AD7992_15 Datasheet, PDF (16/29 Pages) Analog Devices – 2-Channel, 12-Bit ADC with IC-Compatible Interface in 10-Lead MSOP

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.
AD7992
–50
V_REF = 2.5V
–55
–60
–65
V_DD = 2.7V
–70
V_DD = 3.0V
–75
–80
V_DD = 3.3V
–85
–90
–95
–100
1
V_DD = 5.5V ^{VDD = 4.5V}
V_DD = 5.0V
10
100
INPUT FREQUENCY (kHz)
Figure 22. THD vs. Analog Input Frequency for Various
Supply Voltages, FS = 136 kSPS, Mode 1
–40
V_DD = 5.5V
V_REF = 2.5V
–50
–60
R_SOURCE = 1kΩ
–70
R_SOURCE = 100Ω
–80
–90
–100
1
R_SOURCE
=
10Ω
R_SOURCE
=
50Ω ^RSOURCE
=
0Ω
10
100
INPUT FREQUENCY (kHz)
Figure 23. THD vs. Analog Input Frequency for Various
Source Impedances for VDD = 5.5 V, 136 kSPS, Mode 1
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