CS5467 datasheet(22/46 Pages) CIRRUS

22 / 46 page

CS5467

DS714F3

For each enabled input channel, the measured value is

rectified and compared to the associated level register.

Over the duration window, the number of samples

above and below the level are counted. If the number of

samples below the level exceeds the number of sam-

ples above, a Status register bit V1SAG (V2SAG),

I1FAULT (I2FAULT) is set, indicating a sag or fault condi-

tion. (see Figure 8)..

6.12 Epsilon

The Epsilon register is used to set the gain of the 90°

phase shift used in the quadrature power calculation.

The value in the Epsilon register is the ratio of the line

frequency to the output word rate (OWR). It is, by de-

fault, 50/4000 (0.0125), for 50 Hz line and 4000 Hz sam-

ple (OWR) frequencies.

For 60 Hz line frequency, it is 60/4000 (0.015). Other

output word rates (OWR) can be used.

Epsilon

can also be calculated automatically by the

CS5467 by setting the AFC bit in the Mode Control

(Modes) register. The Frequency Update bit (FUP) in

the Status register is set every time the Epsilon register

has been automatically updated.

6.13 Temperature Measurement

The on-chip temperature sensor is designed to mea-

sure temperature and optionally compensate for tem-

perature drift of the voltage reference. It uses the VBE of

a transistor to determine temperature.

In the CS5467, voltage2 and temperature are multi-

plexed on one ADC channel. To initiate a temperature

measurement, write 1 to the Temperature Measure-

ment (TMEAS) register. TMEAS will go through counts 1,

2, 4, and back to 0. Wait for TMEAS to return to 0. When

done, Temperature (T) is updated. The Status register

bit TUP also indicates when the Temperature register is

updated. The Voltage2 register (V2) will not update dur-

ing the temperature measurement, but resume mea-

surement afterwards.

Temperature measurements are stored in the Temper-

ature register (T) which, by default, is configured to a

range of ±128 degrees on the Celsius (°C) scale.

The application program can change both the scale and

range of Temperature (T) by changing the Temperature

Gain (TGAIN) and Temperature Offset (TOFF) registers.

Two values must be known — the transistor’s

VBE per

degree, and the transistor’s VBE at 0 degrees. At the

time of this publication, these values are:

VBE (per degree) = 0.2769523 mV/°C or °K

VBE0 = 79.2604368 mV at 0°C

To determine the values to write to TGAIN and TOFF, use

the following formulae:

TGAIN = ADFS / VBE / TFS x 2

TOFF = -VBE0 / ADFS x 2

In the above equations, ADFS is the full-scale input

range of the temperature A/D converter or 833.333 mV

and TFS is the desired full-scale range of the Tempera-

ture (T) register. The binary exponents are the bit posi-

tions of the binary point of these registers.

To use the Celsius scale (°C) and cover the chip’s oper-

ating temperature range of -40°C to +85°C, the Temper-

ature register range needs to be ±128 degrees. TFS

should be 128 degrees.

TGAIN = 833.333 / 0.2769523 / 128 x 131072

= 3081155 (0x2F03C3)

TOFF = -79.2604368 / 833.333 x 8388608

= -797862 (0xF3D35A)

These are the actual default values for these registers.

TGAIN and TOFF can also be used to calibrate the gain

and/or offset of the temperature sensor or A/D convert-

er. (See Section 9. System Calibration on page 40).

To use the Kelvin (°K) scale, simply add 273 times

VBE

/ ADFS x 2

23 to T

OFF since 0°C = 273°K,. You will also

need more range. Since -40°C to +85°C is 233°K to

358°K, a TFS of 512 degrees should be used in the

TGAIN calculation.

To use the Fahrenheit (°F) scale, multiply

VBE by 5/9

and add 32 times the new

VBE/ AD

FS x 2

23 to T

OFF

since 0°C = 32°F. You will also want to use a

T

FS of 256

degrees to cover the -40°C to +85°C range.

Figure 8. Sag and Fault Detect

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