Siemens Pacemaker UM344-2 Manuale Utente Scaricare il pdf (Pagina 138)

UM344-2 CIRCUIT DESCRIPTION

September 1995

The current from the 104 microamp source is applied to the RTDI(+) pin which is jumpered to the TC(+)

pin. Since the 52 µA sink network draws 52 microamps, the remaining current flows through the RTD

input to ground creating a voltage at the input to the 2-pole filter. 52 microamps is drawn through the TC

(+) pin into the 52 µA sink network, ensuring that when a 3-wire RTD is used , both measurement leads

will have identical current flowing in the same direction into the transmitter.

7.2.3 Signal Conversion

The digitized sensor and reference junction signals are passed from the input circuit’s A/D Converter

through isolated pulse transformers into the microcontroller (µC) for signal linearization and temperature

compensation.

The output signal from the µC is applied to a D/A Converter which outputs two variable width pulses

(PULSE ON/PULSE OFF). The widths of the pulses are modulated in direct proportion to the voltage

level of the sensor input signal. The pulses are simultaneously applied to the V/I Converter and to two P-

channel FETs within the Converter. The output from the FETs is filtered and fed into an operational

amplifier whose output drives the voltage-to-current conversion circuit, a Darlington transistor pair acting

as a pass transistor for the 4 to 20 mA loop current output.

7.2.4 Communication Format

The Model 344 communicates, via the HART protocol, with the portable MXC and any Primary or

Secondary Master connected to the Network.

HART communication uses phase-continuous frequency-shift-keying (FSK) at 1200 bits/sec and

frequencies of 1200 Hz for logic 1 and 2200 Hz for logic 0. HART communications are superimposed

(AC coupled) on the analog 4-20 mA signal. Because the digital signaling is high frequency AC, its DC

average is zero and does not interfere with analog signaling.

A 2-Pole Active Filter connected to the loop input receives HART transmissions. The filter rejects low

frequency analog signaling, and other out-of-band interference, preventing a compromise of the digital

reception. The filtered signal is applied to a Zero Crossing Detector which converts the filtered

information into clean pulses of uniform amplitude before introduction to the Bell 202 Modem.

The Modem receives and processes (e.g., demodulation) the serial FSK signal (R

) and outputs the signal

) to the Microcontroller where serial to parallel conversion is performed.

In response to the received signal, the Microcontroller outputs a signal (T

) to the Modem where it is

modulated and fed into the feedback circuit of the V/I Converter for transmission (T

) over the loop.

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Siemens Pacemaker UM344-2 Manuale Utente Pagina 138