensitivity of an accelerometer is defined as its output voltage per unit input of motion. The unit ofmotion used is the “g”. One “g” is equal to the gravitational acceleration at the Earth’s surface, which is
32.2 ft/(sec)(sec) or 981 cm/(sec)(sec). The output is usually specified in millivolts per “g” (mV/g).
Sensitivity is usually specified under defined conditions (frequency, testing levels, and temperature), forexample: 100 mV/g at a frequency of 100 Hz, level +1 g, at 72°F.
While a given sensor model may have a “typical” sensitivity of 100 mV/g, its actual sensitivities may range
from 95 to 105 mV/g when checked under stated conditions. The manufacturer typically provides sensor
calibration values.
Transverse Sensitivity
An accelerometer is designed to have one major axis of sensitivity, usually perpendicular to the base andco-linear with its major cylindrical axis. The output caused by the motion perpendicular to the sensing axis
is called the transverse sensitivity. This value varies with angle and frequency and typically is less than 5%of the basic sensitivity.
Base-Strain Sensitivity
An accelerometer’s strain sensitivity is the output caused by deformation of the base due to bending in themounting structure. In measurements on large structures with low natural frequencies, significant bending
may occur. Units with low base-strain sensitivity should be selected.
Inserting a washer smaller than the accelerometer base under the base to reduce the contact surface area
can substantially reduce base-strain effects. This technique lowers the usable upper frequency range.
Acoustic Sensitivity
High-level acoustic noise can induce outputs unrelated to vibration input. In general, the effect diminishes
as the accelerometer mass increases. Using a light, foam-rubber boot may reduce this effect.
Frequency Response
An accelerometer’s frequency response is the ratio of the sensitivity measured at frequency f to the basic
sensitivity measured at 100 Hz. This response is usually obtained at a constant acceleration level, typically
1 g or 10 g. Convention defines the usable range of an accelerometer as the frequency band in which thesensitivity remains within 5% of the basic sensitivity. Measurements can be made outside these limits if
corrections are applied. Care should be taken at higher frequencies because mounting conditions greatly
affect the frequency range (see Mounting Effects below).
Dynamic Range
The dynamic measurement range is the ratio of the maximum signal (for a given distortion level) to theminimum detectable signal (for a given signal-to-noise ratio). The dynamic range is determined by several
factors such as sensitivity, bias voltage level, power supply voltage, and noise floor.
Bias Level
Under normal operation, a bias voltage appears from the output signal lead to ground. There are two basic
MOSFET configurations commonly used. One exhibits a 7-8 V bias and the second a 9-12 V bias.
Operation of the two circuits is identical except for the available signal swing. The low-voltage version
typically exhibits 5-10 µVrms versus 10-20 µVrms for the high voltage.
Thermal Shock - Temperature Transients
Piezoelectric accelerometers exhibit a transient output that is a function of a temperature’s “rate-ofchange”. This “thermal shock” is usually expressed in g/°C and is related to:• Non-uniform mechanical stresses set up in the accelerometer structure• A pyroelectric effect in piezoelectric materials—an electrical charge is produced by thetemperature gradient across the crystal.

DBK4, pg. 12887793

DBK Option Cards and Modules

This quasi-static effect produces a low-frequency voltage input to the MOSFET amplifier. This voltage isusually well below the low-frequency corner, but the effect can reduce the peak clipping level and cause
loss of data. This effect does not affect the accelerometer’s basic sensitivity or the data unless the thermal
shift in the operation bias level results in clipping. Where drastic thermal shifts are expected, use 12 V bias
models. The effect’s severity is related to the mass of the accelerometer. In 100 mV/g industrial units, theeffect is usually negligible. Using rubber thermal boots can reduce the effect significantly.
Overload Recovery
Recovery time from clipping due to over-ranging is typically less than 1 ms. Recoveries from quasi-static
overloads that generate high DC bias shifts are controlled by the accelerometer input RC time constant thatis fixed during manufacture.
Power Supply Effects
The nominal power supply voltage recommended by most manufacturers is 15 to 24 V. Units may be usedwith voltages up to 28 volts. Sensitivity variations caused by voltage change is typically 0.05%/volt.
Power supply ripple should be less than 1 mVrms.
Connector
This parameter specifies the connector type and size (4-48, 6-40, 10-32 coaxial etc) and the location on thesensor, i.e., top or side (usually on the hex base). Where there is no connector on the sensor, an integral
cable is specified with the length and the connector, i.e., integral 6-ft to 10-32.

Physical Setup
Mounting Effects
Adding an accelerometer to a test object may influence the object’s mechanical resonance and/or reduce
the resultant vibration level. The mass of the structure and accelerometer as well as the position on the
structure can affect the measurement. For example, an accelerometer with a small mass relative to a car’s
mass will affect measurements on the hood when placed directly in the hood’s center. The sameaccelerometer placed on the more massive frame will have little effect.
The resonance change in a simple spring-mass system caused by adding an accelerometer may beapproximated by:∆f = fn {1 - [m/(m - ma)]½}
Where:∆f = Change in Resonant Frequency
fn = Original Resonance Frequency without Accelerometerm = Spring Mass
ma = Mass of Accelerometer
Threaded Stud Mount
The preferred method for attaching an accelerometer to the test surface is by the threaded stud mount. Thismethod fuses the accelerometer and test surface. When the stud mount is impractical, an adhesive
installation is usually a good alternative.
Adhesive Mounts
Some accelerometers can be glued directly to the test surface. Others must use mounting adapters (bases).
These adapters are normally glued to the test surface and the accelerometers are stud-mounted to them.
When applying a layer of adhesive, keep in mind that the adhesive creates a new spring-mass system withlower high-frequency response. To avoid a thick glue line, Cyanocrylate adhesives work well because:• They cure quickly, within seconds.
• Glue lines can be kept ultra-thin by using glue sparingly and pressing the accelerometer to thesurface with adequate force.
• Cleanup is easy since these adhesives will dissolve with acetone.

DBK Option Cards and Modules

887793

DBK4, pg. 13

Removal of Adhesive Accelerometers
Many accelerometers and adhesive adapters have been damaged by improper removal. A safe removal
method is to torque the accelerometer or its adapter with a wrench using the flats provided. Adhesives are
generally weakest in the shear mode and will yield under steady torque.

Accelerometer Mounting

CAUTION
Never strike an accelerometer to remove it. The trauma will likely damage theaccelerometer and affect calibration.

Magnetic Mounting Adapters
Magnetic mounting adapters can attach accelerometers to ferromagnetic surfaces such as machinery andstructures where the sensor is to be moved quickly from place to place. The accelerometer is attached tothe magnet (usually by stud mount) and the assembly is then applied to the surface.
In general, use magnetic adapters with caution and never beyond 1 kHz. Expect response degradation in
proportion to accelerometer mass. To ensure the best accuracy from the magnetic mount:1.

If possible, attach the magnet to bare metal. Paint will lessen the holding force of the magnet and canaffect response.

2.

Clean mounting area to remove oil, grease and foreign matter, which could reduce the contact for astrong magnetic bond.

3.

Select a flat area to achieve maximum surface contact.

4.

Attach the magnet to the test surface CAREFULLY. Remember that the pull of the magnet rises
sharply just before contact with the ferromagnetic surface. This force could pull the assembly fromyour hand resulting in a severe metal-to-metal shock. This could over-range the accelerometer beyond
its maximum shock limit and damage it.

5.

If possible, calibrate the magnetic/accelerometer assembly.

Electrical Grounding
Case-Grounded Design
In case-grounded designs, the common lead on the impedance-matching circuit is tied to the accelerometercase. The accelerometer base/stud assembly forms the signal common and electrically connects to the shellof the output connector. Case-grounded accelerometers are connected electrically to any conductive
surface on which they are mounted. When using these units, take care to avoid errors due to ground noise.
Isolated-Base Design
To prevent ground-noise error, many accelerometers have a base-isolated design. The outer case/base of
the accelerometer is isolated electrically off ground by means of an isolation stud insert. The proprietary
material used to form the isolation provides strength and stiffness to preserve high-frequency performance.
DBK4, pg. 14887793

DBK Option Cards and Modules

Practical Limitations
Mass Loading
The accelerometer mass should be less than 10% of the rigidly-coupled mass of the test object. The test
object should be rigid at the mounting point, such as a bearing housing rather than a sheet metal cover.
Upper Frequency Response
Piezoelectric accelerometers will attenuate below the low-frequency 3-dB point, but they will amplify at ornear their resonant frequency. The mounted resonant frequency of the accelerometer should be about fivetimes the highest frequency of interest (for ±0.5 dB accuracy). Refer to the frequency response data
supplied with each accelerometer to establish the usable range. For general surveys or broader tolerances,smaller multiples may be used. For example, for frequencies up to 1/2 the mounted resonance, sensitivity
would typically be 12% higher than nominal.

Cable-Connector Handling Precautions
Connectors
Clean the connector ends periodically with a stiff-bristle brush (such as a glue brush or small paintbrushwith clipped bristles) dipped in a solvent. This will remove metal slivers (which can peel off connector
threads and short across contacts), oil and other contaminants.
When connecting 10-32 or 5-44, thread the cables to sensor jacks. Do not use pliers to torque knurledcoupling nuts in place. Hand tightening is sufficient for most applications. If the nut tends to loosen under
severe shock or vibration, you may use a mild thread-locking compound. Use it very sparingly and apply
only to the male thread to avoid contamination of the electrical contact.
Because of their size, 10-32 (and smaller) connectors are relatively fragile. Always hold the transducer
steady and twist off the knurled coupling-nut. Never hold the coupling-nut and twist the transducer. Use
10-32 (and smaller) connectors only when necessary.
Cables
The weakest point on the cable assembly is at the intersection of the wire and connector; so, avoid stress at
this point. Sharp bends can overstress the conductors and insulation causing shorting or breakage.
The figure shows the preferred methods of cablerouting to the connector. The flexure occurs awayfrom the intersection point. Relative cable motion
at the sensor body or connector can cause erroneous
output signals at frequencies where cable motion is
amplified (cable “whip”). Miniature accelerometers
are especially susceptible due to their small size andlow mass. This problem is remedied by proper tie
down as shown. A “strain relief loop” will ensurethat cable flexure does not occur at the connector
when the cable is fastened to a stationary surface.
Accelerometer Connections
Triboelectric Noise
Triboelectric noise is caused by the potential developed in a flexing cable. The cable shield may rub
against the cable’s insulator, and the interior friction generates a charge that produces noise. The
triboelectric noise voltage or charge is coupled to the signal leads by inner conductor capacitance and may
be represented by a voltage source in series with a capacitance. In general, triboelectric noise is associatedwith bending of signal cables when low-frequency vibrations (below 10Hz) are being measured. If moving
or securing the cable alters the output compared to an external reference, triboelectric noise is a likely
suspect.

DBK Option Cards and Modules

887793

DBK4, pg. 15

Cable Driving
Operation over long cables is a concern with all types of sensors. Concerns involve cost, frequency
response, noise, ground loops, and distortion caused by insufficient current available to drive the cablecapacitance.
Coupling a short (e.g., 1m) adapter cable from the accelerometer to a long, low-cost cable like RG-58U or
RG-62U with BNC connectors can reduce the cost of long cables. Since most cable failures occur at theaccelerometer connection where vibration is the greatest, only the short adapter cable would needreplacement.
Capacitive loading in long cables acts like a low-pass, second-order filter and can attenuate or amplify
high-frequency signals depending on the output impedance of the accelerometer electronics. Generally thisis not a problem with low-frequency vibration (10 Hz to 2000 Hz). For measurements above 2000 Hz andcables longer than 100 ft, the possibility of high-frequency amplification or attenuation should beconsidered.
The DBK4 constant-current source provides 2 or 4 mA to integral electronics. Use the higher current
setting (4 mA) for long cables, high peak voltages, and high signal frequencies.
The maximum frequency that can be transmitted over a given length of cable is a function of both the cablecapacitance and the ratio of the maximum peak signal voltage to the current available from the constantcurrent source:

f =

K
⎛ V ⎞
⎟
2πC⎜
⎝ Icc − Ib ⎠Drive Current
(mA)
2
2
2
4
4
4

DBK4, pg. 16

Where:
f = Maximum frequency in Hz
9
K = 3.45 ×10 (K is the scale factor to convert Farads to picoFarads andAmperes to milliAmperes and a factor to allow cable capacitance tocharge to 95% of the final charge.)
C = Cable capacitance in picoFarads
V = Maximum peak measured voltage from sensor in volts
Icc = Constant current from current source in mA
Ib = Current required to bias the internal electronics, typically 1 mA

Cable Length@30 pF/ft (Ft)
10
100
1000
10
100
1000

887793Frequency Response to 5% ofMaximum Output Signal Amplitude±1V±5V
185 kHz37 kHz18.5 kHz
3.7 kHz1.85 kHz370 Hz550 kHz110 kHz
55 kHz
11 kHz5.5 kHz
1.1 kHzDBK Option Cards and Modules