CS5484 datasheet(63/67 Pages) CIRRUS

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CS5484

DS981F3

7.1.1.2 AC Offset Calibration

The AC offset calibration command measures the

residual RMS values 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.

The AC offset register for the channel being calibrated

should first be cleared prior to performing the

calibration. The high-pass filter should be enabled if AC

offset calibration is used. It is recommended that

TSETTLE be set to 2000 ms before performing an AC

offset calibration. Note that the AC offset register holds

the square of RMS value measured during calibration.

Therefore, it can hold a maximum RMS noise

. This is the maximum RMS noise that AC

offset correction can remove.

7.1.2 Gain Calibration

Prior to executing the gain calibration command, gain

registers for any path to be calibrated (VxGAIN, IxGAIN)

should be set to 1.0, and TSETTLE should be set to

2000 ms. For gain calibration, a reference signal must

be applied to the meter. During gain calibration, the

voltage RMS result register (VxRMS) is divided into 0.6,

and the current RMS result register (IxRMS) is divided

into the Scale register. The quotient is put into the

associated gain register. The gain calibration algorithm

attempts to adjust the gain register (VxGAIN, IxGAIN)

such that the voltage RMS result register (VxRMS)

equals 0.6, and the current RMS result register (IxRMS)

equals the Scale register.

Note that for the gain calibration, there are limitations on

choosing the reference level and the Scale register

value. Using a reference or a scale that is too large or

too small can cause register overflow during calibration

or later during normal operation. Either condition can set

Status register bits I1OR (I2OR) V1OR (V2OR). The

maximum value that the gain register can attain is four.

Using inappropriate reference levels or scale values

may also cause the CS5484 to attempt to set the gain

result will be invalid.

The Scale register is 0.6 by default. The maximum

voltage (UMAX Volts) and current (IMAX Amps) of the

meter should be used as the reference signal level if the

Scale register is 0.6. After gain calibration, 0.6 of the

VxRMS (IxRMS) register represents UMAX Volts (IMAX

Amps) for the line voltage (load current); 0.36 of the

PxAVG, QxAVG, or Sx register represents UMAX×IMAX

Watts, Vars, or VAs for the active, reactive, or apparent

power.

If the calibration is performed with UMAX Volts and ICAL

Amps and ICAL<IMAX, the Scale register needs to be

scaled down to 0.6×ICAL/IMAX before performing gain

calibration. After gain calibration, 0.6 of the VxRMS

IxRMS

represents

ICAL

Amps,

and

0.36 x ICAL /IMAX of the PxAVG, QxAVG, or Sx register

represents UMAXxICAL Watts, Vars, or VAs.

7.1.3 Calibration Order

1. If the HPF option is enabled, then any DC

component that may be present in the selected

signal channel will be removed, and a DC offset

calibration is not required. However, if the HPF

option is disabled, the DC offset calibration should

be performed.

When using high-pass filters, it is recommended that

the DC offset register for the corresponding channel

be set to zero. Before performing DC offset calibra-

tion, the DC offset register should be set to 0, and the

corresponding gain register should be set to 1.

2. If there is an AC offset in the IxRMS calculation, the

AC offset calibration should be performed on the

current channel. Before performing AC offset

calibration, the AC offset register should be set to 0.

It is recommended that TSETTLE be set to 2000ms

before performing an AC offset calibration.

3. Perform the gain calibration.

4. If an AC offset calibration was performed (step 2),

then the AC offset may need to be adjusted to

compensate for the change in gain (step 3). This can

be accomplished by restoring zero to the AC offset

The adjustment could also be done by multiplying

the AC offset register value that was calculated in

step 2 by the gain calculated in step 3 and updating

the AC offset register with the product.

7.2 Phase Compensation

A phase compensation mechanism is provided to adjust

for meter-to-meter variation in signal path delays. Phase

offset between a voltage channel and its corresponding

current channel can be calculated by using the power

factor (PF1, PF2) register after a conversion.

1) Apply a reference voltage and current with a lagging

power factor to the meter. The reference current

waveform should lag the voltage with a 60° phase

shift.

2) Start continuous conversion.

0xFFFFFF

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