Optimize a digitizer's analog signal path
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 8/1/2008 2:00:00 AM
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. |
Amplifiers in the analog signal path play a key role in providing gain, differential inputs, and high input impedance while maintaining bandwidths of several hundred megahertz. The signal channel requires gain settings from 0.25 to 4, which adjusts the input signal to match the analog-to-digital converter’s input voltage range (4 Vp-p to 250 mVp-p).
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.
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