CS5467 datasheet(41/46 Pages) CIRRUS

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CS5467

DS714F1

9.1.2 Gain Calibration

During gain calibration, a full-scale reference signal

must be applied to the meter or optionally, scaled to the

VIN1

± (VIN2±), IIN1± (IIN2±) pins of the CS5467. A DC

reference must be used for DC gain calibration. Either

an AC or DC reference can be used for RMS AC calibra-

tions. If DC is used, the associated high-pass filter

(HPF) must be off.

Figure 12 shows the basic setup for gain calibration.

Using a reference that is too large or too small can

cause an over-range condition during calibration. Either

condition can set Status register bits I1OR (I2OR)

V1OR (V2OR) for DC and I1ROR (I2ROR) V1ROR

(V2ROR) for AC calibration.

Full scale (FS) for the voltage input is ±250 mV peak

and for the current inputs is ±250 mV or ±50 mV peak

depending on selected gain range. The normal peak

voltage applied to these pins should not exceed these

levels during calibration or normal operation.

The range of the gain registers limits the gain calibration

range and subsequently the range of the reference level

that can be applied. The reference should not exceed

FS or be lower than FS/4.

9.1.2.1 AC Gain Calibration

Full scale for AC RMS gain calibrations is 60% of the in-

put’s full-scale range, which is either 250 mV or 50 mV

depending on the gain range selected. That’s 150 mV or

30 mV, again depending on range. So the normal refer-

ence input level should be either 150 or 30 mVRMS, AC

or DC.

Prior to executing an AC Gain Calibration command,

gain registers for any channel to be calibrated should be

set to 1.0 if the reference level mentioned above is

used, or to that level divided by the actual reference lev-

el used.

During AC gain calibration the RMS level of the applied

reference is measured with the preset gain, then divided

into 0.6 and the quotient stored back into the corre-

sponding gain register.

9.1.2.2 DC Gain Calibration

With a DC reference applied, the DC Gain Calibration

command measures and averages DC values read on

the specified voltage or current channels and stores the

reciprocal result in the associated gain registers, con-

verting measured voltage into needed gain. Subse-

quent conversions will use the new gain value.

9.1.3 Calibration Order

1. DC offset.

2. DC or AC gain.

3. AC offset (if needed).

If both AC gain and offset calibrations were performed,

it is possible to repeat both to obtain additional accuracy

as AC gain and offset may interact.

9.1.4 Temperature Sensor Calibration

Temperature sensor calibration involves the adjustment

of two parameters -

∆VBE and VBE0. These values must

be known in order to calibrate the temperature sensor.

See Section 6.13 Temperature Measurement on page

22 for an explanation of

∆VBE and VBE0 and how to cal-

culate TGAIN and TOFF register values from them.

9.1.4.1 Temperature Offset Calibration

Offset calibration can be done at any temperature, but

should be done mid-scale if any gain error exists.

Subtract the measured T register temperature from the

actual temperature to determine the offset error. Multi-

ply this error by

∆VBE and add it to VBE0 to yield a new

VBE0 value. Recalculate TOFF using this new value.

9.1.4.2 Temperature Gain Calibration

Two temperature points far enough apart to give rea-

sonable accuracy, for example 25°C and 85°C, are re-

quired to calibrate temperature gain.

Divide the actual temperature difference by the mea-

sured (T register) difference for the two temperatures.

This gives a gain correction factor. Update the TGAIN

Update

∆VBE by dividing its old value by the gain cor-

rection factor. It will be needed for subsequent offset

calibrations.

External

Connections

IN +

IN -

XG AIN

Reference

Signal

Figure 12. System Calibration of Gain.

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