AD4697 datasheet(71/107 Pages) AD

71 / 107 page

Data Sheet

AD4697/AD4698

APPLICATIONS INFORMATION

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Rev. 0 | 71 of 107

Assuming all noise sources are Gaussian and uncorrelated, calcu-

late the total system rms noise (vn_total) as follows:

vn_TOTAL= vn_AFE2+vn_ADC2

(9)

where:

vn_AFE is the referred to output (RTO) rms noise of the AFE.

vn_ADC is the AD4697/AD4698 input referred rms noise.

The estimated system dynamic range (DRTOTAL) is a measure of

the system rms noise and the full-scale input range.

DRTOTAL=20log VREF22

vn_TOTAL

(10)

The AD4697/AD4698 input referred rms noise specification (vn_ADC)

is typically 37.8 µV rms (see Table 1). Figure 114 shows the typical

system dynamic range vs. vn_AFE with vn_ADC = 37.8 µV rms and

VREF = 5 V. For vn_AFE less than 13 µV rms, the overall system dy-

namic range remains within 0.5 dB of the AD4697/AD4698 dynamic

range specification (see Table 1).

Figure 114. AD4697/AD4698 Typical Dynamic Range vs. vn_AFE, VREF = 5 V

The AFE RTO noise (vn_AFE) is equal to the rms noise of each of

the constituent components in the AFE, referred to the output of the

external RC filter (REXT and CEXT in Figure 66 and in the External

RC Filter section). Assuming the RC filter bandwidth is much lower

than the bandwidth of the amplifier circuit, vn_AFE is equal to the

noise spectral density of each of these components (referred to the

amplifier output) multiplied by the effective noise bandwidth of the

RC filter (ENBWRC) as shown in the following equations,

where:

k is the Boltzmann constant.

T is the absolute temperature in Kelvin.

RF and RG are the feedback network resistors, as shown in

Figure 113.

RS is the source resistance, as shown in Figure 113.

in+ and in− represent the amplifier input current noise spectral

density in pA/√Hz.

vn represents the amplifier input voltage noise spectral density in

nV/√Hz.

See MT-049 and MT-050 for detailed derivations of vn_AFE vs.

analog front-end components and configurations.

ENBWRC= π2× 12πREXT CEXT

(11)

vn_AFE=ENBWRC× 4kTRF+ 1+RFRG 24kTRS+ in+×RS2+vn2 + RFRG 24kTRG+ in−×RF2 (12)

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