AD4697 datasheet(72/107 Pages) AD

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Data Sheet

AD4697/AD4698

APPLICATIONS INFORMATION

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Rev. 0 | 72 of 107

Analog Front-End Noise in Pseudobipolar

Mode

When configuring a channel in pseudobipolar mode, typically a

second AFE circuit is required to drive the negative side input

to VRREF/2 V (as described in the Channel Configuration Options

section). In this case, the RTO rms noise of the additional AFE

(vn_AFE2) is added to the rss equation to calculate the total system

rms noise:

vn_TOTAL= vn_AFE2+vn_AFE22+vn_ADC2 (13)

Note that the bandwidth of the RC filter and values of REXT and

CEXT cannot be set arbitrarily low due to the settling requirements

of the AD4697/AD4698 analog inputs. Refer to the Signal Settling

Requirements section for guidelines on selecting the optimal RC

filter components for the target sample rate.

Guidelines for Driver Amplifier Selection

The following is a list of guidelines for selecting the amplifier(s)

used in the AD4697/AD4698 AFE based on the end system re-

quirements.

The amplifier voltage and current noise specifications must be

sufficiently low to achieve the desired rms noise and dynamic

range performance, as described in the Analog Front-End Noise

Considerations section.

The distortion performance of the amplifier must be sufficient to

achieve desired THD performance. To meet the AD4697/AD4698

THD data sheet specification, the amplifier circuit must have lower

or comparable distortion specifications.

The small signal bandwidth of the amplifier must be sufficiently

higher than the minimum bandwidth required to adequately settle

the voltage steps that occur when switching between two analog

input channels, as described in the Signal Settling Requirements

section.

The amplifier must also have sufficient supply headroom to ade-

quately output a full-scale signal to the AD4697/AD4698 analog

inputs (see the input voltage range specification in Table 1). Refer

to the input and output headroom requirements in the amplifier

data sheet to determine the supply voltages required to support the

desired full-scale range for the given channel.

The ADA4805-1 and ADA4807-1 and their dual- and quad-amplifier

models are suitable amplifiers for channels acquiring ac waveforms,

due to their exceptionally low noise and distortion and high band-

width.

The ADA4610-1 and ADA4077-1 and their dual- and quad-amplifier

models are suitable amplifiers for channels monitoring dc or low

frequency signals that require high precision. It is recommended to

enable analog input high-Z mode on the AD4697/AD4698 analog

input channels when driven directly by the ADA4610-1, ADA4077-1,

or amplifiers with similar bandwidth specifications to ensure ade-

quate settling performance (see the Signal Settling Requirements

section and Analog Input High-Z Mode section).

Table 26. Recommended Amplifier and External RC Filter Component Selection

Input Signal Bandwidth (kHz)

Sample Rate

Amplifier

REXT (Ω)

CEXT (pF)

≤10

≤1 MSPS

ADA4805-1/ADA4805-2

390

180

ADA4807-1/ADA4807-2/ADA4807-4

390

180

ADA4610-1/ADA4610-2/ADA4610-4

680

180

ADA4077-1/ADA4077-2/ADA4077-4

680

180

≤500 kSPS

ADA4805-1/ADA4805-2

680

180

ADA4807-1/ADA4807-2/ADA4807-4

680

180

ADA4610-1/ADA4610-2/ADA4610-4

680

470

ADA4077-1/ADA4077-2/ADA4077-4

680

470

>10

≤1 MSPS

ADA4805-1/ADA4805-2

200

180

ADA4807-1/ADA4807-2/ADA4807-4

200

180

ADA4896-2

200

180

≤500 kSPS

ADA4805-1/ADA4805-2

390

180

ADA4807-1/ADA4807-2/ADA4807-4

390

180

ADA4896-2

390

180

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