Amplifiers and Filters

1. How does a differential amplifier work? What is the common mode rejection ratio (CMRR)?
2. How does a high frequency filter differ from a low frequency filter?
3. If the low frequency filter setting is elevated, what will occur?
4. If the high frequency filter is reduced, what will occur?
5. What is the difference between the sweep speed and sensitivity?

Answers:
1. The differential amplifier cancels waveforms recorded at both the active and reference pickups and amplifies the remaining potentials. The CMRR refers to selectively amplifying different signals while rejecting common ones, measured in decibels (should be >90 dB). A more efficient amplifier has a larger CMRR.
2. A high frequency filter removes faster waveforms. For sensory NCS, it is set at 2 kHz, and 10 kHz for motor NCS and EMG. A low frequency filter removes signals with lower frequencies. For sensory NCS and EMG, it is set at 20 Hz, and 2 Hz for motor NCS.
3. This results in a shortened peak latency, lower amplitude, and triphasic potentials, but does not affect distal latency.
4. This results in prolonged peak and onset latency, lower amplitude, and a longer negative spike.
5. Sweep speed is the time allocated for each x-axis division. Sensitivity (gain) is the height allocated for each y-axis division.