AD623

–12–

REV. C

Table I. Required Values of Gain Resistors

Desired

1% Std Table

Calculated Gain

Gain

Using 1% Resistors

2

100 k

2

5

24.9 k

5.02

10

11 k

10.09

20

5.23 k

20.12

33

3.09 k

33.36

40

2.55 k

40.21

50

2.05 k

49.78

65

1.58 k

64.29

100

1.02 k

99.04

200

499

201.4

500

200

501

1000

100

1001

INPUT AND OUTPUT OFFSET VOLTAGE

The low errors of the AD623 are attributed to two sources,

input and output errors. The output error is divided by the

programmed gain when referred to the input. In practice, the

input errors dominate at high gains and the output errors domi-

Total Error RTI = Input Error + (Output Error/G)

Total Error RTO = (Input Error

× G) + Output Error

RTI offset errors and noise voltages for different gains are shown

below in Table II.

Table II. RTI Error Sources

Max

Max

Total Input

Total Input

Total Input

Offset Error

Offset Drift

Referred Noise

Gain

V

V

V/ C

V/ C

(nV/

√Hz)

AD623A AD623B AD623A AD623B AD623A & AD623B

1

1200

600

12

11

62

2

700

350

7

6

45

5

400

200

4

3

38

10

300

150

3

2

35

20

250

125

2.5

1.5

35

50

220

110

2.2

1.2

35

100

210

105

2.1

1.1

35

1000 200

100

2

1

35

INPUT PROTECTION

Internal supply referenced clamping diodes allow the input,

reference, output and gain terminals of the AD623 to safely

withstand overvoltages of 0.3 V above or below the supplies.

This is true for all gains, and for power on and off. This last

case is particularly important since the signal source and ampli-

fier may be powered separately.

If the overvoltage is expected to exceed this value, the current

through these diodes should be limited to about 10 mA using

external current limiting resistors. This is shown in Figure 42.

The size of this resistor is defined by the supply voltage and the

required overvoltage protection.

10mA

1 = 10mA MAX

OUTPUT

AD623

Figure 42. Input Protection

RF INTERFERENCE

All instrumentation amplifiers can rectify high frequency out-of-

band signals. Once rectified, these signals appear as dc offset

errors at the output. The circuit of Figure 43 provides good RFI

suppression without reducing performance within the in amps

pass band. Resistor R1 and capacitor C1 (and likewise, R2 and

C2) form a low-pass RC filter that has a –3 dB BW equal to:

F = 1/(2

π R1C1). Using the component values shown, this

filter has a –3 dB bandwidth of approximately 40 kHz. Resistors

R1 and R2 were selected to be large enough to isolate the

circuit’s input from the capacitors, but not large enough to

significantly increase the circuit’s noise. To preserve common-

mode rejection in the amplifier’s pass band, capacitors C1 and

C2 need to be 5% or better units, or low cost 20% units can be

tested and “binned” to provide closely matched devices.

Capacitor C3 is needed to maintain common-mode rejection at

the low frequencies. R1/R2 and C1/C2 form a bridge circuit

whose output appears across the in amp’s input pins. Any

mismatch between C1 and C2 will unbalance the bridge and

reduce common-mode rejection. C3 ensures that any RF signals

+2.5V TO +6V

REF (INPUT)

0.1 F

10 F

–2.5V TO –6V

REF

OUTPUT

0.1 F

10 F

+3V TO +12V

REF (INPUT)

REF

OUTPUT

0.1 F

10 F

a. Dual Supply

b. Single Supply

Figure 41. Basic Connections