Instrumentation amp has low offset, drift, and low-frequency noise

Analog Devices AD8231 AD8250

Marián Štofka


Analog Devices’ digitally gain-programmable AD8231 instrumentation amplifier exhibits zero offset. It has programmable voltage gains, which are successive powers of two, from 20 = 1 to 27 = 128. The AD825x family also includes some digitally gain-programmable instrumentation amplifiers, which have gain expressed as powers of 10. These amplifiers contain no internal autozero circuitry, however. The composite instrumentation amplifier in Figure 1 suits applications requiring instrumentation amplifiers having voltage gains of a multiple of 10 and requiring low voltage offset, drift, and low-frequency noise.

Instrumentation amp has low offset, drift, and low-frequency noise
Figure 1. By cascading an autozeroed instrumentation amplifier having a gain of 23
and instrumentation amplifiers having gains of five, you get a decade-gain
instrumentation amp whose dc performance is much better than that of
monolithic decade-gain instrumentation amps.

The design exploits the fact that the gain is 10M, where M is an integer, which you can express as 10M = 2M × 5M. The circuit in Figure 1 employs a cascade of the autozeroed AD8231 instrumentation amp, IC1, with a preset voltage gain of eight, IC2, and IC3. The net result is that the input-voltage offset of IC2 causes an RTI (referred-to-input) voltage offset, which decreases by a factor of eight compared with an offset of a stand-alone circuit, IC2. The same holds also for the offset-voltage drift. The auto-zeroing circuitry of the IC1 decimates the low-frequency noise.

Materials on the topic

  1. Datasheet Analog Devices AD8231
  2. Datasheet Analog Devices AD8250


You may have to register before you can post comments and get full access to forum.
User Name
10% Off for PCB and PCB Assembly Order