AS8500 - Data Sheet

austriamicrosystems

Revision 1.2, 08-Junel-06

www.austriamicrosystems.com

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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 with

temperature. 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.

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 POR threshold 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)

For an clock frequency of 8.192 MHz it can vary between

16 000 Hz and 1.95 Hz.

In the dual mode (see 7.4 mode 2) the ASSP is switching automatically between the two channels and it needs at least one measurement for each

polarity to get a valid measurement. In addition the ASSP needs some time to reprogram the internal registers and switches. Therefore the maximum

sampling frequency is limited to 7.5 kHz for the above given clock frequency. The internal averaging is not working in the dual mode, but the sampling

frequency can be different for each channel.

7.3.2

Calibration

The calibration of the ASSP is done by a test setup as follows:

-

room temperature calibration of the internal temperature sensor

-

absolute input-output calibration for all gain settings

-

TC calibration for the measurement path for gain 24

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