SNIU028E February 2016 – February 2025 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.
Figure 8-4 Impedance Test SetupThe 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.
Figure 8-5 ADC12 Channel ImpedanceGenerally, 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.
Figure 8-6 S/H Capacitor Charge vs. Settling Time