Figure 2-2 shows the fully differential drive amplifier (FDA) configurations used for this EVM. The FDA is a very common type of amplifier used in driving fully differential data converters because this amplifier outputs a differential signal with a fixed common-mode voltage as required by the ADC. The common-mode voltage is set on pin 2 of U2. This voltage is generated in Figure 2-3 and can be set to either 2.5 V or 2.6 V with jumper JP3. The THS4551 is used in this example because this device is a very low-noise and low-distortion FDA. The FDA gain is set to 1 V/V using R3, R10, R25, and R22. For best distortion, 0.1% resistors are used. The output filter is optimized for good ADC settling (R8, R17, and C6). The charge bucket capacitor C6 must be a C0G-type capacitor for low distortion. The jumpers JP1 and JP2 are not installed if the input signal is differential. Section 2.3 describes how these jumpers can be used to convert a single-ended signal to a differential signal.