AD7991 datasheet(17/25 Pages) AD

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Preliminary Technical Data

AD7991/AD7995/AD7999

Rev. PrB | Page 17 of 25

For ac applications, removing high frequency components from

the analog input signal is recommended by use of 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 impedance increases, and performance degrades. Figure

14 and Figure 15 shows the THD vs. the analog input signal

frequency for different source impedances. at 3V and 5V

respectively.

Figure 14. THD vs Analog Input Frequency for various source

Impedances for VDD = 3V

Figure 15. THD vs Analog Input Frequency for various source

Impedances for VDD = 5V

AD7991 Datasheet(PDF) 17 Page - Analog Devices