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ADS8028 reading error

Shlomo Perlshtein
Intellectual 290 points
Other Parts Discussed in Thread: ADS8028

I’m using the ADS 8028 to read several signals in my circuit. One input (AIN6) is from a voltage divide, which accepted its voltage from a DAC.

The reading of that input is offsets by ~70mV , by unknown reason. I have to emphasize, that I measured ( with DVM) the voltage in the ADC input, and compared its value to the read value by the ADC. All the other inputs is correct. There is one input which tied to GROUND, and the reading of the ADC is also correct(<1mV).

Following a graph of the measured input versus the reading values:(The Y axis is millivolts)


The ADC operated from external reference, which is its the supply voltage- 3.3Volts

This is the ADC circuit:P2.PDF

 

The source for the AIN6 input( through R15) :P1.PDF

 

Do you have any idea ?

 

Thanks

 

 

Shlomo Perlshtein

Board Designer

Elbit Systems Ltd.


  • 0
    TI__Mastermind 37955 points

    Hello Shlomo,

    Thanks to share the details and let us know your problem, but I can't see your attached files, can you check and upload them again? thanks.

    Best regards

    Dale Li

  • 0 in reply to Dale Li
    Intellectual 290 points

    Hello Dale.

    Thanks for your response.

    I will try to put a picture here.

    I don't know how to upload a file in this site. I can send it to your mail.

    Thanks

    Shlomo

  • 0 in reply to Dale Li
    Intellectual 290 points

    Hello Dale,

     

     

    Thanks for you answer.

     

     

    I tried to put the circuit as a picture and send to you, but seem the link is in difficulties..

     

     

    Anyway, I can send the file to your mail - if you send it to me. Or- if you explain how I can upload files to the forum.

     

     

     

     

     

    Thank

     

     

    Shlomo

  • 0 in reply to Shlomo Perlshtein
    TI__Guru* 93625 points

    Shlomo,

    When you hit the "Reply" button, it will pull up a dialog window to type. At the lower right corner, there will be a "Use rich formatting" button that will pull up some additional options at the top of the dialog window.

    There is a picture icon as an insert media button that can be used to insert pictures or files and a paper clip icon that can insert files to be appended to the post.

    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 290 points

    ADC_Circuit.pdfThanks Joseph for you explanation- it helped/

    Attached the circuit file schematics.

  • 0 in reply to Shlomo Perlshtein
    TI__Mastermind 37955 points

    Hi Shlomo,

    Thanks for your schematics, I apologize I missed your previous response. I'm trying to understand the scematics and your test points because it's not a whole schematics, you connected the input of AIN6(VCSEL_IN_MON) to one end of R15, is that Ground? did you measure at the positions of two red arrows you marked in the schematics? that made me confused. I want to understand your connections and test well, that will be helpful to check, also can do the simulation if necessary. Thanks.

    Best regards

    Dale

  • 0 in reply to Dale Li
    Intellectual 290 points

    Hello Dale,

    Hence the circuit divided between several pages, I cut the separate parts and joined it on one page.

    The AI6 input of the ADC is connected through the 10k R15 resistor. I connected it with a red lines.

    the two red arrows indicating that the measure d voltage, which is the voltage on the junction of R102-r112-U27 , is going to AIN6 of the ADC through R15, as I explained previously.

    By the way, The Rout of the DAC(U28) is less than 1 ohms.

    Attached a simplified schematics of the whole relevant circuit.

    Thank

     

    Shlomo

  • 0 in reply to Shlomo Perlshtein
    TI__Mastermind 37955 points

    Hi Shlomo,

    Thanks for your clarification.It's supposed that you diconnected ADC from front-end DAC(LTC2636) and amplifier(Max9619) for the test you did, if not, please disconnect them with R15, also have some suggestions:

    1. The best solution is using a amplifier buffer to isolate the ADC's front-end with others, that's the interaction connections in three parts of the circuit, Figure 42 in ADS8028 datasheet is a very good application circuit for Buffer with Voltage Scaling.

    2. One high quality(NP0/C0G) capacitor is required before ADC's input as a charge bucket and also low pass filter with resistor together.

    Thanks&Best regards

    Dale

  • 0 in reply to Dale Li
    Intellectual 290 points

    Hi Dale,

    Thanks a lot for your suggestions. I will check again with according that.

    Anyway, I assumed that the input impedance of the ADC is high, am I wrong? From the datasheet is not so clear that you HAVE to use a buffer.

     

    Shlomo

  • 0 in reply to Shlomo Perlshtein
    TI__Mastermind 37955 points
    Hi Shlomo,
    The input impedance of thie switched-capcaitor input ADC depends on the internal input capacitance and the sampling rate. The higher throughout, the lower input impedance.

    Regards
    Dale