Cirrus Logic AN227 User Manual

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 be­ing calibrated. But the CS5460A’s output is always
affected by the values inside the various calibra­tion 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 calibra­tion, 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 cal­ibration is performed. Although the CS5460A will
always exhibit the linearity+variation tolerances
that are specified in Table 1, the exact reference

AN227

Before AC Gain Calibration (Vgain Register = 1)

250 mV

230 mV

DC Si gnal

INPUT

0V

SIGNAL

-250 mV

V

Register =

RMS

After AC Gain Calibration (Vgai n Regi ster changed to ~0.65217)

250 mV

230 mV

DC Si gnal

INPUT

0V

SIGNAL

-250 mV

V

RMS

230

/

=

250

Register = 0.6000...

0.92

Figure 2. Another Example of AC Gain Calibration

0.9999...

0.92

Instantaneous Voltage

Register Values

-1.0000...

0.65217

0.6000

Instantaneous Voltage

Register Values

-0.65217

Before DC Gain Calibration (Vgain Register = 1)

250 mV

230 mV

DC Si gnal

INPUT

0V

SIGNAL

-250 mV

V

Register =

RMS

After DC Gain Calibration (Vgain Register changed to 1.0870)

230 mV

DC Si gnal

INPUT

0V

SIGNAL

V

RMS

230

/

=

250

Register = 0.9999...

0.92

Figure 1. Example of DC Gain Calibration

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0.9999...

0.92

Instantaneous Voltage

Register Values

-1.0000...

voltage and current levels to which this linearity is
referenced will vary from sample to sample. If no
calibration is performed, these voltage/current ref­erence levels exist based on the full-scale DC in­put 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) abili­ties of the sample, with respect to a specific input

0.9999...

Instantaneous Voltage

Register Values

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 (as­sume 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 full­scale digital output code (0x7FFFFF in the Instan-

CopyrightCirrus Logic, Inc. 2003

(All Rights Reserved)

FEB ‘03

AN227REV1

1

Energy Vrms Irms

Range (% of FS)

Max. Differential

Input

Linearity

Table 1. Available range of ±0.1% output linearity, with

default settings in the gain/offset registers.

0.1% - 100% 50% - 100% 0.2% - 100%

not applicable

0.1% of
reading

V-channel:

±250 mV

0.1% of
reading

I-channel:
±250 mV 10x
±50 mV 50x

0.1% of

reading

taneous Current Register) will be obtained when­ever the input voltage across the IIN+ and IIN- pins
is 187.5 mV (DC). Note that this level is ~75% of
the (typical) maximum available input voltage
range [i.e, ~

±250 mV DC.] In this situation, the cur-

rent channel input ranges for which ±0.1% linearity
+ variation are guaranteed will be reduced to be­tween 0.5 mV (DC) and 187.5 mV (DC), as op­posed to what is specified in Table 1 [which would
translate into a voltage range between 0.5 mV
(DC) and 250 mV (DC)].

Also note that using gain calibration signal levels
which cause the CS5460A to set the internal gain
registers to a value that is less than unity will effec­tively decrease the guaranteed “±0.1% of reading”
linearity+variation range (and therefore the accura­cy range) of the RMS calculation results and the
overall energy results. This will occur whenever a
DC gain calibration is performed (on either chan­nel) of a CS5460A sample while applying a DC sig­nal whose value is less than the individual
sample’s inherent maximum differential DC input
voltage level. This will also occur whenever an AC
gain calibration is performed (on either channel)
using an AC signal whose RMS value is less then
60% of the sample’s inherent maximum AC input
voltage levels.

Finally, remember that the ±0.1% (of reading) ac­curacy guarantee is made with the assumption that
the device has been calibrated with MCLK =

4.096 MHz, K = 1, and N = 4000. If MCLK/K be­comes too small, or if N is set too low (or a combi­nation of both), then the CS5460A may not exhibit
±0.1% linearity + variation.

1.1 Order of Calibration Sequences

Should offset calibrations be performed before
gain calibrations? Or vice-versa? This section

AN227

summarizes the recommended order of calibra­tion.

1. If the user intends to measure any DC content
that may be present in the voltage/current and
power/energy signals, then the DC offset calibra­tion sequences should be run (for both channels)
before any other calibration sequences. However if
the user intends to remove the DC content present
in either the voltage or current signals (by turning
on the voltage channel HPF option and/or the cur­rent channel HPF option--in the Status Register)
then DC offset calibration does not need to be ex­ecuted for that channel. Note that if either the volt-
age HPF or current HPF options are turned on,
then any DC component that may be present in the
power/energy signals will be removed from the
CS5460A’s power/energy results.

2. If the user intends to set the energy registration
accuracy to within ±0.1% (with respect to reference
calibration levels on the voltage/current inputs)
then the user should next execute the gain calibra­tions for the voltage/current channels. The user
can execute either the AC or DC gain calibration
sequences (for each channel).

3. Finally, the user should (if desired) run the AC
offset calibration sequences for the voltage and
current channels. Simply ground the “+” and “-” in­puts of both channels and execute the AC offset
calibration sequence.

Note that technically, by following the order of cal­ibrations as suggested above, if DC offset calibra­tion is performed for a given channel, and
afterwards a gain calibration is performed on the
channel, then the DC offset register value for the
channel should be scaled by a factor equal to the
respective channel’s new gain register value. For
example, suppose that execution of DC offset cal­ibration for the voltage channel results in a value of
0x0001AC = 0.0000510(d) in the Voltage Channel
DC Offset Register (and we assume that the value
in the Voltage Channel Gain Register was at its de­fault value of 1.000... during execution of this DC
offset calibration). Then if AC or DC gain calibra­tion is executed for the voltage channel such that
the Voltage Channel Gain Register is changed to
0x4020A3(h) = 1.0019920(d), then the user may
want to modify the value in the Voltage Channel

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