CS5467
9.1.2 Gain Calibration
During gain calibration, a full-scale reference signal
must be applied to the meter or optionally, scaled to the
VIN1 ??? VIN2 ?? , ? IIN1 ? (IIN2 ??? pins of the CS5467. A DC
reference must be used for DC gain calibration. Either
an AC or DC reference can be used for RMS AC calibra-
tions. If DC is used, the associated high-pass filter
(HPF) must be off.
Figure 12 shows the basic setup for gain calibration.
External
Connections
During AC gain calibration the RMS level of the applied
reference is measured with the preset gain, then divided
into 0.6 and the quotient stored back into the corre-
sponding gain register.
9.1.2.2 DC Gain Calibration
With a DC reference applied, the DC Gain Calibration
command measures and averages DC values read on
the specified voltage or current channels and stores the
reciprocal result in the associated gain registers, con-
verting measured voltage into needed gain. Subse-
quent conversions will use the new gain value.
-
-
R eference
Signal
CM
+
+
IN+
IN-
+
-
XG AIN
+
-
9.1.3 Calibration Order
1. DC offset.
2. DC or AC gain.
3. AC offset (if needed).
If both AC gain and offset calibrations were performed,
Figure 12. System Calibration of Gain.
Using a reference that is too large or too small can
cause an over-range condition during calibration. Either
condition can set Status register bits I1OR (I2OR)
V1OR (V2OR) for DC and I1ROR (I2ROR) V1ROR
(V2ROR) for AC calibration.
Full scale (FS) for the voltage input is ±250 mV peak
and for the current inputs is ±250 mV or ±50 mV peak
depending on selected gain range. The normal peak
voltage applied to these pins should not exceed these
levels during calibration or normal operation.
The range of the gain registers limits the gain calibration
range and subsequently the range of the reference level
that can be applied. The reference should not exceed
FS or be lower than FS/4.
9.1.2.1 AC Gain Calibration
Full scale for AC RMS gain calibrations is 60% of the in-
put’s full-scale range, which is either 250 mV or 50 mV
depending on the gain range selected. That’s 150 mV or
30 mV, again depending on range. So the normal refer-
ence input level should be either 150 or 30 mV RMS , AC
or DC.
Prior to executing an AC Gain Calibration command,
gain registers for any channel to be calibrated should be
set to 1.0 if the reference level mentioned above is
used, or to that level divided by the actual reference lev-
el used.
DS714F3
it is possible to repeat both to obtain additional accuracy
as AC gain and offset may interact.
9.1.4 Temperature Sensor Calibration
Temperature sensor calibration involves the adjustment
of two parameters - ? V BE and V BE 0. These values must
be known in order to calibrate the temperature sensor.
22 for an explanation of ? V BE and V BE 0 and how to cal-
culate T GAIN and T OFF register values from them.
9.1.4.1 Temperature Offset Calibration
Offset calibration can be done at any temperature, but
should be done mid-scale if any gain error exists.
Subtract the measured T register temperature from the
actual temperature to determine the offset error. Multi-
ply this error by ? V BE and add it to V BE 0 to yield a new
V BE 0 value. Recalculate T OFF using this new value .
9.1.4.2 Temperature Gain Calibration
Two temperature points far enough apart to give rea-
sonable accuracy, for example 25°C and 85°C, are re-
quired to calibrate temperature gain.
Divide the actual temperature difference by the mea-
sured ( T register) difference for the two temperatures.
This gives a gain correction factor. Update the T GAIN
register by multiplying it’s value by this correction factor.
Update ? V BE by dividing its old value by the gain cor-
rection factor. It will be needed for subsequent offset
calibrations.
41
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