AD7452

Data Sheet

Rev. C | Page 16 of 24

DRIVING DIFFERENTIAL INPUTS

ously driven with two equal signals that are 180° out of phase.

The common mode must be set up externally and has a range

fier used to drive the analog inputs (see Figure 23 and

Figure 24). Differential modes of operation with either an ac or

a dc input provide the best THD performance over a wide

frequency range. Since not all applications have a signal

preconditioned for differential operation, there is often a need

to perform single-ended-to-differential conversion.

Differential Amplifier

An ideal method of applying differential drive to the AD7452 is

to use a differential amplifier such as the AD8138. This part can

be used as a single-ended-to-differential amplifier or as a

differential-to-differential amplifier. In both cases, the analog

input needs to be bipolar. It also provides common-mode level

shifting and buffering of the bipolar input signal. Figure 29

shows how the AD8138 can be used as a single-ended-to-

differential amplifier. The positive and negative outputs of the

AD8138 are connected to the respective inputs on the ADC via

a pair of series resistors to minimize the effects of switched

capacitance on the front end of the ADCs. The RC low-pass

filter on each analog input is recommended in ac applications to

remove high frequency components of the analog input. The

architecture of the AD8138 results in outputs that are very

highly balanced over a wide frequency range without requiring

tightly matched external components. If the analog input source

increased by 25 Ω to balance this parallel impedance on the

input and thus ensure that both the positive and negative analog

inputs have the same gain (see Figure 29). The outputs of the

amplifier are perfectly matched, balanced differential outputs of

identical amplitude, and are exactly 180° out of phase.

The AD8138 is specified with +3 V, +5 V, and ±5 V power

supplies, but the best results are obtained when it is supplied by

±5 V. The AD8132 is a lower cost device that could also be used

in this configuration with slight differences in characteristics to

the AD8138 but with similar performance and operation.

EXTERNAL

+2.5V

GND

–2.5V

C*

C*

AD7452

3.75V

2.5V

1.25V

3.75V

2.5V

1.25V

*MOUNT AS CLOSE TO THE AD7452 AS POSSIBLE

AND ENSURE HIGH PRECISION Rs AND Cs ARE USED.

AD8138

Figure 29. Using the AD8138 as a Single-Ended-to-Differential Amplifier