Instrumentation amp has low offset, drift, and low-frequency noise
By cascading instrumentation amplifiers, you can achieve an instrumentation amplifier with decade-gain having low offset, drift and LF noise.
Marián Štofka, Slovak University of Technology, Bratislava, Slovakia; Edited by Martin Rowe and Fran Granville -- EDN, August 21, 2008
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 (reference 1 and reference 2). 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.
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.
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It appears that there is an error on the schematic.
All of the programming pins (14, 15, & 16) are
connected to 2.5V. Wouldn't this produce a gain of
128?
Kenneth Lundgren - 2008-17-10 05:56:00 PDT -
If you rely on the theory that 10^M = 2^M * 5^M it would be assumed that the in-amps C2, C3 or a combination of the two would have selectable gains which are powers of five. One and five are the only gains that meet this criteria from the AD8250. From these parts the only decades acheivable are 1, 10, and possibly 100. What really throws me off though is the preset gain of eight. Could you explain this further?
Michael Scarpa - 2008-5-9 11:57:00 PDT
