AD621

REV. B

–14–

Gain Selection

The AD621 has accurate, low temperature coefficient (TC),

gains of 10 and 100 available. The gain of the AD621 is nomi-

nally set at 10; this is easily changed to a gain of 100 by simply

connecting a jumper between Pins 1 and 8.

AD621

555.5

5,555.5

Figure 11. Programming the AD621 for Gains Between

10 and 100

As shown in Figure 11, the device can be programmed for any

gain between 10 and 100 by connecting a single external resistor

between Pins 1 and 8. Note that adding the external resistor will

degrade both the gain accuracy and gain TC. Since the gain

equation of the AD621 yields:

G

= 1+

This can be solved for the nominal value of external resistor for

gains between 10 and 100:

(G – 1) 555.555 – 55,000

(10 – G )

Table III gives practical 1% resistor values for several com-

mon gains.

Table III. Practical 1% External Resistor

Values for Gains Between 10 and 100

Desired Recommended

Temperature

Gain

1% Resistor Value

Gain Error Coefficient (TC)

10

∞ (Pins 1 and 8 Open)

*

5 ppm/

°C max

20

4.42 k

Ω±10%

≈0.4 (50 ppm/°C

+ Resistor TC)

50

698

Ω±10%

≈0.4 (50 ppm/°C

+ Resistor TC)

100

0 (Pins 1 and 8 Shorted)

*

5 ppm/

°C max

*Factory trimmed–exact value depends on grade.

A High Performance Programmable Gain Amplifier

The excellent performance of the AD621 at a gain of 10 makes

it a good choice to team up with the AD526 programmable gain

amplifier (PGA) to yield a differential input PGA with gains of

10, 20, 40, 80, 160. As shown in Figure 12, the low offset of the

AD621 allows total circuit offset to be trimmed using the offset

null of the AD526, with only a negligible increase in total drift

error. The total gain TC will be 9 ppm/

°C max, with 2 µV/°C

typical input offset drift. Bandwidth is 600 kHz to gains of 10 to

80, and 350 kHz at G = 160. Settling time is 13

µs to 0.01%

for a 10 V output step for all gains.

–

+

AD621

INPUTS

–

+

0.1 F

G = 10

–

+

AD526

0.1 F

20k

0.1 F

0.1 F

2

OUTPUT

Figure 12. A High Performance Programmable Gain

Amplifier

COMMON-MODE REJECTION

Instrumentation amplifiers like the AD621 offer high CMR

which is a measure of the change in output voltage when both

inputs arc changed by equal amounts. These specifications are

usually given for a full-range input voltage change and a speci-

fied source imbalance.

For optimal CMR, the reference terminal should be tied to a

low impedance point, and differences in capacitance and resis-

tance should be kept to a minimum between the two inputs. In

many applications shielded cables are used to minimize noise,

and for best CMR over frequency the shield should he properly

driven. Figures 13 and 14 show active data guards that are config-

ured to improve ac common-mode rejections by “bootstrapping”

the capacitances of input cable shields, thus minimizing the

capacitance mismatch between the inputs.

–INPUT

+INPUT

100

100

100k

100k

REFERENCE

AD621

AD648

–

+

Figure 13. Differential Shield Driver, G = 10

AD548

100

– INPUT

+ INPUT

REFERENCE

AD621

4

8

3

1

2

7

5

6

Figure 14. Common-Mode Shield Driver, G = 100