AD7997 datasheet(17/32 Pages) AD

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AD7997/AD7998

Rev. 0 | Page 17 of 32

For ac applications, removing high frequency components from

the analog input signal is recommended, by using an RC band-

pass filter on the relevant analog input pin. In applications where

harmonic distortion and signal-to-noise ratio are critical, the

analog input should be driven from a low impedance source.

Large source impedances significantly affect the ac performance

of the ADC. This may necessitate the use of an input buffer

amplifier. The choice of the op amp is a function of the particular

application.

When no amplifier is used to drive the analog input, the source

impedance should be limited to low values. The maximum source

impedance depends on the amount of total harmonic distortion

(THD) that can be tolerated. THD increases as the source imped-

ance increases, and performance degrades. Figure 23 shows the

THD vs. the analog input signal frequency when using supply

voltages of 3 V ± 10% and 5 V ± 10%. Figure 24 shows the THD

vs. the analog input signal frequency for different source

impedances.

–100

–90

–80

–70

–50

–40

1000

INPUT FREQUENCY (kHz)

100

–60

VDD = 5.5V

VDD = 5V

VDD = 4.5V

VDD = 3.3V

VDD = 3V

VDD = 2.7V

Figure 23. THD vs. Analog Input Frequency for Various

Supply Voltages, FS = 136 kSPS, Mode 1

–100

–90

–80

–70

–50

–40

1000

INPUT FREQUENCY (kHz)

100

–60

RIN = 50Ω

RIN = 10Ω

RIN = 100Ω

RIN = 1000Ω

VDD = 5V

Figure 24. THD vs. Analog Input Frequency for Various

Source Impedances for VDD = 5 V, 136 kSPS, Mode 1

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