Datasheet

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
THD (dB)
10 1000
INPUT FREQUENCY(kHz)
03472-0-023
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)
03472-0-024
100
–60
R
IN
= 50 Ω
R
IN
= 10 Ω
R
IN
= 100 Ω
R
IN
= 1000 Ω
V
DD
= 5V
Figure 24. THD vs. Analog Input Frequency for Various
Source Impedances for V
DD
= 5 V, 136 kSPS, Mode 1