Specifications

Section 4. Sensor Support
4-14
The composite transfer function of the instrumentation amplifier, integrator,
and analog-to-digital converter of the CR1000 is described by the following
equation:
COUNTS = G * Vin + B
where COUNTS is the result from an analog-to-digital conversion, G is the
voltage gain for a given input range, and B is the internally measured offset
voltage.
Automatic self-calibration only calibrates the G and B values necessary to run
a given CRBasic program, resulting in a program dependent number of self-
calibration segments ranging from at least 6 to a maximum of 91. A typical
number of segments required in self-calibration is 20 for analog ranges and 1
segment for the panel temperature measurement, totaling 21 segments. So, (21
segments) * (4 s / segment) = 84 s per complete self-calibration. The worst-
case will be (91 segments) * (4 s / segment) = 364 s per complete self-
calibration.
During instrument power-up, the CR1000 computes calibration coefficients by
averaging 10 complete sets of self-calibration measurements. After power up,
newly determined G and B values are low-pass filtered as followed:
Next_Value = (1/5) * New + (4/5) * Old. For a step change of the New value,
the low-pass filter Next_Value = (1/5) * New + (4/5) * Old results in 20%
settling for 1 New value, 49% settling for 3 New values, 67% settling for 5
New values, 89% settling for 10 New values, and 96% settling for 14 New
values. If this rate of update for measurement channels is too slow, a user can
utilize the Calibrate() instruction. The Calibrate() instruction computes the
necessary G and B values every scan without any low-pass filtering.
For a VoltSe() instruction, B is determined as part of self-calibration only if the
parameter MeasOff = 0. An exception is B for VoltSe() on the ±2500 mV
input range with 250 μs integration, which is always determined in self-
calibration for use internally. For a VoltDiff() instruction, B is determined as
part of self-calibration only if the parameter RevDiff = 0.
VoltSe() and VoltDiff() instructions on a given input range with the same
integration durations, utilize the same G, but different B values. The 6 input
voltage ranges (±5000 mV, ±2500 mV, ±250 mV, ±25 mV, ±7.5 mV, and ±2.5
mV) along with the 3 different integration durations (250 μs, _50Hz, & _60Hz)
result in a maximum of 18 different gains (G), and 18 offsets for VoltSe()
measurements (B), and 18 offsets for VoltDiff() measurements (B) to be
determined during CR1000 self-calibration (maximum of 54 values).
The various G and B values can be viewed in the Status Table as CalGain(1)
through CalGain(18), CalSeOffset(1) through CalSeOffset(18), and
CalDiffOffset(1) through CalDiffOffset(18), with an order of 250 μs
integration, _60Hz integration, and _50Hz integration on the following input
voltage ranges: ±5000 mV, ±2500 mV, ±250
mV, ±25 mV, ±7.5 mV, and ±2.5
mV.
An example of the Calibrate instruction for all input ranges is given as
Calibrate(cal(1),true), where Dest is an array of 54 values, and Range ≠ 0 in
order to calibrate all input ranges. TABLE 4.2-9 describes the 54 values
g
enerated from the Calibrate(cal(1),true) instruction.