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db05175e_V3.01_preliminary_2017-07-06

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AS89000

V3.01

The input of the reference channel is connected with an external resistor that will load with an input voltage

which is different to VREF.

The output voltage of the measuring channel is explained in the coming formula:

(1)

 

 

 

T

RTIA

IIN

T

VREF

T

VTIA







IIN is the input current, which is supplied by the external sensor. The output voltage of the channel for the

temperature compensation is defined:

(2)

 

 

 

 

 

 

T

RTK

T

REXT

T

VREF

T

VIN

T

VREF

T

VTK









The following voltages will calculate for temperature compensation with a resistor.

(3)

 

 

 

T

VTIA

T

VREF

T

VTIA







(4)

 

 

 

T

VTK

T

VREF

T

VTK







For example the voltage ∆VTK(T0) will detect during the initialization of the system. The value is equivalent

to a constant for the temperature T0, which prevailed at the time of initialization. All further measurements

will calibrate by this value.

(5)

 

 

 

 

T

VTK

T

VTK

T

VTIA

T

iert

VTIAkorrig













0

All variables of the channel for temperature compensation are affected by temperature effects. Therefore

there is an additional coefficient necessary. That coefficient should be highly reduced opposite to the named

above value of the RTIA (typical -3300 ppm/K).

(6)

)

(

)

(

)

(

VREF

TK

VREF

VIN

VREF

VIN

TK

VREF

VIN

VIN

REXT

TK

TK















„TK(REXT)“ is the temperature coefficient of the external resistor, „TK(VIN)“ is the temperature coefficient

of the input voltage and „TK(VREF)“ is the temperature coefficient of the reference voltage.

Please consider the following interrelationship by the choice of resistors REXT and RTK in term of the

selected voltages VIN and VREF (values from (2) and (4)).

(7)

1





VREF

VIN

RTK

REXT

The adherence of this non-equation ensures, that the voltage VTK is located in the working range. That

means the amplifier of the channel for temperature compensation doesn’t go into saturation.

Furthermore you can calculate the absolute value of the transimpedance resistor RTK for a certain actual

existing temperature.

(8)

 

 

 

 

 

 

T

VREF

T

VIN

T

VTK

T

VREF

T

REXT

T

RTK









6.3 Output Signals VOUT

AS89000 works by the principle of a connected op-amp:

VOUT = VREF – IIn * R {limited by GROUND …VREF}

IIN = 0



VOUT = VREF

IIN = max.  VOUT = 0