### Q

We have a data logger which measure temperature from one RTD in 4 wireconfiguration.

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.

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