AS8500 datasheet(12/39 Pages) AMSCO

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AS8500 - Preliminary Data Sheet

austriamicrosystems

Revision 1.1, 04-April-06

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Page 12 of 39

7.2.3

Internal temperature sensor

The ASIC 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 µC

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

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 ASIC 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 ASIC 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 ASIC 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

The absolute input-output calibration of the gain ranges can be done

that way that for a given input voltage 30 000 digits at the output are

produced:

Table 7.2.2

The TC-value of the output (total measurement path) for G24 can be

trimmed to a minimum value by selecting the best setting of the

TRIMBTC subregister of the TRR register (see 7.5.7).

A factory calibration is done for the amplifier offset (TRIMA).

This data is stored in the ZZR register. ZZR-register mapping is given

in 8.6.2

measurement of internal temperature sensor over oil bath temperature

15000

20000

25000

30000

35000

-50

-25

100

125

150

175

temperature in deg C

-100

-75

-50

-25

100

output signal

linearity deviation

cubic fit

gain

input/mV

output/digits

720

30 000

120

30 000

100

7.5

30 000

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