Design Ideas: June 22, 1995
You can implement signal-chopping techniques, such as those found in CMOS chopper-stabilized amplifiers, at the system level to obtain the offset stability of chopper amplifiers. Moreover, you'll obtain the better noise performance of bipolar amplifiers. The bridge measurement system in Fig 1 uses the MIC4428 driver to "chop" the bridge excitation, with polarity reversal upon command from the system microcontroller (µC). The DG303 multiplexer selects either the bridge output or a voltage proportional to the bridge excitation.
The A/D converter (operating in bipolar mode) yields a conversion word for each phase of the two phases of the excitation: one positive and one negative. The negative conversion word then subtracts from the positive conversion word to yield a result that's free from system offsets. The µC computes offset-corrected samples for each of the multiplexer signals: the bridge output and bridge excitation. The µC then takes the ratio of these two to compute the final output conversion word, which is free of any drift in the excitation supply.
Each time the bridge excitation or the multiplexer switches, allow time for the amplifier to settle before the 20-bit CS5504 A/D converter starts a new conversion. The converter can operate at speeds as high as 100 samples/sec. The system achieves an offset drift lower than 100 nV p-p over slowly varying temperature conditions, and it yields more than 100,000 noise-free counts at two updates/sec after averaging five conversion words. Channel 2 of the A/D converter serves to monitor the temperature of the load cell, whose temperature profile you can record during system calibration. (DI #1720)