Datasheet

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
THD (dB)
10 1000
INPUT FREQUENCY(kHz)
03473-0-023
100
–60
V
DD
= 5.5V
V
DD
= 5V
V
DD
= 4.5V
V
DD
= 3.3V
V
DD
= 3V
V
DD
= 2.7V
Figure 23. THD vs. Analog Input Frequency for Various
Supply Voltages, F
S
= 136 kSPS, Mode 1
–100
–90
–80
–70
–50
–40
THD (dB)
10 1000
INPUT FREQUENCY(kHz)
03473-0-024
100
–60
R
IN
= 50Ω
R
IN
= 10Ω
R
IN
= 100Ω
R
IN
= 1000Ω
V
DD
= 5V
Figure 24. THD vs. Analog Input Frequency for Various
Source Impedances for V
DD
= 5 V, 136 kSPS, Mode 1