AD8421BRMZ-R7 datasheet(27/29 Pages) AD

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AD8421

Data Sheet

Rev. 0 | Page 26 of 28

DRIVING AN ADC

The Class AB output stage, low noise and distortion, and high

bandwidth and slew rate make the AD8421 a good choice for

driving an ADC in a data acquisition system that requires front-

end gain, high CMRR, and dc precision. Figure 71 shows the

AD8421, in a gain-of-10 configuration, driving the AD7685,

a 16-bit, 250 kSPS pseudodifferential SAR ADC. The RC low-pass

filter that is shown between the AD8421 and the AD7685 has

several purposes. It isolates the amplifier output from excessive

loading from the dynamic ADC inputs, reduces the noise

bandwidth of the amplifier, and provides overload protection for

the AD7685 analog inputs. The filter cutoff can be determined

empirically. To achieve the best ac performance, keep the impe-

dance magnitude greater than 1 kΩ at the maximum input signal

frequency, and set the filter cutoff to settle to ½ LSB in one

sampling period for a full-scale step. For additional considerations,

refer to the data sheet of the ADC in use.

In a gain-of-10 configuration, the AD8421 has approximately

8 nV/√Hz voltage noise RTI (See the Calculating the Noise of

the Input Stage section.) The front-end gain makes the system

ten times more sensitive to input signals, with only a 7.5 dB

reduction of SNR. The high current output and load regulation

of the ADR435 allow the AD7685 to be powered directly from the

reference without the need to provide another analog supply rail.

The reference pin buffer may be any low power, unity-gain stable,

dc precision op amp with less than approximately 25 nV/√Hz of

wideband noise, such as the OP1177. Not all proper decoupling is

shown in Figure 71. Take care to follow decoupling guidelines for

both amplifiers and the ADR435.

AD7685

REF

GND

VDD

IN–

IN+

VIO

SDI

SCK

SDO

CNV

3- OR 4-WIRE INTERFACE

0.1µF

+12V

ADR435

3nF

+IN

–IN

AD8421

G = 10

+12V

–12V

1.1kΩ

±250mV

+5V

1µF

2.5V

10kΩ

10Ω

REF

10µF

5kΩ

100Ω

Figure 71. AD8421 Driving an ADC

–20

–40

–160

–140

–120

–100

–80

–60

100

125

FREQUENCY (kHz)

SNR 81.12dB

THD –100.91dB

SFDR 90.71dB

Figure 72. Typical Spectrum of the AD8421 (G = 10) Driving the AD7685

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