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FDC1004: Gain drift and offset drift vs temperature

Nam, Dino
Expert 6445 points
Part Number: FDC1004

Dear team, 

There is an issue related temperature. The measured capacitance increases as device temp increases. Is it related gain drift and offset drift? Or Is the other factor related temp? Please review it and below question and let me know your opinion.

1. Gain Drift

  • We can see below data from datasheet. But I don’t understand the meaning of PPM unit. The -37.5ppm/C seems to mean that the gain is decreased by the slope of 37.5. For example, the gain drift at room temp(25C) is 0ppm, but 5000ppm at -125C. Is it mean the gain at -125C is reduced by 0.5%(5000PPM) than gain at room temp(25C). is it right? Please let me know your opinion.

             

2. Gain 

  • Gain equation is Gain = GAIN_CAL[15:0]/2^14
  • [15:14] 2MSB is integer portion of the Gain. 00 mean gain is 0.xx, 01 mean gain is 1.xx, 10 mean gain is 2.xx, 11 mean gain is 3.xx. Is it right?

         

3. Offset

  • Offset deviation over temperature for CAPACITIVE INPUT is typ 46fF from datasheet like below. Would you please let me know max deviation value

4. Offset drift vs temperature

  • Is Figure 5. Offset Drift vs. Temperature graph for CAPACITIVE INPUT or CAPDAC?
  • The Min/Max value is about over 60fF(Min below 40fF, Max over 20fF)

Thank you. 

  • 0
    TI__Mastermind 21060 points

    Thank you for reaching out with your questions.

    1. The typical spec for the gain drift of -37.5 ppm/C is reflective of the overall trend across the device operating temperature range.  For instance, we see about -5000 ppm variation over the range from -23 C to 110 C.  The plot you have cited gives more detail regarding the typical behavior and shows that the slope may deviate slightly from this -37.5 typical performance.  The trend is only characterized and published for the operating range of the device.  The datasheet for FDC1004 specifies an operating range from -40 to +125 C.  What should be observed is that for temperatures below 25C, there will be an increase in gain due to a positive gain drift, while for temperatures above 25C there will be a negative gain shift due to negative gain drift.

    The minimum storage temperature for this device is -65C.  The device behavior is unknown and not guaranteed at -125C.  

    2.Yes.  The GAIN_CAL[15:14] bits represent the integer portion of this gain setting meaning that the gain can be set from 0.xx - 3.xx

    3. Similar to the Gain drift the offset drift plot will provide the most detail regarding the anticipated drift of the offset parameter.  I do not find any detail beyond what is published in the spec table and the offset drift plot to provide more guidance regarding the absolute maximum drift that may be experienced here, however.

    4.This plot specifies that the offset is taken where CIN is open, so this should be representative of the input capacitance.  This deviation specification of 46 fF appears to be an absolute value of the maximum drift observed with respect to the offset at 25C. 46 fF is therefore representative of the expected worst case offset drift at any temperature compared to the nominal value, which in this case occurs at 120C.  We see that from -40C to 75 C that the offset deviation is <20fF.

    thanks,

    Scott

  • 0 in reply to Scott Bryson
    TI__Expert 6445 points

    Hello Scott, 

    Thank you for your comment. 

    I am sorry for confusing you regarding Q1. 

    My question is 125C not -125C. Is it mean the gain at 125C is reduced by 0.5%(5000PPM) than gain at room temp(25C). 

    For clarity, -5000 ppm variation over the range from -23 C to 110 C you mentioned mean -0.5% variation. Is it right?

    Additional question.

    Is there any factor cause temp related issue except above gain and offset drift?

    Thank you.

  • +1 in reply to Nam, Dino
    TI__Mastermind 21060 points

    Dino,

    Yes, your understanding is correct.  -5000 ppm = -0.005000 = -0.5%.  The plot shows that the 0 ppm change occurs at room temperature.  At 125C we expect -0.5% shift in gain from room temp.  At 110C we expect a -0.5% shift from -23C.

    CAPDAC has it's own offset drift specification similar to the Input capacitance.  Standby and Active currents are also impacted by temperature.

    Thanks,

    Scott