CS5490 datasheet(51/56 Pages) APEX

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CS5490

DS982F2

7. SYSTEM CALIBRATION

Component tolerances, residual ADC offset, and

system noise require a meter that needs to be calibrated

before it meets a specific accuracy requirement. The

CS5490 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 CS5490, a CS5490 meter normally only

needs one calibration at a single load point to achieve

accurate measurements over the full load range.

7.1 Calibration in General

The CS5490 provides DC offset and gain calibration

that can be applied to the instantaneous voltage and

current measurements and AC offset calibration, which

can only 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 19.

Note that in Figure 19 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 early in the signal

path; the DC offset register and gain register values

affect all CS5490 results. This is not true for the AC

offset correction. The AC offset registers only affect the

results of the RMS current calculation.

The CS5490 must be operating in its active state and

ready to accept valid commands. Refer to section 6.1.2

Instructions on page 24 for different calibration

commands. The value in the SampleCount register

determines the number (N) of output word rate (OWR)

samples that are averaged during a 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 the 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; the differential signal on

voltage inputs VIN± or current inputs IIN± of the CS5490

should be 0 volts.

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 AC Offset Calibration

The AC offset calibration applies only to the current

channel. It measures the residual RMS values on the

current channel at zero input and stores the squared

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 19. Calibration Data Flow

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