8.2.1.2 Detailed Design Procedure

The application circuit is illustrated in Figure 105. For simplicity, power-supply decoupling capacitors are not shown in these circuit diagrams; see the Power-Supply Recommendations section for suggested guidelines.

The reference voltage of 4.5 V is generated by the high-precision, low-noise REF5045 circuit. The output broadband noise of the reference is heavily filtered by a low-pass filter with a 3-dB cutoff frequency of 16 Hz.

Generally, the distortion from the input driver must be at least 10 dB less than the ADC distortion. The low-power OPA2625 (a high-bandwidth, low-distortion, high-precision amplifier in an inverting gain configuration) as an input driver provides exceptional ac performance because of its extremely low-distortion and high-bandwidth specifications. The distortion resulting from variation in the common-mode signal is eliminated by using the OPA2625 in an inverting gain configuration. To exercise the complete dynamic range of the device, the common-mode voltage at the ADS892xB inputs is established at a value of 2.25 V (4.5 V / 2) by using the noninverting pins of the OPA2625 amplifiers. In addition, the components of the charge kickback filter keep the noise from the front-end circuit low without adding distortion to the input signal.

For a complete schematic, see the ADS8920BEVM-PDK user's guide located in the ADS8920B SAR Analog to Digital Converter Evaluation Module web folder at www.ti.com.

A similar circuit is used in reference design TIPD211, a step-by-step process to design a 20-Bit, 1-MSPS, 4-Ch Small Form Factor Design for Test and Measurement Applications using four ADS8900B SAR ADCs, four OPA2625 precision amplifiers and one REF5050 precision reference.

	For step-by-step design procedure, circuit schematics, bill of materials, PCB files, simulation results, and test results, refer to TI Precision Design TIPD211, 18-Bit, 1-MSPS, 4-Ch Small Form Factor Design for Test and Measurement Applications (TIDUBW7).