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Composite instrumentation amplifier for biopotentials

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Abstract

We present the design of an instrumentation biopotential amplifier that, (a) combines the ac coupling and high input impedance of an ac-coupled buffer with the CMRR of a simple differential amplifier or a monolithic instrumentation amplifier, (b) improves the CMRR by using a potentiometer without requiring either precision resistors or high-CMRR op amps, (c) illustrates how to calculate the CMRR for differential-input stages for either differential output or single-ended output. With our approach, for the 741 op amp, the CMRR of the differential stage exceeds 126 dB at 10 Hz and 106 dB at 100 Hz, which is higher than the 90 dB of the op amp alone. This is because a potentiometer permits mismatch of the resistor ratio, which compensates for the low CMRR of the op amp. Use of the LF355 op amp, which has a wider bandwidth, yields a CMRR of 126 dB at both 10 and 100 Hz. If the second stage is an instrumentation amplifier then no adjustment is necessary. Mismatch of passive components in the bootstrapped input buffer stage decreases the CMRR from 126 dB to 112 dB at 60 Hz and decreases it further at lower frequencies, but that is not a problem for the usual power-line interference.

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Abbreviations

A c :

op-amp open loop common mode gain

A d :

op-amp open loop differential mode gain

A 0 :

op-amp open loop dc gain

CMRR:

common mode rejection ratio

CMRR B :

common mode rejection ratio for the input stage

CMRR D :

common mode rejection ratio for the differential stage

CMRR IA :

common mode rejection ratio for IC instrumentation amp

CMRR OA :

common mode rejection ratio for the op amp

CMRR OA (0):

common mode rejection ratio for the op amp at dc

CMRR T :

total common mode rejection ratio for the circuit

G c :

common mode gain of second stage

G d :

differential mode gain of second stage

G DC :

gain from the common mode input to differential output

G DD :

gain from the differential mode input to differential output

H(s),H 1(s),H 2(s):

transfer function of input buffers

IMRR:

isolated mode rejection ratio

k 1,k 2 :

resistors' ratios

υ a υ b :

buffers' output voltages

υ’ a υ’ b :

input voltages to the op amp

υ c :

common mode input signal

υ d :

differential mode input signal

υ o :

output voltage

υ C :

common mode output signal for the input stage

υ D :

differential mode output signal for the input stage

υ1υ2 :

input voltages

Z C :

common mode input impedance

Z CE :

effective coupling impedance

αα a :

resistors' ratio mismatch

β:

resistors' tolerance

τ1τ2τ m τ a τ n :

time constants

ω a :

op-amp unity gain frequency

ω n :

low-pass corner frequency for input buffers

References

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Author information

Authors and Affiliations

  1. División de Instrumentación y Bioingenieria Departamento de Ingenieria Electrónica, U.P.C., ETS Ing Telecom, P.O. Box 30002, 08080, Barcelona, Spain

    R. Pallás-Areny

  2. Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, Wisconsin

    J. G. Webster

Authors
  1. R. Pallás-Areny
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  2. J. G. Webster
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Additional information

Until August 1, 1990, address correspondence to R. Pallás-Areny, c/o Prof. John G. Webster, Department of Électrical and Computer Engineering, University of Wisconsin-Madison, 1415 Johnson Drive, Madison, WI 53706-1691.

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Pallás-Areny, R., Webster, J.G. Composite instrumentation amplifier for biopotentials. Ann Biomed Eng 18, 251–262 (1990). https://doi.org/10.1007/BF02368441

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  • DOI: https://doi.org/10.1007/BF02368441

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