ADS1247 Hardware-Compensated, Three-Wire RTD Measurement Example

Hello Saretta,

On the datasheet example of the Hardware-Compensated, Three-Wire RTD Measurement provided on the datasheet; the ADS1247 device will see a differential voltage across its inputs that may be positive or negative depending on temperature.

The PT-100 RTD on the example is about 110 Ohms at 25 C; as the temperature changes, the RTD impedance will change from approximately 119.6Ohms at 50C to 100Ohms at 0 degrees C. The excitation current connected through each input AIN+ and AIN- is 1.5mA.  Connected to AIN- is the hardware compensation resistor  (RCOMP)  110 Ohms. 

Since the RTD is approximately impedance (110Ohm) matches RCOMP at 25C; the ADC will see approximately 0mV differential voltage at 25C.  Since the RTD is approximately 119Ohm at 50C; the impedance difference between the RTD and RCOMP (~9.5Ohm) multiplied by the excitation current (1.5mA) will produce a positive differential voltage (~+14mV).  The ADC will see a negative differential voltage when the temperature is at 0C (~-14.7mV).

Provided that both inputs are in the allowed full-scale differential and common-mode range of the device; the ADS1247 can convert positive or negative differential voltages.

Attached is a link to the ADS1248/7 application note measuring RTD's and a link to a design note discussing common-mode and differential input range for the ADS1248 families.

Best Regards,

Luis

http://www.ti.com/lit/an/sbaa180/sbaa180.pdf

http://e2e.ti.com/support/data_converters/precision_data_converters/w/design_notes/input-voltage-range-requirements-for-the-ads1248-and-ads1148-families.aspx

Hi Saretta,

Yes, if one of the inputs is required to have an absolute negative voltage;  the device must be set up with bipolar supplies +2.5V/-2.5V.   The input common mode voltage range is limited by this equation:

AVSS + 0.1 + [(PGAGAIN)(VINdifferential)/2]  <   VCM   <   AVDD - 0.1 - [(PGAGAIN)(VINdifferential)/2]   

The common mode is the average voltage of the inputs: VCM = (AINP+AINN)/2  and the differential input voltage is the difference of the positive and negative input voltage:

  VINdifferential =  AINP – AINN   

This means that  both the positive input AINP and negative input AINN absolute voltage must be inside  the range of the analog supplies (AVDD, AVSS) and at least 0.1V away from the supplies (or more than 0.1V) depending on the PGA setting and the maximum differential voltage expected.   Ideally, the differential voltage must be centered between the supplies. As long as the signal is in the allowed  common-mode range and differential range limits; the device will work as expected. The link to the input voltage range requirements design note gives some examples that may help.  The application note on RTD's SBAA180 explains how the RBIAS resistor connected to AVSS and VREFP positions the RTD differential signal optimally in the middle of the supplies.

Regards,

Luis 

http://e2e.ti.com/support/data_converters/precision_data_converters/w/design_notes/input-voltage-range-requirements-for-the-ads1248-and-ads1148-families.aspx

http://www.ti.com/lit/an/sbaa180/sbaa180.pdf