Cirrus Logic CS5461A - Manual (Page 36)

CS5461A
36 DS661F3
component present in the system during conversion
commands.
7.1.2.2 AC Offset Calibration Sequence
Corresponding offset registers I
ACoff
and/or V
ACoff
should be cleared prior to initiating AC offset calibra-
tions. Initiate an AC offset calibration. The AC offset reg-
isters are updated with an offset value that reflects the
RMS output level. Upon completion of the AC offset cal-
ibration the AC offset is stored in the corresponding AC
offset register. The AC offset register value is subtract-
ed from each successive V
RMS
and I
RMS
calculation.
7.1.3 Gain Calibration Sequence
When performing gain calibrations, a reference signal
should be applied to the VIN
 pins of the voltage and
IIN
 pins of the current channels that represents the de-
sired maximum signal level. Figure 11 shows the basic
setup for gain calibration.
For gain calibrations, there is an absolute limit on the
RMS voltage levels that are selected for the gain-cali-
bration input signals. The maximum value that the gain
registers can attain is 4. Therefore, if the signal level of
the applied input is low enough that it causes the
CS5461A to attempt to set either gain register higher
than 4, the gain calibration result will be invalid and all
CS5461A results obtained while performing measure-
ments will be invalid.
If the channel gain registers are initially set to a gain oth-
er then 1.0, AC gain calibration should be used.
7.1.3.1 AC Gain Calibration Sequence
The corresponding gain register should be set to 1.0,
unless a different initial gain value is desired. Initiate an
AC gain calibration. The AC gain calibration algorithm
computes the RMS value of the reference signal applied
to the channel inputs. The RMS register value is then di-
vided into 0.6 and the quotient is stored in the corre-
sponding gain register. Each instantaneous
measurement will be multiplied by its corresponding AC
gain value.
A typical rms calibration value which allows for reason-
able over-range margin would be 0.6 or 60% of the volt-
age and current channel’s maximum input voltage level.
Two examples of AC gain calibration and the updated
digital output codes of the channel’s instantaneous data
registers are shown in Figures 12 and 13. Figure 13
shows that a positive (or negative), DC-level signal can
be used even though an AC gain calibration is being ex-
+
-
+
-
External
Connections
IN +
IN -
CM
+
-
+
- XG AIN
R eference
S ignal
Figure 11. System Calibration of Gain
V
RMS
Register = 230 /

x 1 /
250
0.65054
250 mV
230 mV
0 V
-230 mV
-250 mV
0.9999...
0.92
-0.92
-1.0000...
V
RMS
Register = 0.600000
250 mV
230 mV
0 V
-230 mV
-250 mV
0.84853
-0.84853
Before AC Gain Calibration (Vgn Register = 1)
After AC Gain Calibration (Vgn Register changed to approx. 0.9223)
Instantaneous Voltage
Register Values
Instantaneous Voltage
Register Values
Sinewave
Sinewave
0.92231
-0.92231
INPUT
SIGNAL
INPUT
SIGNAL
Figure 12. Example of AC Gain Calibration
V
RMS
Register = 230 = 0.92
250 mV
230 mV
0 V
-250 mV
0.9999...
0.92
-1.0000...
V
RMS
Register = 0.600000
250 mV
230 mV
0 V
-250 mV
0.6000
Before AC Gain Calibration (Vgain Register = 1)
After AC Gain Calibration (Vgain Register changed to approx. 0.65217)
Instantaneous Voltage
Register Values
Instantaneous Voltage
Register Values
DC Signal
DC Signal
0.65217
-0.65217
INPUT
SIGNAL
INPUT
SIGNAL
250
Figure 13. Another Example of AC Gain Calibration