Cirrus Logic AN227 - Manual

1
Copyright  Cirrus Logic, Inc. 2003
(All Rights Reserved)
www.cirrus.com
AN227
Application Note
CALIBRATING THE CS5460A
1. Is Calibration Required?
The CS5460A does not have to be calibrated. After
CS5460A is powered on and then reset, the device
is functional. This is called the active state. Upon
receiving
a ‘Start Conversions’ command,
CS5460A can perform measurements without be-
ing calibrated. But the CS5460A’s output is always
affected by the values inside the various calibra-
tion registers. If no calibrations are executed by the
user, then these registers will contain the default
values
(Gains = 1.0, DC Offsets = 0.0,
AC Offsets = 0). Although the CS5460A can be
used without performing an offset or gain calibra-
tion, the guaranteed ranges for accuracy of ±0.1%
of reading (with respect to a known voltage and
current level) will not be valid until a gain/offset cal-
ibration is performed. Although the CS5460A will
always exhibit the linearity+variation tolerances
that are specified in Table 1, the exact reference
voltage and current levels to which this linearity is
referenced will vary from sample to sample. If no
calibration is performed, these voltage/current ref-
erence levels exist based on the full-scale DC in-
put voltage limits for each channel, which are
approximately equal to the voltages specified in
the “Max Input” row of Table 1. But these voltages
will have a variation from part to part. Any given
CS5460A sample must be calibrated to insure the
guaranteed accuracy = (linearity+variation) abili-
ties of the sample, with respect to a specific input
voltage signal levels at the voltage/current channel
inputs.
As an example, suppose the user runs the DC gain
calibration sequence on the current channel (as-
sume PGA gain set for “10x”) using a calibration
signal level across the IIN+/IIN- pins of 187.5 mV
(DC). After this calibration is performed, the full-
scale digital output code (0x7FFFFF in the Instan-
V
RMS
Register = 230 /
250
= 0.92
250 mV
230 mV
0 V
-250 mV
0.9999...
0.92
-1.0000...
V
RMS
Register = 0.9999...
230 mV
0 V
0.9999...
Before DC Gain Calibration (Vgain Register = 1)
After DC Gain Calibration (Vgain Register changed to 1.0870)
Instantaneous Voltage
Register Values
Instantaneous Voltage
Register Values
DC Signal
DC Signal
INPUT
SIGNAL
INPUT
SIGNAL
Figure 1. Example of DC Gain Calibration
V
RMS
Register = 230 /
250
= 0.92
250 mV
230 mV
0 V
-250 mV
0.9999...
0.92
-1.0000...
V
RMS
Register = 0.6000...
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 ~0.65217)
Instantaneous Voltage
Register Values
Instantaneous Voltage
Register Values
DC Signal
DC Signal
0.65217
-0.65217
INPUT
SIGNAL
INPUT
SIGNAL
Figure 2. Another Example of AC Gain Calibration
FEB ‘03
AN227REV1