ADC12 Impedance vs. Sampling Frequency Data
300
SNIU028A – February 2016 – Revised April 2016
Copyright © 2016, Texas Instruments Incorporated
ADC12 Overview
8.2
ADC12 Impedance vs. Sampling Frequency Data
The ADC12 channel impedance measurement setup is shown in
. Two 1Mohm resistors are
connected as a voltage divider. Vsource is 2V DC. The ADC channel impedance is modeled as Z and in
parallel with R2. The value of Z is calculated based on the voltage measured at node A. No external
filtering capacitance is added to the ADC input channel.
Figure 8-4. Impedance Test Setup
The ADC12 channel input impedance vs. sampling frequency data is shown in the
. At highest
sampling frequency that UCD3138 ADC12 can, the input impedance is around 300kohm; at a sampling
frequency of below 150kHz, the channel impedance is above 3Mohm. This data can be used as a general
guideline of designing proper input R for the UCD3138 ADC12, or determine a proper operation frequency
for a given R.
Figure 8-5. ADC12 Channel Impedance
Generally, ADC conversion should be done after the voltage on the S/H capacitor is charged up to less
than 1/2 LSB error. But at high conversion speed and in the condition of large external R, it can be difficult
to charge the S/H capacitor to within 1/2 LSB error in the allotted conversion time. That’s the reason we
see impedance rolls off.