AS8500 - Preliminary Data Sheet

austriamicrosystems

Revision 1.1, 04-April-06

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The TC trimming also opens the unique possibility to change the TC-value within the time of reprogramming of the TRR-register (i.e. within µsec) to

allow the compensation of

different TC-values of the external circuitry for different channels.

In addition it can be used for very fast autocalibration of the total TC of a given channel. An external reference voltage is applied to the channel to be

checked. Then all numbers from 0 to 31 are written into subregister TRIMBTC and a reading is done for the input voltage and the internal temperature

as well. The same is repeated at any temperature above RT. From these data the TRIMBTC setting for a minimum drift can be easily calculated.

7.2.2

Current sources

The AS8500 contains several current sources which can be used for checking all input lines for wire brake, to control external circuitry or to activate

external sensors.

Main current source

The main current source can be digitally controlled via the content of the CRG register in 31 steps of 8 µA in the range of 0 to 248 µV. Its absolute

value can be calibrated by writing in the subregister TRIMC of TRR.

The current source can be switched to the inputs VBAT, ETR or ETS to activate external sensors like RTDs, NTCs or resistance briges and strain

gages. It can also be used to detect a wire breake of external connected sensors. Performing a measurement with a high and a low (or zero) current

opens the possiblity to eliminate thermal EMF voltages in external sensors.

Secondary current sources

The ASIC contains two other high quality current sources supplying a current of approx. 2µA at the inputs RSHL and RSHH. These current sources can

be switched on and off at any time to check the correct connection of both terminals. During off state they must not interfere with the high sensitive

voltage inputs, especially the noise level should not be increased. If one of the terminals is an open connection the amplifier goes into saturation and

the overflow bit is set.

temperature coefficient as function of TRIMBTC setting

-250

-200

-150

-100

-50

0

50

100

150

200

0

5

10

15

20

25

30

35

setting of subregister TRIMBTC of TRR

change 12.7 ppm/K

per step