SM73201 Datasheet, PDF (14/21 Pages) Texas Instruments – 16-Bit, 50 to 250 kSPS, Differential Input, MicroPower ADC

and a series resistor (R_SAMPLE), as shown in Figure 13. Typ-
ical values for C_SAMPLE and R_SAMPLE are 20 pF and 200 ohms
respectively. If there is not a large external capacitor (C_EXT)
at the analog input of the ADC, a voltage spike will be ob-
served at the input pins. This is a result of C_SAMPLE and
C_EXT being at different voltage potentials. The magnitude and
direction of the voltage spike depend on the difference be-
tween the voltage of C_SAMPLE and C_EXT. If the voltage at
C_SAMPLE is greater than the voltage at C_EXT, a positive voltage
spike will occur. If the opposite is true, a negative voltage
spike will occur. It is not critical for the performance of the
SM73201 to filter out the voltage spike. Rather, ensure that
the transient of the spike settles out within t_ACQ; for recom-
mended solutions, see Section 4.0 in the Application Infor-
mation.
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FIGURE 12. Analog Input CMRR vs. Frequency
2.5 Input Settling
When the SM73201 enters acquisition (t_ACQ) mode at the end
of the conversion window, the internal sampling capacitor
(C_SAMPLE) is connected to the ADC input via an internal switch
FIGURE 13. ADC Input Capacitors
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