SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
The ADC12 channel impedance measurement setup is shown in Figure 8-4. 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.
The ADC12 channel input impedance vs. sampling frequency data is shown in the Figure 8-5. 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.
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.