CS5461A datasheet(35/44 Pages) CIRRUS

35 / 44 page

CS5461A

DS661F2

7. SYSTEM CALIBRATION

7.1 Channel Offset and Gain Calibration

The CS5461A provides digital DC offset and gain com-

pensation that can be applied to the instantaneous volt-

age

and current

measurements, and AC offset

compensation to the voltage and current RMS calcula-

tions.

Since the voltage and current channels have indepen-

dent offset and gain registers, system offset and/or

gain can be performed on either channel without the

calibration results from one channel affecting the oth-

er.

The computational flow of the calibration sequences are

illustrated in Figure 9. The flow applies to both the volt-

age channel and current channel.

7.1.1 Calibration Sequence

The CS5461A must be operating in its active state and

ready to accept valid commands. Refer to Section 5.14

Commands on page 23. The calibration algorithms are

dependent on the value N in the Cycle Count Register

(see Figure 9). Upon completion, the results of the cali-

bration are available in their corresponding register. The

DRDY bit in the Status Register will be set. If the DRDY

bit is to be output on the INT pin, then DRDY bit in the

Mask Register must be set. The initial values stored in

the AC gain and offset registers do affect the calibration

results.

7.1.1.1 Duration of Calibration Sequence

The value of the Cycle Count Register (N) determines

the number of conversions performed by the CS5461A

during a given calibration sequence. For DC offset and

gain calibrations, the calibration sequence takes at least

N + 30 conversion cycles to complete. For AC offset

calibrations, the sequence takes at least 6N + 30 ADC

cycles to complete, (about 6 computation cycles). As N

is increased, the accuracy of calibration results will in-

crease.

7.1.2 Offset Calibration Sequence

For DC- and AC offset calibrations, the VIN

± pins of the

voltage and IIN

± pins of the current channels should be

connected to their ground-reference level.

See Figure 10.

The AC offset registers must be set to the default

(0x000000).

7.1.2.1 DC Offset Calibration Sequence

Channel gain should be set to 1.0 when performing DC

offset calibration. Initiate a DC offset calibration. The DC

offset registers are updated with the negative of the av-

erage of the instantaneous samples taken over a com-

putational cycle. Upon completion of the DC offset

calibration the DC offset is stored in the corresponding

DC offset register. The DC offset value will be added to

each instantaneous measurement to cancel out the DC

Figure 9. Calibration Data Flow

Modulator

to V*, I* Registers

Filter

RMS*, IRMS*

Registers

DC Offset*

Gain*

0.6

* Denotes readable/writable register

√

Inverse

-1

RMS

AC Offset*

ΣN

-1

XGAIN

External

Connections

AIN+

AIN-

CM +

Figure 10. System Calibration of Offset.

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