BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 33

6. Typical peak-to-peak noise measurements are found by reading the maximum
peak-to-peak voltage noise of the circuit's output for three observation periods of
10 seconds each, then dividing by the gain of the ampli

fi er (i.e., 100,000).

er
described in the earlier project. Use a procedure similar to the one used earlier to measure
the CMR of the instrumentation biopotential ampli

You may want to compare the CMR of the ICIA to that of the di ff erential ampli fi

fi er:

erential inputs as shown in Figure 1.24.
2. Adjust the signal generator to produce a 60-Hz, 5-V p-p input signal V in CM .
3. Measure the corresponding common-mode output voltage V out CM .
4. Calculate the common-mode gain G CM

1. Connect the equipment, shorting the di ff

V out CM / V in CM .

Using the measured gain of the biopotential ampli fi er, calculate the common-mode

rejection:

G d
G

i ff e

C

ential r

CMR(dB) 20 log 10

M

Switched-Capacitor Instrumentation Biopotential Amplifier

Di erential biopotential signal recordings can be done through circuits other than classi-

cal op-amp di

ff

ff erential or instrumentation ampli fi ers. The simpli fi ed circuit in Figure 1.29

implements a simple but precise instrumentation ampli fi er that uses a switched-capacitor

building block. It converts di ff erential signals from a preampli fi ed electrode pair to a sin-

gle-ended output while rejecting common-mode signals in an e ective manner. In this cir-

cuit, a solid-state dual-pole dual-throw (DPDT) switch block converts the di

ff

ff erential input

to a ground-referred single-ended signal which may then be ampli fi ed by a noninverting

follower op-amp con guration.

During the time that the input switches are closed (

fi

odd), sampling capacitor C S
acquires the input signal V in diff . When the input switches open (

Φ

Φ even), hold capacitor

C H receives the sampled charge. Switching C S continuously between the input

Figure 1.29 In this switched-capacitor instrumentation amplifier, a solid-state DPDT switch block
converts the differential input to a ground-referred single-ended signal which is then amplified by a
noninverting follower operational amplifier configuration.