9.2.2.1 Low Distortion Charge Kickback Filter Design

Figure 41 shows the input circuit of a typical SAR ADC. During the acquisition phase, the SW switch closes and connects the sampling capacitor (C_SH) to the input driver circuit. This action introduces a transient on the input pins of the SAR ADC. An ideal amplifier with 0 Ω of output impedance and infinite current drive can settle this transient in zero time. For a real amplifier with non-zero output impedance and finite drive strength, this switched capacitor load can create stability issues.

Figure 41. Charge Kickback Filter

For ac signals, the filter bandwidth must be kept low to band limit the noise fed into the ADC input, thereby increasing the SNR of the system. Besides filtering the noise from the front-end drive circuitry, the RC filter also helps attenuate the sampling charge injection from the switched-capacitor input stage of the ADC. A filter capacitor, C_FLT, is connected across the ADC inputs. 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. As a rule of thumb, the value of this capacitor is at least 20 times the specified value of the ADC sampling capacitance. For this device, the input sampling capacitance is equal to 15 pF. Thus, the value of C_FLT is greater than 300 pF. Select a COG- or NPO-type capacitor because these capacitor types have a high-Q, low-temperature coefficient, and stable electrical characteristics under varying voltages, frequency, and time.

Note that driving capacitive loads can degrade the phase margin of the input amplifiers, thus making the amplifier marginally unstable. To avoid amplifier stability issues, series isolation resistors (R_FLT) are used at the output of the amplifiers. A higher value of R_FLT is helpful from the amplifier stability perspective, 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 R_FLT requires balancing the stability and distortion of the design.

The input amplifier bandwidth is typically much higher than the cutoff frequency of the antialiasing filter. Thus, a SPICE simulation is strongly recommended to be performed to confirm that the amplifier has more than 40° phase margin with the selected filter. Simulation is critical because even with high-bandwidth amplifiers, some amplifiers can require more bandwidth than others to drive similar filters.

9.2.2.2 Input Amplifier Selection

To achieve a SINAD greater than 70 dB, the operational amplifier must have high bandwidth in order to settle the input signal within the acquisition time of the ADC. The operational amplifier must have low noise to keep the total system noise below 20% of the input-referred noise of the ADC. For the application circuit illustrated in Figure 40, the OPA365-Q1 is selected for its high bandwidth (50 MHz) and low noise (4.5 nV/√Hz).

For a step-by-step design procedure for a low-power, small form-factor digital acquisition (DAQ) circuit based on similar SAR ADCs, see the Three 12-Bit Data Acquisition Reference Designs Optimized for Low Power and Ultra-Small Form Factor TI Precision Design.

9.2.2.3 Reference Circuit

The analog supply voltage of the device is also used as a voltage reference for conversion. The AVDD pin is recommended to be decoupled with a 3.3-µF, low-ESR ceramic capacitor.