Siemens Pacemaker UM344-2 Manual do Utilizador descarregar pdf (Página 136)

UM344-2 CIRCUIT DESCRIPTION

September 1995

7.2 THEORY OF OPERATION

The transmitter input signal, obtained from a Thermocouple (TC), Millivolt (mV), Resistance Temperature

Detector (RTD) , or Resistive (Ohm) source is wired to the transmitter’s signal input terminal block. The

standard calibration curves for J, K, E, T, R, S, B and N type Thermocouples and US/DIN curves for 100,

200, and 500 Platinum RTDs are stored in memory. The input is corrected for ambient temperature

effects, linearized, and converted to a 4 to 20 mA or HART digital output signal. The specific type of

input signal is selected during configuration.

7.2.1 Thermocouple/Millivolt (TC/mV) Input

The range limit for a millivolt input signal from a TC or mV source is -10 mV to +100 mV. Since input

circuits for mV and TC inputs operate identically, a TC input will be described.

The input from the thermocouple is applied to a 2-pole filter which reduces RFI and 60 Hz noise. The

filter’s output is fed to a high gain amplifier and applied to INPUT 1 of the A/D Converter. An amplified

reference junction input from an Integrated Circuit Temperature Sensor is fed to INPUT 2 of the A/D

Converter. The Sensor measures the temperature of the TC (+) screw terminal and regulates the current

at a rate of 1 microamp per degree Kelvin. The Microcontroller (µC) periodically reads the Sensor signal

and compensates the TC input for changes in ambient temperature.

UPSCALE/DOWNSCALE indicates thermocouple burnout (open circuit) by driving the output to a high

or low limit depending on the selected configuration parameter. When the transmitter input is configured

for a TC, the RTD SEL output from the A/D Converter goes logic low. This turns on the P-channel FET

and turns off the CMOS Analog Switch, disabling the 52 µA Current Sink used for RTD/OHMS input.

The V BURNOUT output from the A/D Converter, either a plus or minus voltage depending on the

upscale/downscale choice, passes through the FET and is converted to a trickle current. If the TC opens,

the trickle current will charge the input filter in the appropriate direction and drive the perceived

temperature to the high or low limit.

The input circuitry communicates with the microcontroller (µC) through three pulse transformers

(DATA/CONTROL ISOLATION) which provide isolation with minimum current consumption.

7.2.2 RTD/OHMS Input

The range input for an OHM type input is 0 to 1875 ohms (from an RTD or potentiometer source). Input

circuits for a potentiometer or RTD input operate identically; here, an RTD input will be described.

The operation of the input circuitry with an RTD/OHM input is essentially the same as with a TC/mV

input with the following exception; an RTD/OHM input is read by forcing a 52 microamp current through

the sensing device and measuring the resulting voltage drop that is applied to the input filter (52 microamps

was chosen to generate standard TC voltages when used with a TC input) .

When the transmitter is configured for an RTD input, the RTD SEL output from the A/D Converter goes

logic high turning off the P-channel FET and disabling the burnout trickle current network. The CMOS

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