Fluke Biomedical 945A User Manual
Page 44
Table 4-10. Electrometer Control Bit Assignments
Bit Position
7 6 5 4 3 2 1 0 Description
X X X X X 0 0 0 Gain
=
1
X X X X X 0 0 1 Gain
=
2
X X X X X 0 1 0 Gain
=
4
X X X X X 0 1 1 Gain
=
8
X X X X X 1 0 0 Gain
=
16
X X X X X 1 0 1 Gain
=
32
X X X X X 1 1 0 Gain
=
64
X X X X X 1 1 1 Gain
=
128
X X X X 0 X X X Unshort
capacitor
X X X X 1 X X X Short
capacitor
X X X 0 X X X X Digitize
electrometer
X X X 1 X X X X Digitize
high
voltage
X = Bit position does not matter (1 or 0)
The integrating electrometer is formed by the combination of current source Z6, dual MOSFET Q4,
operational amplifier Z12, transistor Q3, and analog switch Z17, pin 15.
Counter Z15 and ladder network RN3 form the auto-zero portion of the integrating electrometer. Refer to
section 5 for auto-zero adjustment instructions.
Dual MOSFET Q4 acts as a low leakage input buffer to the electrometer. C17 is an integrating capacitor,
while Z17, pin 15 and Q3 are the shorting elements of the integrator.
Refer to High Voltage/Communications Interface, P/N 977-200-15, schematic diagram 977-210-13 in
Appendix B. U12 is an asynchronous communications interface adapter (ACIA) which communicates
with the UDR. U1 is an analog switch used to select either the Victoreen loop or the RS232
driver/receiver for external communications. Optical isolation U10 isolates data transmitted on the
Victoreen loop while the circuitry comprised of Q6, Q7, Q14, and Q18 are the actual loop drivers. Optical
isolator U8 isolates the receive data from the Victoreen loop.
U2 and U3 are, respectively, the receiver and driver circuits for the RS232 port. U14 decodes address
block A000 to provide chip enables to the ACIA switch input register U6.
Power for the detector, analog, and digital outputs is derived from three terminal regulators VR3 and VR4.
The ±5 Vdc logic power is derived from the three terminal regulators VR1 and VR2. Regulator VR5
provides +12 Vdc for the high voltage power supply. Operational amplifier U5, transformer TX1, and
transistors Q21 & Q23 form the high voltage power supply. Nominal operating point voltage for the power
supply is 500 Volts. Transistor Q3 is a switch under control of port bit B2 of the microprocessor. This
switch is toggled to induce a pulse on the high voltage output, activating current flow due to the
capacitance of the ion chamber. This operation occurs during the check source mode and during integral
cable check.
4-20