SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
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 Figure 8-7, the external impedance becomes smaller. Thus, larger external capacitance helps reducing the charge time and makes the measurement more accurate.