AD7993 datasheet(17/32 Pages) AD

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AD7993/AD7994

Rev. 0 | Page 17 of 32

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 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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