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8.3
Effect of External Capacitance
The above experiment measures DC voltage without external filtering capacitor. In this chapter, the effect
of external capacitance is discussed. It is obvious that the external capacitor affects the cutoff frequency of
the first order filter; and this cut off frequency should be determined by the frequency range of the
interested signal. But it may not be obvious that the external C also plays the role in adjusting the source
impedance and should be determined by the sampling frequency of the ADC as well.
Take the above experiment setup as an example; if an external capacitor is added as shown in
the external impedance becomes smaller. Thus, larger external capacitance helps reducing the charge
time and makes the measurement more accurate.
Figure 8-7. External Capacitance Makes Lower Source Impedance
8.4
Channel to Channel Crosstalk
The 16 multiplexed ADC channels share the same S/H capacitor. Channel to channel crosstalk is
negligible when source impedance and sampling frequency are low. But when source impedance is high,
channel to channel crosstalk can be observed easily.
SNIU028A – February 2016 – Revised April 2016
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Figure 8-6. S/H Capacitor Charge vs. Settling Time
Low source impedance
Figure 8-8. Channel to Channel Crosstalk
Copyright © 2016, Texas Instruments Incorporated
High source impedance
Effect of External Capacitance
Figure
8-7,
301
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