AD627

–11–

REV. A

Table I. Recommended Values of Gain Resistors

Desired

1% Std Table

Resulting

Gain

Gain

5

∞

5

6

200 k

6

7

100 k

7

8

68.1 k

7.93

9

51.1 k

8.91

10

40.2 k

9.98

15

20 k

15

20

13.7 k

19.6

25

10 k

25

30

8.06 k

29.81

40

5.76 k

39.72

50

4.53 k

49.15

60

3.65 k

59.79

70

3.09 k

69.73

80

2.67 k

79.9

90

2.37 k

89.39

100

2.1 k

99.24

200

1.05 k

195.48

500

412

489.44

1000

205

980.61

Reference Terminal

The reference terminal potential defines the zero output voltage

and is especially useful when the load does not share a precise

ground with the rest of the system. It provides a direct means of

injecting a precise offset to the output. The reference terminal is

also useful when bipolar signals are being amplified as it can be

used to provide a virtual ground voltage.

Since the AD627 output voltage is developed with respect to the

potential on the reference terminal, it can solve many grounding

problems by simply tying the REF pin to the appropriate “local

ground.” The REF pin should however be tied to a low imped-

ance point for optimal CMR.

Input Range Limitations in Single Supply Applications

In general, the maximum achievable gain is determined by the

available output signal range. However, in single supply applica-

tions where the input common mode voltage is close to or equal

to zero, some limitations on the gain can be set. While the In-

put, Output and Reference Pins have ranges that are nominally

defined on the specification pages, there is a mutual interdepen-

dence between the voltage ranges on these pins. Figure 34 shows

the simplified schematic of the AD627, driven by a differential

the voltage on the REF pin and the programmed gain. This

voltage is given by the equation:

Ω/R

We can also express the voltage on A1 as a function of the ac-

tual voltages on the –IN and +IN pins (V– and V+)

Ω/R

G

A1’s output is capable of swinging to within 50 mV of the nega-

tive rail and to within 200 mV of the positive rail. From either of

acts as a positive offset at the output of the AD627), tends to

decrease the voltage on A1. Figures 35 and 36 show the maxi-

mum voltages that can be applied to the REF pin, for a gain of

five for both the single and dual supply cases. Raising the input

common-mode voltage will increase the voltage on the output of

A1. However, in single supply applications where the common-

mode voltage is low, a differential input voltage or a voltage on

REF that is too high can drive the output of A1 into the ground

rail. Some low side headroom is added by virtue of both inputs

and Q2). The above equations can be used to check that the

voltage on amplifier A1 is within its operating range.

Table II gives values for the maximum gains for various single

supply input conditions. The resulting output swings shown

refer to 0 V. The voltages on the REF pins has been set to either

A1

200k

OUTPUT

2k

+IN

Q2

Q1

2k

–IN

100k

REF

EXTERNAL GAIN RESISTOR

100k

25k

25k

200k

–IN

+IN

V+

V–

2

2

A2

Figure 34. Amplifying Differential Signals with a Common-Mode Component

Table II. Maximum Gain for Low Common-Mode Single Supply Applications

REF

Supply

Resulting

Output Swing

Pin

Voltage

Tolerance)

Max Gain

WRT 0 V

±100 mV, V

2 V

+5 V to +15 V

28.7 k

Ω

12.0

0.8 V to 3.2 V

±50 mV, V

2 V

+5 V to +15 V

10.7 k

Ω

23.7

0.8 V to 3.2 V

±10 mV, V

2 V

+5 V to +15 V

1.74 k

Ω

119.9

0.8 V to 3.2 V

V– = 0 V, V+ = 0 V to 1 V

1 V

+10 V to +15 V

78.7 k

Ω

7.5

1 V to 8.5 V

V– = 0 V, V+ = 0 mV to 100 mV

1 V

+5 V to +15 V

7.87 k

Ω

31

1 V to 4.1 V

V– = 0 V, V+ = 0 mV to 10 mV

1 V

+5 V to +15 V

7.87

Ω

259.1

1 V to 3.6 V