AD628 datasheet(17/24 Pages) AD

17 / 24 page

AD8278/AD8279

Rev. C | Page 17 of 24

POWER SUPPLIES

The AD8278 and AD8279 operate extremely well over a very

wide range of supply voltages. They can operate on a single

supply as low as 2 V and as high as 36 V, under appropriate

setup conditions.

For best performance, the user should ensure that the internal

op amp is biased correctly. The internal input terminals of the

op amp must have sufficient voltage headroom to operate

properly. Proper operation of the part requires at least 1.5 V

between the positive supply rail and the op amp input terminals.

This relationship is expressed in the following equation:

−

REF

For example, when operating on a +VS= 2 V single supply and

VREF = 0 V, it can be seen from Figure 50 that the op amp input

terminals are biased at 0 V, allowing more than the required 1.5 V

headroom. However, if VREF = 1 V under the same conditions, the

input terminals of the op amp are biased at 0.66 V (G = ½). Now

the op amp does not have the required 1.5 V headroom and

cannot function. Therefore, the user must increase the supply

voltage or decrease VREF to restore proper operation.

The AD8278 and AD8279 are typically specified at single and

dual supplies, but they can be used with unbalanced supplies as

well; for example, −VS = −5 V, +VS = +20 V. The difference between

the two supplies must be kept below 36 V. The positive supply

rail must be at least 2 V above the negative supply.

R1 + R2

(VREF)

R1 + R2

(VREF)

VREF

Figure 50. Ensure Sufficient Voltage Headroom on the Internal Op Amp

Inputs

Use a stable dc voltage to power the AD8278 and AD8279. Noise

on the supply pins can adversely affect performance. Place a

bypass capacitor of 0.1 μF between each supply pin and ground,

as close as possible to each supply pin. Use a tantalum capacitor

of 10 μF between each supply and ground. It can be farther

away from the supply pins and, typically, it can be shared by

other precision integrated circuits.

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