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AD623

Rev. D | Page 20 of 24

Amplifying Signals with Low Common-Mode Voltage

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.

0.1µF

AD623

1.02kΩ

REF

J-TYPE

THERMOCOUPLE

OUTPUT

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 DIFFERENTIAL AND COMMON-MODE RANGE

vs. SUPPLY AND GAIN

Figure 55 shows a simplified block diagram of the AD623. The

voltages at the outputs of Amplifier A1 and Amplifier A2 are

given by

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

POSITIVE SUPPLY

INVERTING

NEGATIVE SUPPLY

NONINVERTING

50kΩ

OUTPUT

REF

GAIN

VDIFF

–

VDIFF

–

VCM

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

the previous equations, the maximum and minimum input

common-mode voltages are given by the following equations:

VCMMAX

= V+ − 0.7 V − VDIFF × Gain/2

VCMMIN

= 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.

|VDIFFMAX| = 2 (V+ − 0.7 V − VCM/Gain

|VDIFFMAX| = 2 (VCM − V− +0.590 V/Gain

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.

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.

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.

GainMAX

= 2 (V+ − 0.7 V − VCM)/VDIFF

GainMAX

= 2 (VCM − V− +0.590 V)/VDIFF

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

GainMAX

= Available Output Swing/Input Range

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.

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.

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).

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