AD7992 datasheet(16/29 Pages) AD

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AD7992

Rev. 0 | Page 15 of 28

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 22 shows the THD vs. the analog input signal frequency

when using supply voltages of 3 V ± 10% and 5 V ± 10%.

Figure 23 shows the THD vs. the analog input signal frequency

for different source impedances.

–100

–95

–90

–85

–80

–75

–70

–65

–60

–55

–50

100

INPUT FREQUENCY (kHz)

VREF = 2.5V

VDD = 2.7V

VDD = 3.0V

VDD = 3.3V

VDD = 5.5V

VDD = 5.0V

VDD = 4.5V

Figure 22. THD vs. Analog Input Frequency for Various

Supply Voltages, FS = 136 kSPS, Mode 1

–100

–90

–80

–70

–60

–50

–40

100

INPUT FREQUENCY (kHz)

VDD = 5.5V

VREF = 2.5V

RSOURCE = 1kΩ

RSOURCE = 100Ω

RSOURCE = 0Ω

RSOURCE = 10Ω

RSOURCE = 50Ω

Figure 23. THD vs. Analog Input Frequency for Various

Source Impedances for VDD = 5.5 V, 136 kSPS, Mode 1

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