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AMC3330: Maximum ADC resolution

Theo Keinarth
Prodigy 250 points
Part Number: AMC3330
Other Parts Discussed in Thread: ADS127L18, , ADS127L11, ADS127L21, ADS124S06, ADS122U04, ISOW7821, TPS7A20

Tool/software:

Hi TI-Team

We would like to use the AMC3330(-Q1) to isolate our ten +/-20mA current loops from each other. As ADC we are planning to use 2 of the ADS127L18 to also filter 50/60Hz noise. I have a eval board of both devices and connected them to make some tests and I'm not so sure about the results. What would be the maximum resolution I could achieve with the AMC3330 in combination with the ADS127L18 with a 2-Point calibration without temperature? Temp-Range is 0-60°C.

Thank you in advance!

Regards Theo

  • 0
    TI__Guru 53992 points

    Hi Theo,

    I would expect the resolution to be limited by the noise of the AMC3330-Q1 as the ADS127L18 is a very precise data converter. 

    In terms of accuracy, this excel calculator can be used for the AMC3330-Q1: https://www.ti.com/lit/zip/sbar013

    This tool can be used to estimate the accuracy of the ADS127L11, likely the closest device to the L18: https://dev.ti.com/gallery/view/PADC/PADC_Design_Calculator_Tool/ver/1.20.0/

    I'm sure the precision data converter team plans to update this calculator once the device is released and the specifications are finalized. 

  • 0 in reply to Alexander Smith
    Prodigy 250 points

    Hi Alexander

    I did some new testing with the ADS127L18 and it seems activating the diagnostics (CRCs...) with the EVM tool did not work and was the main issue. After turning them off I got noise free data to about 18bit. I think the 50/60Hz sinc3 filter makes it possible to get rid of most of the noise from the AMC3330. With the excel document I get a typical error of 0,03% an a max error of 0,16% for the AMC3330 as a current sens amplifier. The gain drift over temperature seems to be the main problem here. Is there another more precise solution to get an anlog value over an isolation? We need at least 0,1% (it can be unidirectional). Or do i need to isolate the whole ADC to achieve such a high precision? I taught of a solution with a ADS127L21, but it would be much more expensive and would need a lot more space!

    Thank you in advance!

    Regards Theo

  • 0 in reply to Theo Keinarth
    TI__Mastermind 43640 points

    Hello Theo,

    In order to significantly improve accuracy, I would suggest using a single channel ADC for each 4-20mA input, and isolate using digital isolators.  You will then be able to get the accuracy of the ADC without additional signal chain errors.

    The ADS122U04 has very good DC specs:  gain drift of 2ppm/C maximum and offset drift of 0.6uV/C maximum.  Operating at 20sps, you will get both 50Hz and 60Hz noise rejection, and nearly 18b of noise free resolution.  If you need lower noise, there are other single channel ADC options that can be used, such as ADS124S06, for a slightly higher cost that can get closer to 20b noise free resolution.

    Pairing up an ISOW7821 dual channel isolator with integrated power will provide a similar solution size and will be very close to the same cost for 8 channels (8x ADS122U04 and 8x ISOW7821) verses 1x ADS127L18 and 8x AMC3330.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 250 points

    Hi Keith

    This ADC (ADS122U04) seems perfect since a 0.01% overall accuracy was our goal at the beginning.

    I finished my design. could you give a short feedback if the circuit is ok?! (Please ignore the 200 Ohm shunt. We still need to figure out what we will use instead).

    Thank you all for your help.

    Regards Theo

  • +1 in reply to Theo Keinarth
    TI__Mastermind 43640 points

    Hello Theo,

    I reviewed your schematic; all device pinouts are correct.  I only have a few comments.

    1.  For the LDO's, I suggest using the TPS7A20.  These are very low cost LDOs and provide very high PSRR which will clean up any switching noise generated by the ISOW device.

    2.  Since you are not using the GPIO pins 1 and 2 on the ADS122U04, these will always default as inputs.  In this case, you should connect these pins to either DVDD or DGND and not leave floating.

    Regards,
    Keith