4.1 Measurement of Electrical Biosignals

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101

OPV2

LT1179

OPV1

LT1179

OPV3

LT1179

R1

22k

R2

22k

R3

220

R4

22k

R5

22k

R6

22k

R7

22k

Rü1

2k

Rü2

2k

Ugl

Uekg

R8

2k

R9

200k

R10

200k

+15V

+15V

+15V

-15V

-15V

-15V

Ue1

Ue2

Ua1

Ua2

Fig. 4.8: Two-stage instrumentation amplifier with consideration of the biosignal UEKG, the common

mode signal UGL and the contact resistances Ru1,2.

actly the specified value. Such a condition is usually well satisfied in integrated cir-

cuits. However, if resistance tolerances have to be taken into account, as is the case

with lumped resistors R4,5,6,7, the common-mode rejection is massively degraded. The

resistance ratios ^R5

R4 ^{and R7}

R6 ^{in the circuit according to Figure 4.8 determine the gain at}

which the common mode signal is applied to the inputs of the subtractor. For ^R5

R4

̸= ^R7

R6

the signal amplitudes at the two inputs are different, resulting in a residual signal

at the subtractor output. To come as close as possible to the requirement ^R5

R4 ⁼

R7

R6 ^,

precision resistors must be selected for R4,5,6,7. The remaining difference in the res-

istor ratio can be eliminated by a subsequent circuit adjustment. For this purpose, the

resistor R5 in Figure 4.8 must be replaced by the potentiometer circuit according to

Figure 4.9.

A measure of the common mode rejection capability is the CMRR value in dB

(CMRR: Common Mode Rejection Ratio). CMRR relates the differential gain Vdiff to the

common mode gain VCM:

CMRR = 20 log10 ( ^Vdiff

VCM

) dB .

(4.6)

Commercially available operational amplifiers have a CMRR of more than 90 dB. For

example, if the amplitude of the common mode signal is 5 V, then CMRR = 100 dB

leaves 50 μV at the output of OPV3, because for the second stage of the instrumenta-

tion amplifier after Figure 4.8 Vdiff = 1. The biosignal, on the other hand, is amplified

by the first stage by a factor of 201. Thus, an ECG with an amplitude of 1 mV at the

input of the instrumentation amplifier is approximately 200 mV at the output, which

is 4000 times than the common mode signal with an input amplitude of 5 V, which is

quite sufficient for most applications.