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Theory of Operation (2635A)
Detailed Circuit Description
2A
2A-33
2A-62. Input Connector PCA
The Input Connector assembly, which plugs into the A/D Converter PCA from the rear
of the instrument, provides 20 pairs of channel terminals for connecting measurement
sensors. This assembly also provides the reference junction temperature sensor circuitry
used when making thermocouple measurements.
Circuit connections between the Input Connector and A/D Converter PCAs are made via
connectors A4P1 and A4P2. Input channel and earth ground connections are made via
A4P1, while temperature sensor connections are made through A4P2.
Input connections to channels 1 through 20 are made through terminal blocks TB1 and
TB2. Channel 1 and 11 HI and LO terminals incorporate larger creepage and clearance
distances and each have a metal oxide varistor (MOV) to earth ground in order to clamp
voltage transients. MOVs A4RV1 through A4RV4 limit transient impulses to the more
reasonable level of approximately 1800V peak instead of the 2500V peak that can be
expected on 240 VAC, IEC 664 Installation Category II, ac mains. In this way, higher
voltage ratings can be applied to channels 1 and 11 than can be applied to the other rear
channels.
Strain relief for the user’s sensor wiring is provided both by the Connector PCA housing
and the two round pin headers. Each pin of the strain relief headers is electrically
isolated from all other pins and circuitry.
Temperature sensor transistor A4Q1 outputs a voltage inversely proportional to the
temperature of the input channel terminals. This voltage is 0.6V dc at 25 ºC, increasing 2
mV with each degree decrease in temperature, or decreasing 2 mV with each degree
increase in temperature. For high accuracy, A4Q1 is physically centered within and
thermally linked to the 20 input terminals. Local voltage reference A4VR1 and resistors
A4R1 through A4R3 set the calibrated operating current of the temperature sensor.
Capacitor A4C1 shunts noise and EMI to ground.
2A-63. Display PCA
Display Assembly operation is classified into six functional circuit blocks: the Main
PCA Connector, the Front Panel Switches, the Display, the Beeper Drive Circuit, the
Watchdog Timer/Reset Circuit, and the Display Controller. These blocks are described
in the following paragraphs.
2A-64. Main PCA Connector
The 20-pin Main PCA Connector (A2J1) provides the interface between the Main PCA
and the other functional blocks on the Display PCA. Seven of the connector pins provide
the necessary connections to the four power supply voltages. (See the following table.)
Power Supply A2J1 Pins Nominal Voltage
VCC
VEE
VLOAD
FIL1 to FIL2
8
6
7
2 to 3
+5.0V dc
-5.0V dc
-30V dc
5.4V ac
Six pins are used to provide the interface to the Front Panel Switches (A2SWR1 through
A2SWR6). The other seven signals interface the Microprocessor (A1U1) to the Display
Controller (A2U1) and pass the reset signals between the assemblies.