67.10.7 ADC Channel Input Impedance

Figure 67-7. 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:

Z IN = 1 f S × C IN

where:
  • fS is the sampling frequency of the ADC channel
  • Typ values are used to compute ADC input impedance ZIN
Table 67-38. ZIN Input Impedance
fS (MHz) 1 0.5 0.25 0.125 0.0625 0.03125 0.015625 0.007813
CIN = 2 pF
ZIN (MΩ) 0.5 1 2 4 8 16 32 64

Track and Hold Time versus Source Output Impedance

The figure below shows a simplified acquisition path.

Figure 67-8. 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 67-39. Number of Tau:n
Resolution (bits) 12
RES 0
n 8

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