Effect Of Source Capacitance And Parasitic Capacitance Of The Pcb; Figure 15. Analog Signal Source Resistance Effect - ST STM32 Application Note

AN2834
The effective charging of C
becomes t
charge the C
less than the actual value.
1. t is the time taken by the C
c
V : capacitor (C
c
t = (R
c ADC
2.2.7

Effect of source capacitance and parasitic capacitance of the PCB

When converting analog signals, it is necessary to account for the capacitance at the source
and the parasitic capacitance seen on the analog input pin (refer to
resistance and capacitance form an RC network. In addition, the ADC conversion results
may not be accurate unless the external capacitor (C
the input voltage. The greater value of (C
The external capacitance at the source and the parasitic capacitance are denoted by C
and C , respectively.
p
is governed by R
ADC
= (R +R ) × C
c ADC AIN ADC
through R
ADC ADC

Figure 15. Analog signal source resistance effect

capacitor to fully charge: V
ADC
) voltage
ADC
+ R ) × C
AIN ADC
DocID15067 Rev 3
+ R
ADC
. If the sampling time is less than the time required to fully
+ R (t < t ), the digital value converted by the ADC is
AIN s c
= V
c
AIN
+ C ), the more limited the source frequency.
AIN p
, so the charging time constant
AIN
(with max.1/2 LSB error)
AIN
Figure 16). The source
+ C ) is fully charged to the level of
p
ADC errors
AIN
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