Optimize a digitizer's analog signal path

Mixed-signal ICs used in cellphones, DVD players, HDTVs, and set-top boxes require analog signal measurements during production. Therefore, the automated test equipment (ATE) systems used to test such products need amplifiers that provide a combination of high bandwidth and accuracy when digitizing signals.

A differential, adjustable-gain amplifier consists of three op amps.

A differential input amplifier typically consists of three current-feedback operational amplifiers surrounded by gain-setting resistors and relays. The amplifiers, configured to provide a differential input, feed into a third amplifier that provides high common-mode rejection for the signal path (figure).

To maintain bandwidth, the amplifier circuits distribute the gain over the circuit’s two stages. U1 and U2 make up the differential-input stage, while U3 is the second stage. By minimizing the gain in any stage, the circuit minimizes variations in closed-loop bandwidth. The circuit maintains its bandwidth because the feedback resistors across each amplifier remain unchanged as gain changes.

To learn the details of the amplifier-circuit topology, you can download the paper “Selecting Op Amps for High-Speed ATE Digitizers” by Maurizio Gavardoni, product definer for Maxim Integrated Products.