MT-055 TUTORIAL

Chopper Stabilized (Auto-Zero) Precision Op Amps
CHOPPER AMPLIFIERS
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.