SBAS569B May 2013 – February 2019 ADS8860
The charge-kickback filter is an RC filter at the input pins of the ADC that filters the broadband noise from the front-end drive circuitry and attenuates the sampling charge injection from the switched-capacitor input stage of the ADC. As shown in Figure 62, a filter capacitor (CFLT) is connected from each input pin of the ADC to ground. This capacitor helps reduce the sampling charge injection and provides a charge bucket to quickly charge the internal sample-and-hold capacitors during the acquisition process. Generally, the value of this capacitor must be at least 20 times the specified value of the ADC sampling capacitance. For the ADS8860, the input sampling capacitance is equal to 59 pF; therefore, for optimal performance, keep CFLT greater than 590 pF. This capacitor must be a COG- or NPO-type. The type of dielectric used in COG or NPO ceramic capacitors provides the most stable electrical properties over voltage, frequency, and temperature changes.
Driving capacitive loads can degrade the phase margin of the input amplifier, thus making the amplifier marginally unstable. To avoid amplifier stability issues, series isolation resistors (RFLT) are used at the output of the amplifiers. A higher value of RFLT helps with amplifier stability, but adds distortion as a result of interactions with the nonlinear input impedance of the ADC. Distortion increases with source impedance, input signal frequency, and input signal amplitude. Therefore, the selection of RFLT requires balancing the stability of the driver amplifier and distortion performance of the design. Always verify the stability and settling behavior of the driving amplifier and charge-kickback filter by a TINA-TI™ SPICE simulation. Keep the tolerance of the selected resistors less than 1% to keep the inputs balanced.
This section describes some common application circuits using the ADS8860. These data acquisition (DAQ) blocks are optimized for specific input types and performance requirements of the system. For simplicity, power-supply decoupling capacitors are not shown in these circuit diagrams; see the Power-Supply Decoupling section for suggested guidelines.