May 22, 1997

Chopped amplifier exacts only 5 µA

Jim Williams, Linear Technology Corp, Milpitas, CA

The chopped amplifier in Figure 1 combines low supply current of 5.5 µA with high accuracy. The offset voltage is 5 µV with a drift of 0.05 µV/°C. The gain exceeds 10⁸, which affords high accuracy, even at large closed-loop gains.

Micropower comparators IC_2A and IC_2B form a biphase 5-Hz clock. This clock drives the input-related switches, causing an amplitude-modulated version of the dc input to appear at IC_1A's input. AC-coupled IC_1A has a gain of 1000, and its output drives a switched demodulator that's similar to the modulator.

The demodulator output, a reconstructed, dc-amplified version of the circuit's input, drives a dc-gain stage, IC_1B. IC_1B's output feeds back through gain-setting resistors to the input modulator, closing the feedback loop around the entire amplifier. The ratio of the feedback resistors, which in this case is 1000, sets the configuration's dc gain.

The circuit's internal ac coupling prevents IC₁'s dc characteristics from influencing overall dc performance, which results in the extremely low offset drift. The high open-loop gain permits 10-ppm gain accuracy at a closed-loop gain of 1000.

The desired micropower operation and IC₁'s bandwidth dictate the 5-Hz clock rate, so the resultant overall bandwidth is low. Full-power bandwidth is 0.05 Hz with a slew rate of approximately 1V/sec. Clock-related noise is about 5 µV. You can reduce this noise by increasing C_COMP, but increasing C_COMP proportionally reduces the bandwidth. (DI #2031)

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