66.9.7 ADC Channel Input Impedance

Figure 66-7. Input Channel Model

where:

  • ZIN is the input impedance in Single-ended or Differential mode
  • CIN = 2 pF ±20% depending on the gain value and mode (SE or DIFF); temperature dependency is negligible
  • RON is typical 2 kΩ and 8 kΩ max (worst case process and high temperature)

The following formula is used to calculate input impedance:

ZIN=1fS×CIN

where:
  • fS is the sampling frequency of the ADC channel
  • Typ values are used to compute ADC input impedance ZIN
Table 66-37. ZIN Input Impedance
fS (MHz)10.50.250.1250.06250.031250.0156250.007813
CIN = 2 pF
ZIN (MΩ)0.51248163264

Track and Hold Time versus Source Output Impedance

The figure below shows a simplified acquisition path.

Figure 66-8. Simplified Acquisition Path

During the tracking phase, the ADC tracks the input signal during the tracking time shown below:

tTRACK = n × CIN × (RON + ZSOURCE) / 1000

where

  • Tracking time expressed in ns and ZSOURCE expressed in Ω
  • n = 8 for 12-bit accuracy
  • RON = 2 kΩ
Table 66-38. Number of Tau:n
Resolution (bits)12
RES0
n8

The ADC already includes a tracking time of 15 tADC Clock.