SBOS778D April 2016 – April 2021 THS4551
Operating the THS4551 with a dc-coupled differential input source is very simple and only requires that the input pins stay in range for the dc common-mode operating voltage. The example in Figure 9-11 takes the output of a dual precision op amp (such as the OPA2192) where a high differential input signal is attenuated by the THS4551 down into the range of an 18-bit SAR ADC such as the 2-MSPS ADS9110. The input stage provides a differential gain of 21 V/V with a common-mode gain of 1 V/V. This example amplifies a small differential signal on top of a very-wide range common-mode voltage. The input common-mode voltage appears at the outputs of the OPA2192. The input common-mode voltage is level shifted by the FDA common-mode control to be at the required output common-mode voltage to drive the ADS9110 SAR ADC (with a 4.096-V reference, as shown in Figure 9-11); the FDA output common-mode voltage must be at the 2.048 V shown in Figure 9-11. This design offers a very high CMRR using the common-mode control loop of the FDA to reset the output common-mode voltage from that delivered to the inputs of the OPA2192. The actual CMRR from the OPA2192 inputs to the FDA outputs is dominated by the resistor mismatches in the FDA. The feedback and differential input capacitors are included to shape the noise gain as described in Section 10.1.6. This full example circuit is available as a TINA-TI™ simulation file.