The input unit consists of a differential-type preamplifier, a high frequency filter, a 50 Hz
notch filter, a low-pass filter, and a variable gain control. ECG signal are picked up by three electrodes and are fed to
the high frequency filter to limit noise from electro-surgical equipment. The preamplifier is protected from over
voltage by diodes and is a differential type with CMRR (Common-mode rejection ratio) better than 60 dB. This is
achieved by a matching filter and the use of close-tolerance resistors in a dual-in-line package. Both high frequency
filter and preamplifier are screened against high-frequency interference.
The signal is also passed through a second-order low-pass filter and variable gain amplifier controlled by the
microcontroller to obtain a normalized output. The frequency response for the amplifier section is 0.3-35 Hz at -3 dB,
which is capable of eliminating any muscle artifacts caused by the patient moving