100

|

4 Measurement of Biosignals and Analog Signal Processing

Fig. 4.7: Two-stage instrumentation amplifier with the gain 201.

If in Equation 4.4 UR3 is replaced by Equation 4.3 and furthermore it is considered that

R1 and R2 are equal for symmetry reasons, then the following holds for the difference

of the two output voltages:

Ua1 −Ua2 = (1 + ^2R1

R3

) (Ue1 −Ue2) .

(4.5)

With Equation 4.5 and the values R1 = 22 kΩand R3 = 220 Ωthe gain for the first

stage is V = 201. The two outputs of OPV1 and OPV2 are symmetrically connected via

R4,6 to OPV3. The output of OPV3 is the potential difference Ue1 −Ue2 with respect to

ground. By increasing the relation between R5,7 and R4,6 an additional amplification

of the second stage can be achieved which was not done here.

Let us now consider common-mode rejection. For this purpose, in the circuit after

Figure 4.7 the biosignal UEKG and the common mode signal UGL are added to the circuit

input. Ru1,2 in Figure 4.8 represent the contact resistances at the electrodes, which are

assumed to be equivalent here.

According to the superposition principle, the common mode signal can be con-

sidered independently of the differential voltage. Due to the circuit symmetry, it is

present with the same amplitude and phase at the input of the operational amplifiers

OPV1 and OPV2 and consequently also above and below R3. So there is no current

flowing through R3, which is why this resistor can be omitted for the common mode

consideration. If R3 is replaced by open terminals, OPV1 and OPV2 form in conjunc-

tion with R1 and R2 an impedance transformer. Thus, UGL passes unamplified (gain

is 1) the first stage of the instrumentation amplifier and is applied to the positive and

negative inputs of OPV3. The common mode rejection is entirely determined by this

operational amplifier. This consideration assumes that the resistors R4,5,6,7 have ex-