AD627ARZ-R7 datasheet(16/24 Pages) AD

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AD627

Rev. D | Page 16 of 24

Table 6. Recommended Values of Gain Resistors

Desired Gain

1% Standard Table

Value of RG

Resulting Gain

∞

5.00

200 kΩ

6.00

100 kΩ

7.00

68.1 kΩ

7.94

51.1 kΩ

8.91

40.2 kΩ

9.98

20 kΩ

15.00

13.7 kΩ

19.60

10 kΩ

25.00

8.06 kΩ

29.81

5.76 kΩ

39.72

4.53 kΩ

49.15

3.65 kΩ

59.79

3.09 kΩ

69.72

2.67 kΩ

79.91

2.37 kΩ

89.39

100

2.1 kΩ

100.24

200

1.05 kΩ

195.48

500

412 Ω

490.44

1000

205 Ω

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 amplifying bipolar signals, because it provides

a virtual ground voltage.

The AD627 output voltage is developed with respect to the poten-

tial on the reference terminal; therefore, tying the REF pin to the

appropriate local ground solves many grounding problems. For

optimal CMR, tie the REF pin to a low impedance point.

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 nearly or equal

to 0, some limitations on the gain can be set. Although the

Specifications section nominally defines the input, output, and

reference pin ranges, the voltage ranges on these pins are

mutually interdependent. Figure 37 shows the simplified

schematic of the AD627, driven by a differential voltage (VDIFF)

that has a common-mode component, VCM. The voltage on the

A1 op amp output is a function of VDIFF, VCM, the voltage on the

REF pin, and the programmed gain. This voltage is given by

VA1 = 1.25 (VCM + 0.5 V) − 0.25 VREF − VDIFF (25 kΩ/RG − 0.625) (3)

The voltage on A1 can also be expressed as a function of the

actual voltages on the –IN and +IN pins (V− and V+) such that

VA1 = 1.25 ((V−) + 0.5 V) − 0.25 VREF − ((V+) − (V−)) 25 kΩ/RG (4)

The output of A1 is capable of swinging to within 50 mV of the

negative rail and to within 200 mV of the positive rail. It is clear,

from either Equation 3 or Equation 4, that an increasing VREF

(while it acts as a positive offset at the output of the AD627)

tends to decrease the voltage on A1. Figure 38 and Figure 39

show the maximum voltages that can be applied to the REF pin

for a gain of 5 for both the single-supply and dual-supply cases.

VIN(–) (V)

–6

–1

–2

–3

–4

–5

–4

–3

–2

–1

234

MAXIMUM VREF

MINIMUM VREF

Figure 38. Reference Input Voltage vs. Negative Input Voltage,

VS = ±5 V, G = +5

VIN(–) (V)

–0.5

0.5

1.0

2.0

1.5

2.5

3.0

3.5

4.0

4.5

MAXIMUM VREF

MINIMUM VREF

Figure 39. Reference Input Voltage vs. Negative Input Voltage,

VS = 5 V, G = +5

Raising the input common-mode voltage increases 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

because both inputs are shifted upwards by about 0.5 V (that is,

by the VBE of Q1 and Q2). Use Equation 3 and Equation 4 to

check whether the voltage on Amplifier A1 is within its

operating range.

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