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DDC316: Full Scalre Range and Bit Resolution - Calibration

Bernhard Gaus
Prodigy 50 points
Part Number: DDC316

On page 12 the data sheet of the DDC316 states:

Table 1:

Range3 -> Input Range = –0.1876pC to 12pC

Table 2:

–1.7857% FS = 0x0000 with –1.7857% * 12pC = -0.214284pC

How should I understand the difference of the two numbers –0.1876pC != -0.214284pC for the same input value?

Is there a hidden recommendation to always calibrate the DDC316 channels to get a precise current measurement?

Regards,

Bernhard

  • 0
    TI__Expert 4590 points

    Hi Bernhard,

    There is certainly something strange on the documentation... I got my theory but running it by other folks. 

    On your second question, no, there is no hidden recommendation... The precision of the measurement (once the above is cleared out) depends on the specs, like offset, gain error, their drifts... So, you can get as good as the spec says or further do calibration to get to the precision you need. For instance, offset calibration is super simple. Gain is a bit harder... etc...

    I'll be back,
    Edu

  • 0 in reply to Eduardo Bartolome
    TI__Expert 4590 points

    Hi again,

    Discussed with another colleague (none of us worked on this project) and we think that there are some mistakes on those tables but can't point to the source.

    The closest theory to what is right is the following:

    • I think you can trust that ideal zero input current is at 0b0000010010010010 (1170).
    • In a sense, the -1.7853% is correct when computed as 1170/65535. But the table says FS which makes it confusing (more below). 
    • Basically as the negative swing is at "-1170", that would correspond to 1170/(65535-1170)*12pC = 0.2181pC.
    • There is no way to straighten/interpret the first column on Table 2 without some modification...
      • One could think that FS in that table would be 12.2181. Notice that  -1.7853% * FS = 0.2181pC. But the wording wouldn't make sense in other rows (like 100% FS would then be 12.2181pC, not 12pC).
      • I think the author was trying to say 100% FS = 12pC, 0% FS = 0pC. But when he/she took the other percents (for the other rows), went for a percent of the 65535.

    • The table 1 value -0.1876pC would therefore be also wrong. And strange thing is that it is no typo as it actually scales perfectly with the other ranges on that table. So, whoever typed that had some thought but can’t tell what.

    Bottom line, the negative swing should be theoretically 0.2181pC. Nevertheless, as mentioned in the previous post, if you care about absolute accuracy you probably need no matter what a calibration to get there (for instance, gain error is +/-20% in 12pC range).

    Best regards,
    Eduardo