Preamp Timing Description

Figure 1–5 Preamp Timing

Instruction Decoding.

The pulse train commands from the System microprocessor are coded by the number of pulses as listed in Table 1-3. The number of pulses represents a binary number which corresponds to a particular command function. Each set of pulses performs two tasks: application of a specific preamp setup, and initiation of ECG signal conversion to a digital equivalent. The rising edge of the first pulse is detected by U31 A which responds by sending a pulse to U34A. This initiates the ECG conversion. Monostable multivibrator U31 B detects the falling edge of the first pulse and clears U31A so that ECG conversion is not initiated with every pulse on the pulse train.

Flip-flop U36A and counter U32 form a five-stage counter that responds to the pulses from the pulse train (see TP17, Figure 1-5). The first rising edge of the pulse train causes the five-stage counter to load from the preset inputs (pins 6, 5, 3, and 4). Subsequent edges count up from the preset inputs. Because of the timing of RC network R101 and C74, U31 B times out at about 1.5ms and sends a positive edge to U33, latching the decoded command signals. These command signals select between leads and paddles.

Instrumentation Amplifier.

The typical ECG signal at U41 and U42 is 16mV p-p maximum and requires amplification for further signal processing. U44A and U44B together form a differential input amplifier with the gain set by RN2 pins 1 and 2, R 144, and RN2 pins 3 and 4. Op amp U44D amplifies the differential signal and rejects the common-mode signal from U44A and U44B. This common-mode signal (primarily 60Hz) is generated from the outputs of U44A and U44B through voltage dividers R148 and R149. The common-mode signal is then inverted and amplified by U35D and passes through analog switch 1141 and Kl. Relay Kl routes the signal to the unused patient lead (LL) cancelling common-mode signals at the patient source.

Next Page