Voltage measuring circuit

A voltage indicating meter is connected between the outputs of two high impedance operational amplifiers, the first of which receives an input signal to be measured through a range setting resistance dividing network and having a gain which can be set to one of a plurality of different values. A zero control network applies a bias voltage to the inverting input of the second operational amplifier for zeroing the meter. The non-inverting input of the second amplifier can be connected either to the output of the first operational amplifier or to a source of reference potential. When, during operation, the non-inverting input of this amplifier is connected to the source of reference potential, the one terminal of the meter is clamped to the bias level and the other receives a voltage proportional to the voltage of given polarity being measured and the meter needle swings in a given direction. When the non-inverting input of the second amplifier is connected to the output of the first operational amplifier, the meter still reads in the same direction for input voltage opposite in polarity to the given polarity.

The present invention relates to apparatus for measuring direct current 
voltages. 
High impedance voltmeters for measuring direct current voltages of one 
polarity may employ a bipolar or field effect transistor amplifier 
operated in a differential mode, and a sensitive microammeter which 
measures the difference in currents sensed by the amplifier. Voltages of 
opposite polarity may be measured by reversing the terminals of the meter 
movement. A zero control is provided to allow zero adjustment of the 
meter. Such meter adjustment usually drifts with change in scales since 
the impedance "seen" by the input stage is varied. Further, adjustment of 
the zero control setting increases the meter reading for one voltage 
polarity and decreases it for the opposite voltage polarity. 
Other high input impedance voltage measuring apparatuses include 
operational amplifier circuits with a meter connected in a feedback 
arrangement across a diode bridge. Zero adjustment is made by applying a 
bias voltage to an input of the operational amplifier. A large number of 
input divider resistances are employed to provide a full range of input 
voltages to the circuits. Such divider resistances are usually precision 
resistances of relatively low tolerance and are relatively expensive. The 
resistances are switched by a corresponding large number of switching 
devices for making the desired scale changes which also adds to the 
complexity and cost of the apparatus. 
A voltage measuring circuit embodying the present invention includes 
indicating means having a pair of inputs. During the calibration phase, a 
reference voltage is applied to one of the inputs and an adjustable bias 
to the other. To measure voltage of one polarity, a voltage proportional 
to the voltage being measured is added to the reference voltage at one 
input while the other input is held at the bias level. To measure voltage 
of the opposite polarity, a voltage proportional to this voltage is added 
to the reference voltage at the one input and a second voltage, at an 
amplified level proportional to the opposite polarity voltage, is added to 
the bias voltage at the other input.

In the drawing, an input voltage to be measured is applied between input 
terminals 16 and 16'. Network 14 includes a plurality of range selecting 
resistances 18, 18' and 18" which can be selectively connected between 
terminal 16 and terminal 20 by wiper 22 of switch S3. A second set of 
range selecting resistances 24, 24' and 24" are selectively connected 
between terminal 20 and common lead 28 via wiper 26 of switch S3. While a 
plurality of resistances 24, 24', and 24" are shown, a single resistance 
could be used instead. The wipers 22 and 26 are ganged, as shown by the 
dashed line, so that there is concurrent selection of a particular one of 
the range resistances 18, 18' and 18" and a corresponding one of 
resistances 24, 24' and 24". Terminal 16' is connected to common lead 28. 
The terminal 20 is connected to the the non-inverting input of 
high-impedance, high-gain operational amplifier 10 through resistance 30 
of filter 12. The capacitor 32 of filter 12 is connected between the 
non-inverting input of amplifier 10 and common lead 28. Lead 28 is at a 
reference level between 0 and +V volts as will be discussed later. The 
filter 12 has a long-time constant and its purpose is to filter switching 
transients produced by wipers 22 and 26. The values of resistances 18, 18' 
and 18" and 24, 24' and 24" are selected in accordance with the ranges of 
voltage it is desired that the circuit measure. In this particular 
example, there are three ranges determined by switch S3; however, by 
adding more resistors to each bank, the number of ranges would be 
increased. 
The inverting input of amplifier 10 is connected to its output through 
feedback resistance 34 and to common lead 28 through series connected 
resistances 36 and 38. A switch S1 is connected across resistance 38. The 
values of resistances 34, 36 and 38 in the present example are such that 
the gain of amplifier 10 is two when switch S1 is open and six when switch 
S1 is closed. These gain values are given by way of example and in other 
implementations can have other values. Further, in some embodiments, 
switch S1 may be omitted. In the quiescent state, amplifier 10 draws 
negligible current. Thus the input and output V.sub.10 of amplifier 10 are 
at V.sub.ref (the potential at common lead 28). 
A second high-impedance, high-gain operational amplifier 40 has its 
non-inverting input connected to a switching transient filter 42 
comprising capacitance 44 and resistance 46 which may have the same values 
as capacitance 32 and resistance 30, respectively. Filter 42 filters 
transients produced by the operation of switch S2. Wiper 48 of switch S2 
is connected to the non-inverting input of amplifier 40 through resistance 
46 of filter 42. Capacitance 44 is connected between the non-inverting 
input of amplifier 40 and common lead 28. Terminal 50 of switch S2 is 
connected to the output of amplifier 10 while terminal 52 is connected to 
common lead 28. The output of amplifier 10 is connected to one input of 
milliammeter 74 through variable resistance 76. The output of amplifier 40 
is connected to the other input of milliammeter 74. Meter 74 has, for this 
example, a zero reading at a and a full scale reading at b and a center 
scale reading at c. 
When switch S2 is connected to terminal 52, reference potential V.sub.ref 
of some given value produced by power supply 60 is applied via common lead 
28 and resistance 46 to the non-inverting input of amplifier 40. V.sub.ref 
appears at the non-inverting input due to the high impedance of amplifier 
40, that is, negligible current flows through amplifier 40 in the 
quiescent state of amplifier 40. When switch S2 is connected to terminal 
50, the output voltage V.sub.10 of amplifier 10 (at V.sub.ref in the 
quiescent state) is applied as an input to the non-inverting input of 
amplifier 40. Therefore the reference potential V.sub.ref applied to the 
non-inverting input of amplifier 40 does not change when wiper 48 is 
switched. 
In response to wiper 48 connected to terminal 50, the output voltage 
V.sub.40 of amplifier 40 follows the output voltage V.sub.10 of amplifier 
10 and has a value dependent on the gain of amplifier 40. The gain of 
amplifier 40 is determined by the feedback voltage divider resistances 54 
and 56. Resistance 54 is connected between the output and inverting input 
of amplifier 40 and resistance 56 is connected between the inverting input 
of amplifier 40 and power supply 60. The values of resistances 54 and 56 
are chosen in this example to make the gain of amplifier 40 two. 
Therefore, the value of any signal appearing at the output of amplifier 40 
will be double the value of the signal appearing at the output of 
amplifier 10 when wiper 48 is connected to terminal 50 and of the same 
polarity as the output signal of amplifier 10. Since the gain of amplifier 
10 with switch S1 open is made two and the gain of amplifier 40 is also 
made two, by way of example, then the value of resistance 34 in this case 
equals the value of resistance 54 and resistances 36 and 38 have a sum 
which equals the value of resistance 56. 
Power supply 60 provides a bias voltage for the inverting terminal of 
amplifier 40 and a reference voltage for the common lead 28. The power 
supply 60 comprises a resistor 64, Zener diode 72 and resistor 66, 
connected in that order between a zero volt source and a positive voltage 
source +V. (In the alternative, the zero volt source may be replaced by a 
negative voltage source.) Tapped resistor 62 is connected across the Zener 
diode 72 as are the series connected resistors 68 and 70. Resistor 56 is 
connected at one end to the tap of resistor 62 and the common lead 28 
connects to the circuit point between resistors 68 and 70 which may be of 
equal value. 
In the operation of the power supply 60, Zener diode 72 conducts and places 
a constant voltage across resistor 62 and across the series connected 
resistors 68 and 70. A bias voltage thereby develops at the tap of 
resistor 62 and a reference voltage V.sub.ref at the common lead 28. 
In operation of the system, assume that when there is zero volts across the 
meter, the needle is at a. With wiper 48 connected to terminal 52 the 
non-inverting input of amplifier 40 is at V.sub.ref with no voltage 
applied between input terminals 16 and 16', the output of amplifier 10 
also is at V.sub.ref. Now the tap at 62 is adjusted to provide a bias 
voltage at the inverting input of amplifier 40 and at the output of 
amplifier 40 which sets the pointer of meter 74 to the desired position, 
which in this example is a. To accomplish this, it is clear that the tap 
on 62 must be set to V.sub.ref as at this setting both terminals of the 
meter will be at V.sub.ref so no current flow through the meter. If it is 
desired to zero adjust the meter to a different setting, such as c, for 
example, then the tap on 62 is adjusted to a different setting such that 
current flows through the meter proportional to V.sub.ref -V.sub.62 (where 
V.sub.62 is the voltage at the tap along resistor 62) which is sufficient 
to move the meter needle to position c. After this initial adjustment and 
before the voltage it is desired to measure is applied between terminals 
16 and 16' the range scale for the circuit is set by setting the wipers 22 
and 26 of switch S3. 
The switch S1 is either closed or open in accordance with the desired range 
of voltages to be measured. As described above, for this example, with the 
switch S1 open, the gain of the amplifier 10 and its feedback network is 
made two, whereas with switch S1 closed the gain is made six. Therefore, 
by way of example, a 1.5 VDC input signal at terminal 20 will produce a 3 
VDC at the output of amplifier 10 with the switch S1 open and a 0.5 VDC 
input signal at terminal 20 will produce a 3 VDC output signal at the 
output of amplifier 10 with switch S1 closed. 
In the present description, it is assumed that the offset voltage of 
amplifier 40 is negligible. That is, this offset voltage has a value which 
may be in the order of millivolts as compared to a full scale measuring 
range of meter 74 in volts. Such low offset voltages would not be measured 
by meter 74 for this range. 
In the quiescent condition of the circuit, that is, in the absence of an 
input voltage, when wiper 48 of switch S2 is connected to terminal 50, the 
reference potential V.sub.ref appears at the non-inverting input of 
amplifier 40, (as V.sub.ref is the output of amplifier 10). The same holds 
when wiper 48 of switch S2 is connected to terminal 52. The open or closed 
state of switch S1 does not affect the value of the reference potential 
applied to the inverting input of amplifier 10, in view of this 
amplifier's high impedance. Therefore the amplifier 10 output V.sub.10 is 
not affected by any switching of switch S1 to alter the gain of amplifier 
10, assuming as before, negligible offset voltage. 
In summary, the following relationship exists between the outputs of 
amplifiers 10 and 40. 
EQU (1) V.sub.10 -V.sub.40 =.+-.V.sub.10 +V.sub.62 
where V.sub.10 and V.sub.40 are the respective output voltages of 
amplifiers 10 and 40 and V.sub.62 is the voltage at the tap of resistor 62 
and the polarity of the .+-.V.sub.10 term is determined by the position of 
wiper 48 of switch S2 (terminal 50-, terminal 52+). It is apparent that 
polarity reversal occurs when switch S2 is switched from + to -, 
remembering, that the output V.sub.40 is 2 V.sub.10 -V.sub.62 in the S2 
(=) position. In practice switch S2 is placed in the (=) position when 
V.sub.10 is negative so that V.sub.10 -V.sub.40 is positive. This results 
in the V.sub.40 output being more negative that V.sub.10 and meter 74 
reading in the upscale (clockwise) direction from position a regardless 
the polarity of the input voltage to be measured. 
Resistance 76 is a calibration resistor which is adjusted in accordance 
with the full scale current sensitivity of the meter 74. For example, with 
a full scale input range of 1.5 volts, the nominal voltage difference 
between amplifiers 10 and 40 is 3 volts. A meter with full scale 
sensitivity at one milliampere would require a total resistance of 3 k 
ohms. 
When the voltage to be measured V.sub.IN applied between terminals 16 and 
16' is positive, switch S2 wiper 48 is connected to terminal 52. This 
applies the constant reference potential V.sub.ref at common lead 28 to 
the non-inverting input of amplifier 40. With no voltage applied to 
terminals 16, -16', a relatively small offset error voltage may appear at 
the output of amplifier 10 due to its operating characteristics. Power 
supply 60 provides a bias signal voltage which can compensate for that 
offset error should it in certain low voltage instances be significant. 
That is, in those cases where full scale on meter 74 is a fraction of a 
volt, a millivolt offset may be significant with respect to that fraction. 
In the absence of input voltage, no change in the meter 74 zero setting 
occurs when S2 is switched from one position to another. The values of 
resistances 62.gtoreq.70 are made such that the down side 
(counterclockwise) zero adjustment on the meter (controlled by making 
V.sub.62 more positive than V.sub.ref), is about one third of the up scale 
(clockwise) zero adjustment on the meter (controlled by making V.sub.62 
less positive than V.sub.ref). This is because it is desired, for purposes 
of convenience of operation, that the meter 74 initially be zeroed at a, 
for most input signals, and that it be deflected clockwise for voltages 
applied to terminal 16 of either polarity. A zero adjustment to c, the 
center of scale is used less often, it being of interest, for example, in 
certain alignment applications. 
Once having set the zero adjustment of the meter 74 with the switch S2 
wiper connected to terminal 52, a positive voltage applied to terminal 16 
will cause the meter 74 to deflect in the clockwise direction. V.sub.62 is 
present at one input to the meter; V.sub.10, which is proportional to the 
input voltage is present at the other input to the meter, and the meter 
measures the difference between these voltages. If, for example, the meter 
is zeroed at a, when V.sub.10 becomes more positive in response to a 
positive input signal (V.sub.62 remaining constant) the meter needle 
deflects from a toward b. Closing switch S1 multiplies the gain of 
amplifier 10 by a given factor, three in this example (as determined by 
the relative values of resistances 36 and 38) and this increases the meter 
reading for a given input voltage. The change in gain produces a change in 
the offset error voltage of amplifier 10; however, this is generally 
negligible, as already mentioned. If it isn't, a new zero adjustment can 
be made at 62 with switch S1 closed, prior to measuring the voltage. 
When a negative voltage is desired to be measured, switch S2 is switched to 
terminal 50 so that the reading on meter 74 is in the same direction as 
before. The output voltage V.sub.10 of amplifier 10 is applied to the 
non-inverting input of amplifier 40 and the latter multiplies this voltage 
by a given factor, for example, two. Any negative voltage appearing at the 
output of amplifier 10 will also be negative at the output of amplifier 
40, but at twice the magnitude (assuming a gain of two for the amplifier 
40). This negative voltage V.sub.40 will cause the meter 74 to read in a 
clockwise direction since V.sub.40 is more negative than V.sub.10. The 
following values are given by way of example for one implementation of the 
circuit 
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Resistance 18 9 Meg .OMEGA. for 15 
Resistance 24 1 Meg .OMEGA. volt scale 
Resistance 30 1 Meg .OMEGA. 
Capacitance 32 0.01 .mu.F 
Resistance 34 1 Meg .OMEGA. 
Resistance 36 200 k .OMEGA. 
Resistance 38 800 k .OMEGA. 
Resistance 46 1 Meg .OMEGA. 
Capacitance 42 0.01 .mu.F 
Resistance 54 1 Meg .OMEGA. 
Resistance 56 1 Meg .OMEGA. 
Resistance 62 5 k .OMEGA. 
Resistance 64 15 k .OMEGA. 
Resistance 66 10 k .OMEGA. 
Resistance 68 4.7 k .OMEGA. 
Resistance 70 15 k .OMEGA. 
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While a single power supply is illustrated herein having an exemplary range 
between 0 and 18 volts with the common lead floating, it is possible to 
employ alternative forms of power supplies. For example, one could use two 
fixed power supplies such as ones delivering +12 and -12 volts, 
respectively, and a common ground. The +12 volts may be employed as +V and 
the -12 volts may replace OV. Resistances 68 and 70 are not used, common 
lead 28 being connected to ground (zero volts). 
If the power supply 60 comprises a regulated power supply, then the Zener 
diode 72 need not be employed. While one gain selection range is shown via 
switch S1 it is apparent that many resistances and corresponding switches 
in series with resistance 38 and switch S1, respectively, may be provided 
to provide other gain factors for amplifier 10. For example, a number of 
resistors could be placed in series with resistor 38 and a corresponding 
number of switches placed across the resistors, one switch for each 
resistor. The more different values of gain, the fewer resistors 18 and a 
correspondingly simpler switch would be required for network 14. For 
implementations where only input voltages within the voltage range of 
amplifier 10 are employed, network 14 may be aliminated completely and 
range selection may be provided by a number of gain selecting resistors 
and switches such as resistor 38 and switch S1. For high voltage inputs, 
network 14 provides attenuation to protect circuit 10 against overloads. 
Switch S1 may also be ganged with switch S3. 
Once the zero bias adjust voltage is set for amplifier 40 by the tap at 62 
it need not be readjusted when the ranges are switched by network 14. For 
a given gain of the amplifier 10 the bias voltage produced by power supply 
60 at the tap of 62 remains constant. Only when certain voltages are 
switched by the gain switch S1 need the zero bias control be reset. For 
all other operations, a single adjustment is all that is required. There 
thus has been shown a simple circuit in which a meter of relatively low 
cost can be provided and which can be made to read in the same direction 
for either polarity of voltage to be measured. 
For very low voltage implementations where the offset voltage of the 
amplifier 10 is a factor, a second adjustable voltage source may be 
employed. The connection between lead 28 and the point where switch S1 and 
resistor 38 join this lead may be opened and this point then connected to 
a tap on a resistor (not shown). This resistor may be connected in 
parallel with resistor 62. This second voltage source then can be used to 
zero the meter 74 so that switching S1 (or S2) will not affect the 
position of the needle of meter 74. 
In operation, meter 74 is zeroed as before with wiper 48 connected to 
terminal 52. Switch S1 is open, wiper 48 is then connected to terminal 50. 
If the zero position of the meter is affected an amount sufficient to 
warrant re-zeroing the meter then the second tap to the second resistor is 
displaced to alter the bias supplied to amplifier 10 until the meter 74 is 
at the zero position. 
At this time, switching S2 will result in no change in the meter reading. 
That is, the output V.sub.10 is the same s the output V.sub.40 (assuming a 
zero position at a on the meter) and both are now at V.sub.ref. Since the 
offset voltage is no longer effectively present, a change in gain of 
amplifier 10 by switching S2 will also not result in any other offset 
voltage errors. At this time any of the switches may be operated without 
offset voltage errors.