AD623

–15–

REV. C

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. Fig-

ure 50 shows a thermocouple application where one side of the

J-type thermocouple is grounded.

J-TYPE

THERMOCOUPLE

2V

+5V

0.1 F

AD623

REF

1.02k

Figure 50. Amplifying Bipolar Signals with Low Common-

Mode Voltage

Over a temperature range from –200

°C to +200°C, the J-type

thermocouple delivers a voltage ranging from –7.890 mV to

1.02 k

Ω) and a voltage on the AD623 REF pin of 2 V, results in

the AD623’s 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 51 shows a simplified block diagram of the AD623. The

voltages at the outputs of the amplifiers A1 and A2 are given by

the equations

50k

6

REF

5

50k

50k

50k

1

POS SUPPLY

7

INVERTING

2

4

NEG SUPPLY

3

NONINVERTING

7

GAIN

2

50k

A1

4

8

50k

A3

A2

2

Figure 51. Simplified Block Diagram

The voltages on these internal nodes are critical in determining

whether or not the output voltage will be clipped. The voltages

supply (V– or Ground) to within about 100 mV of the positive

rail before clipping occurs. Based on this and from the above

equations, the maximum and minimum input common-mode

voltages are given by the equations

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

will be the lesser of the two equations.

However, the range on the differential input voltage range is also

to be lower according the equation.

Input Range ≤ Available Output Swing/Gain

For a bipolar input voltage with a common-mode voltage that is

value that the above equations yield because the REF pin will be

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.

Again, we must ensure 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,

mum gain will be half the value yielded by the above equations

because the REF pin must be at midsupply.

The maximum gain and resulting output swing for different

input conditions is given in Table IV. Output voltages are refer-

enced 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 compo-

nent. So 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

50. The inverting input on the AD623 is grounded. So when the

input voltage is –10 mV, the voltage on the noninverting input is

–10 mV. For the purposes of signal swing calculations, this input

voltage should be considered to be composed of a common-mode

voltage of –5 mV (i.e., (+IN + –IN)/2) and a differential input

voltage of –10 mV (i.e., +IN – –IN).