Fluke 943-227-15 - Manual (Page 15)

Theory of Operation
Main Circuit Boards
2
2-3
Table 2-2. 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 4 for auto-zero adjustment instructions (note this is a factory only adjustment).
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.
There is a hysteresis built into the range changing function of the electrometer. The range change-up
occurs at the decade points whereas the range change-down occurs at 80% of the decade points. This
hysteresis results in the display of only two significant digits when in the upper 20% of a decade.
Whether or not this occurs depends on which direction the radiation level is trending. An increasing trend
will result in three significant digits while a decreasing trend will result in two significant digits.
The response times for a change in reading within the same decade are listed in Table 2-1. If a change
of 100 to 1000 times the current reading occurs, the instrument will range change within a single 50-
millisecond clock cycle.
Table 2-3. Response Time
DECADE RANGE RESPONSE TIME
1 0.1 to 1.0 mR/h 80 sec.
2 1.0 to 10 mR/h 32 sec.
3 10 to 100 mR/h 16 sec.
4 100 to 1000 mR/h 4 sec.
5 1.0 to 10 R/h 2 sec.
6 10 to 100 R/h 0.9 sec.
7 100 to 1000 R/h 0.2 sec.
8 1.0 k to 10 kR/h 0.2 sec.