CS5464
40 DS682F3
9.1.2 Gain Calibration
During gain calibration, a full-scale reference signal
must be applied to the meter or optionally, scaled to the
VIN
,IIN1 (IIN2pins of the CS5464. A DC reference
must be used for DC gain calibration. Either an AC or
DC reference can be used for RMS AC calibrations. If
DC is used, the associated high-pass filter (HPF) must
be off.
Figure 12 shows the basic setup for gain calibration.
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 ±250mV peak and
for the current inputs is ±250mV or ±50mV peak de-
pending on selected gain range. The normal peak volt-
age 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 250mV or 50mV
depending on the gain range selected. That’s 150mV or
30mV, 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.
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.
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,
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 -
VBE and VBE0. These values must
be known in order to calibrate the temperature sensor.
See Section 6.13
Temperature Measurement on page
21 for an explanation of
VBE and VBE0 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
VBE and add it to VBE0 to yield a new
V
BE0 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
VBE by dividing its old value by the gain cor-
rection factor. It will be needed for subsequent offset
calibrations.
+
-
+
-
External
Connections
IN+
IN-
CM
+
-
+
-
XGAIN
Reference
Signal
Figure 12. System Calibration of Gain.
