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ADS130B04-Q1: Data Conversion Issue

Peter Blank
Prodigy 20 points
Part Number: ADS130B04-Q1

Tool/software:

Hi all,

I have a question about the data conversion for ADS130B04.

I used all 4 channels as single-ended channels, with negative inputs connected to ground.

Communication and setup are working fine, also the voltage values seem to be correct, but on channel 1 and 3, the sign is swapped.

I have on all channels an input voltage of around 0.6V, but channel 1 and 3 say -0.6V.

In the datasheet it says that the data is represented as two's complement, so my conversion is:

int16_t data = (int16_t)(((uint16_t)msb << 8) | lsb);

float volts = ((float)data * 1.2f) / 32768.0f;

MSB and LSB are the first and second byte read by NULL command from each data channel.

And the 1.2 / 32768 come from the reduces scale range.

What do I do wrong here? Any suggestions? 

Thanks in advance,

Peter 

  • 0
    TI__Mastermind 41230 points

    Hello Peter,

    If I understand how you have the inputs connected, you have AIN0N, AIN1N, AIN2N, and AIN3N connected to GND (GND=DGND=AGND).  You are then trying to connect a negative voltage with respect to GND on the AIN1P and AIN3P inputs.

    The inputs of the ADS131B04 are limited (relative to GND) to no less than -0.3V for gains greater than 2, and -0.1V for gains of 1 and 2.  Any negative voltage lower than this is an invalid input.

    In order to force a negative differential input voltage, for example on channel 1, connect AIN1P to GND and AIN1N to +0.6V.  These will be valid input voltages to the ADC, and the ADC will interpret this as a negative differential input voltage of -0.6V.

    V-IN = V-AINxP - V-AINxN = 0V - 0.6V = -0.6V.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • +1 in reply to Keith Nicholas
    TI__Mastermind 22355 points

    Hello Peter,

    you might want to double check your schematic. The order of the positive and negative inputs on the package changes between channels. That can be easily mixed up.

    Regards,
    Joachim Wuerker

  • 0 in reply to Keith Nicholas
    Prodigy 20 points

    Hi Keith,

    no, I dont want to measure negative voltages, I read out negative voltages from the registers, but the reply below from Joachim solved the issue..

    Thanks anyways!

  • 0 in reply to Peter Blank
    Prodigy 20 points

    Hi Joachim, 

    indeed, this is the problem, our PCB designer mixed up the inputs.

    So, now we connected, e.g. AIN3N with 0.6V and AIN3P with GND. The absolute values stay the same, is it fine to just swap the sign, or is this electrically an issue?

    Thanks again,

    Peter 

  • 0 in reply to Peter Blank
    TI__Mastermind 22355 points

    Hallo Peter,

    glad we figured that out so quickly.

    There is no issue at all. You only need to swap the sign of the conversion result and you are good to go.

    Schönes Wochenende,
    Joachim Wuerker