8.2.2 Detailed Design Procedure

Figure 80 shows an application circuit for this example. The device incorporates an internal 2.5-V reference voltage and independent matched reference buffers for each ADC. The internal reference output (REFOUT) is decoupled with a 1-µF capacitor. The matched reference buffers provide a gain of 1.6384 V/V and generate a high-precision, 4.096-V reference voltage for each ADC channel. Decouple the reference buffer outputs (the REFP_A and REFP_B pins) with the REFM_A and REFM_B pins, respectively, with 10-µF decoupling capacitors. The circuit in Figure 80 shows a fully-differential data acquisition (DAQ) block optimized for low distortion and noise using the THS4551 and the ADS92x4R. Both differential ADC inputs are driven using a high-bandwidth, low-distortion, fully differential amplifier (FDA) designed in a gain of 1 V/V and an optimal RC charge-kickback filter before going to the ADC. Generally, the distortion from the input driver must be at least 10 dB less than the ADC distortion. Therefore, these circuits use the low-power THS4551 as an input driver that provides exceptional AC performance because of its extremely low-distortion and high bandwidth specifications. In addition, the components of the charge kickback filter are selected to keep the noise from the front-end circuit low without adding distortion. This front-end circuit configuration requires a differential signal at the input of the FDA and provides a differential output to drive the ADC inputs. The FDA establishes a fixed common-mode voltage at the ADC inputs using the VOCM input pin from the FDA. The ADS92x4R incorporates a REFby2 buffer output for setting the common-mode voltage. The ADS92x4R REFby2 output is decoupled using a 1-µF capacitor and connected to each FDA VOCM input pin. Each VOCM pin is decoupled using a 0.1-µF capacitor. For a complete schematic, see the ADS9224REVM-PDK user's guide located in the ADS9224R SAR analog to digital converter evaluation module tool folder.