Fluke Biomedical 945A - Manual (Page 38)

Analog Output
The analog outputs are a logarithmic function of the current UDR reading. The outputs are scaled by the
Full Scale Value and Low Scale Value, positions 4 and 7 respectively of the FUNCTION switch. An 8 bit
DAC is used to convert the displayed dose rate to a 4-20 mA or 0-10 Vdc output on connector P6 (on the
rear panel). Other outputs may be provided through the use of the Analog Output Option Board (942-
200-95). Refer to the option board manual for adjustment and value selection. The option board output
is also a logarithmic value. Output current or voltage is calculated using the following equations (shown
below):
P = log (R/LSV)/ [ log (FSV) - log (LSV) ]
and
V = P (VMax - VMin) + VMin or I = P (IMax - IMin) + IMin
where:
P = Percent of scale, expressed in a decimal number
R = Current reading
LSV = Low Scale Value
FSV = Full Scale Value
V = Voltage output
I = Current output
VMax = Maximum voltage available (usually 10 Vdc)
IMax = Maximum current available (usually 20 mA)
VMin = Minimum voltage available (usually 0 Vdc)
IMin = Minimum current available (usually 4 mA)
If the current reading (R) is greater than the full scale value (FSV) the output is limited to the MAX value (typically 10
Vdc or 20 mA).
As an example:
Assume: LSV = 1E-1, FSV = 1E7, R = 1E3, IMax = 20 mA, and IMin = 4 mA.
Then: P = [log (1E3/1E-1) ] / [ log (1E7) - log (1E-1)]
= log (1E4) / log (1E8)
= 4/8 = 0.5
and: I = 0.5 (20 mA - 4 mA) + 4 mA
= 0.5 (16 mA) + 4 mA
= 12 mA
To aid in calibrating the analog output, the high scale or low scale setpoint may be set to zero to force the
output high or low. If the low scale setpoint is set to zero, the analog output goes to low scale. If the high
scale value is set to zero, the analog output goes to full scale. If both are set to zero, the output goes to
full scale. Error code E0008 will be displayed if either setpoint is set to zero. The procedure for
calibrating the analog outputs is contained in Section 5.
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