The data acquisition system shown in Figure 54 has three major contributors to the noise and accuracy in the system: the input driver, the reference with driver, and the data converter. Each analog block is carefully designed so that the data converter specifications limit the system specifications. The THS4551, a fully differential operational amplifier is used to drive the 18-bit ADC (ADS8881). The charge-kickback RC filter at the output of the THS4551 is used to reduce the charge kickback created by the opening and closing of the sampling switch inside the ADC. Design the RC filter so that the voltage at the sampling capacitor settles to 18-bit accuracy within the acquisition time of the ADC.
Data-acquisition systems require stable and accurate voltage references in order to perform the most accurate data conversion. The REF61xx family of voltage references have integrated an ADC drive buffer, and can therefore drive the REF pin of the ADS8881 directly, without the need for an external reference buffer. See the Integrated ADC Drive Buffer section for more details about reference-buffer requirements. Correct output capacitor selection for the REF61xx is very important in this design. The Stability section describes the ESR requirements of the output capacitor for stability and burst-mode requirements. A capacitance of 1 μF is connected to the FILT pin to reduce broadband noise of the REF61xx.