AD7991 datasheet(19/28 Pages) AD

19 / 28 page

AD7991/AD7995/AD7999

Rev. 0 | Page 19 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 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 for different source impedances at a

supply voltage of 5 V.

56Ω

5.1kΩ

2kΩ

1.3kΩ

240Ω

100

ANALOG INPUT FREQUENCY (kHz)

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

VDD = 5V

VREF = VDD

TEMPERATURE = TA

fSCL = 1.7MHz

Figure 23. THD vs. Analog Input Frequency for Various Source Impedances

for VDD = 5 V, fSCL = 1.7 MHz Without Clock Stretching

AD7991 Datasheet(PDF) 19 Page - Analog Devices