AD7793: Digital reading

Q 

We have a data logger which measure temperature from one RTD  in 4 wire
configuration.
As AD converter we use AD7793.
pin nb.4 from AD7793 (IOUT1) is connected to the RTD and function as excitation
current. Output from RTD is connected directly to AIN1 channel of the AD7793.
AD7793 is configured :
Configuration register :
Bias generator disabled,BornOut current disabled,Unipolar Mode, No Boost,Gain
= 16, Internal reference enabled.Channel bits to AIN1.
IO register : IEXC1 connected to pin IOUT1, Courent Source Value = 210microA.
Mode Register:
Single conversion mode,Internal clock not available to CLK pin,Update rate
fADC=50 Hz.

At startup of the system we perform 1 internal zero scale calibration and 1
internal full scale calibration.

Our Problem :
After we start conversion, we read from AD7793 the following value (in
hexadecimal): 0x0652F0.
Using formula Ain =( code*1170)/2^24 we obtain AIN =28,90254 mV.
This is the equivalent of 137,631
Ohm (AIN [mv] / 0.21 [mA]).
In reality we should read around 110 Ohm respectively 0X050753 as code(if our
calculus were correct).
What could be the problem?

In the formula from Datasheet code=2^N * AIN / Vref  is no reference to the
gain value .
If we consider VIN the "real input" is AIN from your formula AIN = VIN * gain?

 

A 

The correct formula is shown on page 24 of the rev A datasheet.

Code = (2^N × AIN × GAIN)/VREF for unipolar operation

Or AIN = (VREF/GAIN) * (CODE /2^N)

So the digital reading of 0x0652F0 with a gain of 16 and a  Vref of 1170mV
corresponds to an input voltage of 1.8mV.

To calculate the resistance based on Ohms law and the current source is not the
correct way to do this. The current source has a tolerance of +/-5% which is
like a 4 or 5 bit converter!

Figure 21 shows the correct way to do RTD measurements. Disable the internal
reference and use refin+ and refin- pins to generate a ratiometric reference
based on the current source.

In this way, errors in the excitation current sources cancel out and your
reading is simply the ratio of the unknown impedance (RTD) (multiplied by the
INAMP Gain) to the known impedance (RREF). This is a 3-wire ratiometric
measurement.