AD628

Rev. C | Page 15 of 20

APPLICATIONS

GAIN ADJUSTMENT

The AD628 system gain is provided by an architecture

consisting of two amplifiers. The gain of the input stage

is fixed at 0.1; the output buffer is user adjustable as

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

×

=

EXT2

EXT1

TOTAL

R

R

G

1

1

.

0

(1)

At a 2 nA maximum, the input bias current of the buffer amplifier

is very low and any offset voltage induced at the buffer amplifier by

its bias current may be neglected (2 nA × 10 kΩ = 20 µV). However,

selected so that their parallel combination is 10 kΩ. If practical

the difference. Table 5 lists several values of gain and corresponding

resistor values.

Table 5. Nearest Standard 1% Resistor Values for Various

Gains (See Figure 29)

Total Gain

(V/V)

A2 Gain

(V/V)

0.1

1

10 k

∞

0

0.2

2

20 k

20 k

0

0.25

2.5

25.9 k

18.7 k

0

0.5

5

49.9 k

12.4 k

0

1

10

100 k

11 k

0

2

20

200 k

10.5 k

0

5

50

499 k

10.2 k

0

10

100

1 M

10.2 k

0

To set the system gain to less than 0.1, an attenuator may be

reference voltage. A divider would be formed by the 10 kΩ

resistor which is in series with the positive input of A2 and

Using a divider and setting A2 to unity gain yields

1

kΩ

10

0.1

/

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

×

=

EXT4

EXT4

DIVIDER

W

R

R

G

INPUT VOLTAGE RANGE

and the supply voltage. The relation is expressed by

REF

S

CM

REF

S

CM

V

V

V

V

V

UPPER

10

)

V

2

.

1

(

11

V

10

)

V

2

.

1

–

(

11

LOWER

−

+

≥

−

≤

−

+

(2)

where

+

S

V

is the positive supply,

−

S

V

is the negative supply

and 1.2 V is the headroom needed for suitable performance.

Equation 2 provides a general formula for calculating the

common-mode input voltage range. However, the AD628

should be kept within the maximum limits listed in the

Specifications table (Table 1) to maintain optimal performance.

This is illustrated in Figure 30 where the maximum common-

mode input voltage is limited to ±120 V. Figure 31 shows the

common-mode input voltage bounds for single-supply voltages.

–200

–150

–100

–50

0

50

100

150

200

8

6

24

010

12

14

16

SUPPLY VOLTAGE (±V)

MAXIMUM INPUT COMMON-MODE

Figure 30. Input Common-Mode Voltage vs. Supply Voltage for Dual Supplies

–80

–60

–40

–20

0

20

40

60

80

100

8

6

24

010

12

14

16

SINGLE-SUPPLY VOLTAGE (V)

MAXIMUM INPUT COMMON-MODE

Figure 31. Input Common-Mode Voltage vs.

Supply Voltage for Single Supplies