AD7982 datasheet(20/24 Pages) AD

20 / 24 page

AD8475

Rev. B | Page 20 of 24

–OUT

+OUT

VOCM

+IN 0.4x

–IN 0.8x

+IN 0.8x

–IN 0.4x

1kΩ

1.25kΩ

1kΩ

AD8475

1.25kΩ

+VS

–VS

0.1µF

VOUT = (V+OUT – V–OUT)

REF

0.1µF

10µF

LOW

IMPEDANCE

INPUT SOURCE

VIN

0.1µF

10µF

Figure 53. Typical Configuration—16-Lead LFCSP

HIGH PERFORMANCE ADC DRIVING

The AD8475 is ideally suited for broadband dc-coupled and

industrial applications. The circuit in Figure 55 shows an

industrial front-end connection for an AD8475 driving an

AD7982, a 18-bit, 1 MSPS ADC, with dc coupling on the

AD8475 input and output. (The AD7982 achieves its optimum

performance when driven differentially.) The AD8475 performs

the attenuation of a 20 V p-p input signal, level shifts it, and

converts it to a differential signal without the need for any

external components. The AD8475 eliminates the need for dual

supplies at the front end to accept large bipolar signals. It also

eliminates the need for a precision resistor network for attenua-

tion, and a transformer to drive the ADC and perform the single-

ended-to-differential conversion.

The ac and dc performance of the AD8475 are compatible with

the 18-bit, 1 MSPS AD7982 PulSAR® ADC and other 16-bit and

18-bit members of the family, which have sampling rates up to

4 MSPS. Some suitable high performance differential ADCs are

listed in Table 6.

Table 6. High Performance SAR ADCs

Part

Resolution

Sample

Rate

Description

AD7984

18 Bits

1.33 MSPS

True differential input,

14 mW, 2.5 V ADC

AD7982

18 Bits

1 MSPS

True differential Input,

7.0 mW, 2.5 V ADC

AD7690

18 Bits

400 kSPS

True differential input,

4.5 mW, 5 V ADC

AD7641

18 Bits

2 MSPS

True differential input,

75 mW, 2.5 V ADC

In this example, the AD8475 is powered with a single 5 V

supply and used in a gain of 0.4, with a single-ended input

converted to a differential output. The input is a 20 V p-p

symmetric, ground-referenced bipolar signal. With an output

common-mode voltage of 2.5 V, each AD8475 output swings

between 0.5 V and 4.5 V, opposite in phase, providing an 8 V p-p

differential signal to the ADC input.

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