AD7949 temperature measurement

I've designed in a couple of AD7949 ADCs. In a typical 20-25C environment my boards give values that translate to temperatures in the range of 5C. The fact that they are all low makes me wonder if my calculation is wrong, but it looks fine. The parts are being used with internal 2.5V reference, b[5:3]=000.

Since there are no min/max limits for temperature measurement in the datasheet, I was wondering if this kind of offset is allowable. If so, then it would be appropriate for the datasheet to at least warn the user that calibration is required in order to get a usable temperature measurement.

George

  • 0
    •  Analog Employees 
    on Jul 14, 2012 12:59 AM

    George,

    For applications requiring the use of the temperature sensor, the internal reference must be active (internal buffer can be disabled in this case). So, the temperature sensor is simply enabled by setting the CFG register bits INCC [12:10] = ‘011’. Enabling the internal reference (as you have done) also enables the internal temperature sensor, which measures the internal temperature of the AD7949.

    Note that the AD7949 temp sensor conversions are forced to be single-ended conversions referenced to GND (unipolar) and are therefore straight binary and this temp sensor output is typical (283 mV @ 25 C) and it’s NOT a guaranteed spec. So, using the 2.5V internal reference, the temp sensor output is simply the (2.5V/65535)*output code that you read from the ADC and it changes as the temp increases of decreases.  I wouldn’t recommend  to use the internal temp sensor if you require guaranteed tighter accuracy spec. We don’t calibrate this internally due to offset and gain trim issues.

     

    As per the attached AD7949 characterization data, the ball-park range for the temperature sensor’s typical output voltage (283mV) is around +/-4% for the VDD = 5V.

    Regards,
    Maithil

  • Your quick response and data are much appreciated. A +/-4% range on the nominal 25C output works out to +/- 11 C, so I might have to calibrate or do something else for temperature in this specific application. I do like the part and am getting good performance on the muxed inputs.

  • I would like to try to make a temperature measurement on the AD7949 when configured with external REF.  I have configured the following -

    CFG register bits

    INCC = Temperature sensor. [12:10] = ‘011’.

    REF = External reference, temperature enabled. [5:3] = '010'

     

    My external REF pin is 3.0 V with VDD = 3.3V, and REFIN is no connected.

     

      My temperature values calculated at 274 mV - assuming 3.0V is the reference for the temperature channel.   The confusing part is that voltage values fall as the part is heated.   That doesn't seem correct based on the graph in the previous message.

     

    Q: Is it possible to measure temperature when configured with external REF as in my configuration?

     

      Thanks

  • Hi Maithil,

    Please let me know the formulae for calculating Internal temperature sensor value from AD7949 chp

    and also the temperature sensor maximum and minimum output voltage level or plot..

    I am using external 5V reference voltage.

    Voltage= (5000* adc_reading)/ 2^14
    Temperature= (Voltage * 25)/ 0.283      Celcius

    Thanks in Advance

    Tamilarasan.

  • 0
    •  Analog Employees 
    on Jul 20, 2015 7:25 PM

    Hello Tamilarasan,

    The AD7949 temperature sensor voltage can be calculated as: Temp Sense Voltage = Temp Sense Output Code×(Reference Voltage) / (2^14-1).

    We don't guarantee this spec due to offset and gain trim issues, so you can use a typical plot available at the top of this post.

    Regards,

    Maithil