AS8501 - Data Sheet

austriamicrosystems

Revision 1.1, 04-April-06

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Page 12 of 40

7.2.3

Internal temperature sensor

The ASSP contains a high sensitive precision temperature sensor which can be used at any time. The sensor supplies a very linear voltage signal with

an offset at 23 degC, which is calibrated and stored in the ZZR-register. The voltage can be measured using the internal circuitry with gain 6, with free

selection of all other parameters defining the sampling rate.

The slope of the curve is approx. 75 digits per degC.

The calculation of the temperature has to be done in the external µ C acc. to the following simple formula:

Tint=( Uint(T)-Uint(23) ) / 75 + 23°C

Uint(T) is the measured result and Uint(23) is the reference value at 23°C, which is stored as an 11 bit-word in the ZZR-register.

Bits 15, 14 ,13 and 12 will always be the same at room temperature (0101 bin or 20480 dec), therefore it makes no sense to store them for each single

part. In addition we dont need the high resolution of one digit, which means 1/71.3 = 14 milli Kelvin. Therefore we cut the last bit and achieve a word of

11 bit length, which finally is stored in the ZZR-register as shown in the ‚stored ZZR-register mapping‘ given in 8.6.2

Example:

value stored in the ZZR-register:

1060 dez or 10000100100 bin

Uint(23) = 0101 10000100100

0

= 22600 dec

Add register content Add

If your measured value is : Uint(T) = 23767 dec

Ti[°C]= ((23767-22600) / (75 digits / °C)) + 23 °C

= 15.6 °C + 23 °C = 38.6 °C

7.3

Digital part

In the digital part the result of the AD-converter is processed, i.e. calibration, active offset cancellation and filtering is done. In addition the

communication via the serial SDI interface is handled and all circuit functions (like voltage and current path settings, chopping, dechopping) are

controlled.

Whenever the power supply line returns from below 2.0 V to above 3.5 V a power-up circuitry is activated which loads the internal calibration registers

from the Zener-Zap memory into the working register and starts the chip in a special default mode.

7.3.1

Sampling rate

the sampling rate (SR) is defined by the setting of parameters in register CRA or CRB. The oversampling frequency (OSF), the oversampling ratio

(OSR), the chopping ratio (MM) and the averaging number (AV). The sampling rate can be calculated acc. to the following formula:

SR= OSF/(OSR*MM*AV)

Figure 7: Measurement of internal temperature sensor over oil bath temperature

15000

20000

25000

30000

35000

-50

-25

0

25

50

75

100

125

150

175

Temperature in deg C

-100

-75

-50

-25

0

25

50

75

100

output signal

linearity deviation

cubic fit