Differential amp drives high-speed ADC

Chau Tran, Analog Devices Inc, Wilmington, MA -- EDN, 6/21/2001

The schematic in Figure 1 is the discrete-element version of an A/D-converter drive circuit. The circuit converts a single-ended input to a differential output. The ADC's reference voltage determines the common-mode range of the differential outputs. The circuit contains two AD9631 amplifiers—one connected in noninverting mode, and the other connected in inverting mode. The OP279 amplifiers buffer and scale the ADC's reference voltage to set the common-mode range of the two outputs. The circuit in Figure 1 requires many resistors. The two 15W resistors help prevent oscillation arising from the capacitive inputs of the ADC. The circuit has several disadvantages, such as poor gain accuracy, high distortion, and limited speed. The circuit in Figure 2 is an improved ADC driver.

The circuit consists of one AD8132 amplifier and four resistors. You can set the gain of the system by adjusting the ratio of R_F to R_G. The input accommodates both single-ended and differential signals. The circuit in Figure 2 is a low-distortion, high-speed (300-MHz-bandwidth) driver. You can also use it to drive precision delta-sigma ADCs. Figure 3 shows the performance at 10 MHz and unity gain (R_F=R_G=499W). Figure 4 shows the gain error and the low distortion in the circuit of Figure 2. The waveform at the node V_OCM indicates the output-balance error. The topology of Figure 2's circuit also improves the common-mode rejection ratio, because it provides level-shifting to the reference voltage of the ADC.