AD623
Amplifying Signals with Low Common-Mode Voltage
the previous equations, the maximum and minimum input
common-mode voltages are given by the following equations:
Because the common-mode input range of the AD623 extends
0.1 V below ground, it is possible to measure small differential
signals which have low, or no, common-mode component.
Figure 54 shows a thermocouple application where one side
of the J-type thermocouple is grounded.
5V
VCMMAX = V+ − 0.7 V − VDIFF × Gain/2
V
CMMIN = V− − 0.590 V + VDIFF × Gain/2
These equations can be rearranged to give the maximum
possible differential voltage (positive or negative) for a
particular common-mode voltage, gain, and power supply.
Because the signals on A1 and A2 can clip on either rail, the
maximum differential voltage are the lesser of the two equations.
0.1µF
R
J-TYPE
THERMOCOUPLE
G
AD623
OUTPUT
2V
1.02kΩ
|VDIFFMAX| = 2 (V+ − 0.7 V − VCM/Gain
|VDIFFMAX| = 2 (VCM − V− +0.590 V/Gain
REF
However, the range on the differential input voltage range is
also constrained by the output swing. Therefore, the range of
VDIFF may have to be lower according the following equation.
Figure 54. Amplifying Bipolar Signals with Low Common-Mode Voltage
Over a temperature range of −200°C to +200°C, the J-type thermo-
couple delivers a voltage ranging from −7.890 mV to +10.777 mV.
A programmed gain on the AD623 of 100 (RG = 1.02 kΩ) and a
voltage on the REF pin of 2 V, results in the output voltage ranging
from 1.110 V to 3.077 V relative to ground.
Input Range ≤ Available Output Swing/Gain
For a bipolar input voltage with a common-mode voltage that is
roughly half way between the rails, VDIFFMAX is half the value that
the previous equations yield because the REF pin is at midsupply.
Note that the available output swing is given for different supply
conditions in the Specifications section.
INPUT DIFFERENTIAL AND COMMON-MODE RANGE
vs. SUPPLY AND GAIN
The equations can be rearranged to give the maximum gain for
a fixed set of input conditions. Again, the maximum gain will be
the lesser of the two equations.
Figure 55 shows a simplified block diagram of the AD623. The
voltages at the outputs of Amplifier A1 and Amplifier A2 are
given by
GainMAX = 2 (V+ − 0.7 V − VCM)/VDIFF
GainMAX = 2 (VCM − V− +0.590 V)/VDIFF
VA2 = VCM + VDIFF/2 + 0.6 V + VDIFF × RF/RG
= VCM + 0.6 V + VDIFF × Gain/2
VA1 = VCM + VDIFF/2 + 0.6 V + VDIFF × RF/RG
= VCM + 0.6 V − VDIFF × Gain/2
Again, it is recommended that the resulting gain times the input
range is less than the available output swing. If this is not the
case, the maximum gain is given by
POSITIVE SUPPLY
7
GainMAX = Available Output Swing/Input Range
INVERTING
Also for bipolar inputs (that is, input range = 2 VDIFF), the
maximum gain is half the value yielded by the previous equations
because the REF pin must be at midsupply.
2
A1
R
F
4
1
8
50kΩ
50kΩ
50kΩ
50kΩ
–
+
V
DIFF
2
The maximum gain and resulting output swing for different
input conditions is given in Table 8. Output voltages are
referenced to the voltage on the REF pin.
OUTPUT
6
GAIN
R
A3
50kΩ
G
V
CM
R
F
50kΩ
REF
5
7
For the purposes of computation, it is necessary to break down the
input voltage into its differential and common-mode component.
Therefore, when one of the inputs is grounded or at a fixed
voltage, the common-mode voltage changes as the differential
voltage changes. Take the case of the thermocouple amplifier
in Figure 54. The inverting input on the AD623 is grounded;
therefore, when the input voltage is −10 mV, the voltage on the
noninverting input is −10 mV. For the purpose of the signal swing
calculations, this input voltage should be composed of a common-
mode voltage of −5 mV (that is, (+IN + −IN)/2) and a differential
input voltage of −10 mV (that is, +IN − −IN).
–
+
V
DIFF
2
A2
NONINVERTING
3
4
NEGATIVE SUPPLY
Figure 55. Simplified Block Diagram
The voltages on these internal nodes are critical in determining
whether the output voltage will be clipped. The VA1 and VA2
voltages can swing from approximately 10 mV above the negative
supply (V− or ground) to within approximately 100 mV of the
positive rail before clipping occurs. Based on this and from
Rev. D | Page 20 of 24
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