CS5480 datasheet(63/70 Pages) CIRRUS

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CS5480

DS980F3

7. SYSTEM CALIBRATION

Component tolerances, residual ADC offset, and

system noise require a meter to be calibrated before it

meets a specific accuracy requirement. The CS5480

provides an on-chip calibration algorithm to operate the

system calibration quickly and easily. Benefiting from

the excellent linearity and low noise level of the

CS5480, normally a CS5480 meter only needs one

calibration at a single load point to achieve accurate

measurements over the full load range.

7.1 Calibration in General

The CS5480 provides DC offset and gain calibration

that can be applied to the instantaneous voltage and

current measurements and AC offset calibration, which

can be applied to the current RMS calculation.

Since the voltage and current channels have

independent offset and gain registers, offset and gain

calibration can be performed on any channel

independently.

The data flow of the calibration is shown in Figure 25.

Note that in Figure 25 the AC offset registers and gain

registers affect the output results differently than the DC

offset registers. The DC offset and gain values are

applied to the voltage/current signals very early in the

signal path; the DC offset register and gain register

values affect all CS5480 results. This is not true for the

AC offset correction. The AC offset registers only affect

the results of the RMS current calculation.

The CS5480 must be operating in its active state and

ready to accept valid commands. Refer to section 6.1.2

Instructions on page 29 for different calibration

commands. The value in the SampleCount register

determines the number (N) of OWR samples that are

averaged

during

calibration.

The

calibration

procedure takes the time of N +TSETTLE OWR samples.

As N is increased, the calibration takes more time but

the accuracy of calibration results tends to increase.

The DRDY bit in the Status0 register will be set at the

completion of calibration commands. If an overflow

occurs during calibration, other Status0 bits may be set

as well.

7.1.1 Offset Calibration

During offset calibrations, no line voltage or current

should be applied to the meter. In other words, the

differential signal on voltage inputs VIN± or current

inputs IIN1± (IIN2±) of the CS5480 should be 0V.

7.1.1.1 DC Offset Calibration

The DC offset calibration command measures and

averages DC values read on specified voltage or

current channels at zero input and stores the inverse

result in the associated offset registers. This DC offset

will be added to instantaneous measurements in

subsequent conversions, removing the offset.

The gain register for the channel being calibrated

should be set to 1.0 prior to performing DC offset

calibration.

DC offset calibration is not required if the high-pass filter

is enabled on that channel because the DC component

will be removed by the high-pass filter.

7.1.1.2 Current Channel AC Offset Calibration

The AC offset calibration command measures the

residual RMS value on the current channel at zero input

and stores the squared result in the associated AC

offset register. This AC offset will be subtracted from

RMS measurements in subsequent conversions,

removing the AC offset on the associated current

channel.

VRMS*, IRMS*

Registers

Modulator

Filter



* Denotes readable/writable register

Ϯ Applies only to the current path (I1, I2)

N

N

-1

N

RMS

-1

RMS

0.6(Scale

*Ϯ)

*, I*, P*, Q*

Registers

IGAIN*, VGAIN*

Registers

IDCOFF*, VDCOFF*

Registers

IACOFF*Ϯ Register

Figure 25. Calibration Data Flow

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