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By S Sample, Z Auto-zero and V Out


For the lowest offset and drift performance, chopper-stabilized (auto-zero) amplifiers may be the only solution. The best bipolar amplifiers offer offset voltages of 25 µV and 0.1 µV/ºC drift. Offset voltages less than 5 µV with practically no measurable offset drift are obtainable with choppers, albeit with some penalties. A basic chopper amplifier circuit is shown in Figure 1 below. When the switches are in the "Z" (auto-zero) position, capacitors C2 and C3 are charged to the amplifier input and output offset voltage, respectively. When the switches are in the "S " (sample) position, VIN is connected to VOUT through the path comprised of R1, R2, C2, the amplifier, C3, and R3. The chopping frequency is usually between a few hundred Hz and several kHz, and it should be noted that because this is a sampling system, the input frequency must be much less than one-half the chopping frequency in order to prevent errors due to aliasing. The R1-C1 combination serves as an antialiasing filter. It is also assumed that after a steady state condition is reached, there is only a minimal amount of charge transferred during the switching cycles. The output capacitor, C4, and the load, RL, must be chosen such that there is minimal VOUT droop during the auto-zero cycle

Year: 2011
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