This section demonstrates multiplexing several analog input signals to a high-performance driver amplifier that subsequently drives a single, high-resolution, high-speed successive-approximation-register (SAR) analog-to-digital converter (ADC). This example uses the ADS8411 and the TS5A3159 or TS5A3359 as the ADC and the multiplexer, respectively. This application uses the OPAx891 as the operational amplifier.

Figure 7-4 details how the example system consists of an ADC (ADS8411), a driving operational amplifier (OPA891), a multiplexer (TS5A3159), an ac source, a dc source, and two driving operational amplifiers.

The driving amplifiers OPA1 and OPA2 are shown as two OPA891 amplifiers. Alternatively, use a single OPA2891 to save on cost and board space. The purpose of these op-amps is to make the input sources present a low impedance to rest of the circuit. Additionally, to maintain signal fidelity, these operational amplifiers must have low noise and distortion. The third OPA891, labeled OPA3 in Figure 7-4, is used to maintain switching speed and drive the ADC. The passive band-pass filter before the ADC reduces unwanted noise.