9.2.2 Detailed Design Procedure

The theory of operation for the chopper amplifier relies on the concept of converting a DC input signal to AC before feeding it into an AC-coupled wideband amplifier. The conversion utilizes a SPDT switches to “chop” the input DC signal into an AC voltage. The output of the amplifier is then modulated by another SPDT switch to convert the signal back to DC. The output of the switch is then low-pass filtered (or integrated) to smooth and produce the final DC output.

The operation of the chopper amplifier consists of 2 phases, the sampling (S) phase and the auto-zero (Z) phase. During the auto-zero phase, the switches are toggled to the Z position, and capacitors C1 and C2 are charged to the amplifier input and output offset voltage, respectively. During the sampling phase, the switches are toggled to the S position, during which VIN is connected to VOUT through C1, the wideband amplifier, C2, and the integrator. Input DC voltage is AC-coupled by capacitor C1 and amplified by the wideband amplifier A1. C2 helps reduce any DC component caused by the amplifier’s input offset voltage, and the integrator helps smooth out the output signals to produce desired DC voltage output.

Several mechanisms helps reduce overall noise of the chopper-amplifier design. The DC gain, being the product of the AC stage and the DC gain of the integrator, can easily reach an open-loop gain of 160 dB or higher and therefore reduce the gain error, V_OUT/ (A1×A2) to almost zero. The offset and drift in the output integrator stage are nulled by the DC gain of the preceding AC stage. DC drifts in the AC stage are also non-factors because the amplification stage is AC-coupled. The 1/f noise of the wideband amplifier is modulated to higher frequencies by the demodulator.

Note that the input signal frequency shall be much less than one-half of the chopping frequency to prevent aliasing errors in this chopper amplifier implementation. The chopper frequency, in turn, is restricted by the wideband amplifier’s gain-phase limitations as well as errors induced by switch transition time and charge injection. The TMUX6119 ‘s switch transition time is only 68 ns (typ) and average charge injection is less than 0.19pC, making it ideal for the chopper amplifier implementation. However, the input signal frequency is still limited by the amplifier’s performance. If higher sampling frequency is required, a chopper-stabilized amplifier, or an integrated zero-drift amplifier (such as the OPA2188), can be used to satisfy the requirement.