Microsoft LPF-00004 Manuel de l'opérateur télécharger pdf (Page 57)

PRINCIPLES OF OPERATION Page 39

YBER

380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

resistor. Knowing the measured current (I), the resistance can be simply calculated from

Ohm's law, viz.:

(GΩ) = 1 V

p-p

/ I (nA

p-p

)

Common-Mode Rejection Ratio

In general, the information that the researcher wants to record is the difference between two signals

connected to the positive and negative inputs of a differential amplifier. The two signals often contain

a common component that does not contain relevant information. For example, both outputs of a

strain gauge might include a common DC potential of 2.5 volts that arises from the excitation voltage.

However, the strain on the gauge generates a microvolt-size difference between the two outputs; it

does not affect the 2.5 V "common-mode" voltage. Another example is often seen when recording

EMG signals from an animal. Both the positive and the negative electrodes pick up line-frequency

hum from the animal. The hum picked up by the electrodes may be as large as 10 mV, but is identical

on both electrodes. The EMG signal of interest is the small difference between the potentials on the

two electrodes; it may be as small as ten microvolts.

To prevent the common-mode signal from swamping the much smaller differential signal, the gains of

the positive and negative inputs must be nearly identical. In the above EMG example, if both

amplifier inputs have identical gains, the 10 mV of hum that appears equally on both electrodes is

eliminated from the amplifier output. The only signal to appear on the output is the small signal

proportional to the EMG potential generated between the two electrodes.

In practice the positive and negative inputs of a differential amplifier never have exactly equal gains.

The quality of their matching is measured by the common-mode rejection ratio (CMRR). This is

normally quoted in dB, where 20 dB corresponds to a factor of ten. Returning to the EMG example,

if the amplifier operates at unity gain with a CMRR of 60 dB (i.e., one part in a thousand), the 10 mV

of common-mode hum results in 10 µV of hum appearing on the amplifier output. This is small but

may be significant with the smallest EMG signals, so an amplifier with higher CMRR, e.g., 80 dB,

may be desirable.

The CMRR of an amplifier varies with the frequency of the common-mode signal. It is best at very

low frequencies, while above a certain frequency it diminishes steadily as the frequency of the

common-mode signal increases. It is therefore important to verify the CMRR of the amplifier at a

frequency that exceeds the expected frequency of the common-mode signal.

The CMRR of the recording system is adversely affected by imbalances in the source resistances of

the recording electrodes. This is because the source resistance of each electrode forms a voltage

divider with the input resistance of the amplifier. If the source resistances of the two electrodes are

not identical, the voltage dividers at the positive and negative inputs of the amplifier are not equal.

Returning to the EMG example, if one electrode resistance is 9 kΩ while the other is 10 kΩ, and the

amplifier inputs are 1 MΩ, the gain for one electrode is 0.9901 instead of unity, while the gain for the

other electrode is 0.9911. The difference is 0.001. Thus, even though the amplifier may have a

CMRR of 80 dB or more, the system CMRR is only 60 dB. In some cases 60 dB is acceptable but in

others it is not. The solution to this problem is the use of an amplifier with very high input

resistances, typically 100 MΩ or more.

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Microsoft LPF-00004 Manuel de l'opérateur Page 57

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