AD627BRZ-R7 datasheet(16/25 Pages) AD

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Data Sheet

AD627

Rev. E | Page 15 of 24

USING THE AD627

BASIC CONNECTIONS

Figure 36 shows the basic connection circuit for the AD627.

The +VS and −VS terminals connect to the power supply. The

supply can be either bipolar (VS = ±1.1 V to ±18 V) or single

supply (−VS = 0 V, +VS = 2.2 V to 36 V). Capacitively decouple

the power supplies close to the power pins of the device. For

best results, use surface-mount 0.1 µF ceramic chip capacitors.

The input voltage can be single-ended (tie either −IN or +IN to

ground) or differential. The difference between the voltage on the

inverting and noninverting pins is amplified by the programmed

gain. The gain resistor programs the gain as described in the

Setting the Gain and Reference Terminal sections. Basic

connections are shown in Figure 36. The output signal appears

as the voltage difference between the output pin and the

externally applied voltage on the REF pin, as shown in Figure 37.

SETTING THE GAIN

The gain of the AD627 is resistor programmed by RG, or, more

precisely, by whatever impedance appears between Pin 1 and Pin 8.

The gain is set according to

Gain = 5 + (200 kΩ/RG) or RG = 200 kΩ/(Gain − 5)

(2)

Therefore, the minimum achievable gain is 5 (for 200 kΩ/

(Gain − 5)). With an internal gain accuracy of between 0.05%

and 0.7%, depending on gain and grade, a 0.1% external gain

resistor is appropriate to prevent significant degradation of the

overall gain error. However, 0.1% resistors are not available in a

wide range of values and are quite expensive. Table 6 shows

recommended gain resistor values using 1% resistors. For all

gains, the size of the gain resistor is conservatively chosen as the

closest value from the standard resistor table that is higher than

the ideal value. This results in a gain that is always slightly less

than the desired gain, thereby preventing clipping of the signal

at the output due to resistor tolerance.

The internal resistors on the AD627 have a negative temperature

coefficient of −75 ppm/°C maximum for gains > 5. Using a

gain resistor that also has a negative temperature coefficient

of −75 ppm/°C or less tends to reduce the overall gain drift of

the circuit.

+IN

VOUT

VIN

–IN

REF (INPUT)

–1.1V TO –18V

+1.1V TO +18V

+VS

–VS

OUTPUT

REF

0.1µF

+IN

VOUT

VIN

–IN

REF (INPUT)

GAIN = 5 + (200kΩ/RG)

+2.2V TO +36V

+VS

OUTPUT

REF

0.1µF

Figure 36. Basic Connections for Single and Dual Supplies

EXTERNAL GAIN RESISTOR

REF

–IN

+IN

–VS

+VS

100kΩ

25kΩ

200kΩ

2kΩ

0.1V

OUTPUT

–VS

100kΩ

–IN

+IN

V+

V–

VDIFF

VCM

Figure 37. Amplifying Differential Signals with a Common-Mode Component

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