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ADS114S06B: Single ended unipolar measurement howto

Part Number: ADS114S06B
Other Parts Discussed in Thread: ADS114S06

Hello,

I am not sure I understand the proper way to take single ended input measurements. Here is how I have tried:

  byte addr, data, aux;

  addr = CMD_WREG | REF_R;                
  aux = B00001010;                          // bypass both reference buffers; 2.5V internal ref; internal ref always on
  SPI.transfer16((uint16_t) (addr << 8 | aux));

  addr = CMD_WREG | INPMUX_R;                
  aux = B01011100;                          // set input P AIN5 and input N AINCOM
  SPI.transfer16((uint16_t) (addr << 8 | aux));

AINCOMM is not tied to anything (GND ?) so I figured that for my [ 0.5 ~ 4.5 ] V signal I could use the 2.5V bias:

  addr = CMD_WREG | VBIAS_R;                
  aux = B01000000;                          // V bias to AINCOM
  SPI.transfer16((uint16_t) (addr << 8 | aux));

This last setting  to VBIAS_Register made no difference though. I have stuck a wire on the AIN5 input and - 

Result:

yesterday night input was quite noisy around some fixed value, a fixed value that would depend on how the wire was bent, my finger on it, etc. Left it running overnight and now the read data only alternates between 32767 and 32768 no matter what. Where are the problems here ?

thank you

  • Hi Catalin,

    What is your voltage source? What voltage output is connected to AIN5? And how do you have your voltage source connected the ADS114S06B? One side, which I assume is '+' is connected to AINN, but where does the '-' connect? You cannot measure a floating input.

    Also, remember that the output code from the ADC is binary 2's complement and must be properly signed. When you see the codes shifting between 32767 and 32768 you are actually seeing the output shift between positive and negative full-scale.

    Best regards,
    Bob B
  • Hi,

    source voltage: Using AVDD = 5V, AVSS = 0V, both from the arduino

    AIN5 connected to: Tried with GND and 3.3 V of the same arduino

    how is the source connected: by a 0.33 uF capacitor as shown in figure 94 of "ADS114S0xB Low-Power, Low-Noise, Highly Integrated, 6- and 12-Channel, 4-kSPS, 16-Bit, Delta-Sigma ADC With PGA and Voltage Reference" document. I don't know what else to answer here.

    I am attempting a single ended measurement, so I have set up input AINp to AIN5 and input AINn to AINCOM and then applied V_bias on AINCOM as described above. Probably this is where I am doing wrong...

    Good to know about the sign!

    Thank you
  • Hi Catalin,

    What are you using as the reference source for the ADC? By default this is the REF0 inputs. If this is open, then you will get some odd values returned. I would suggest using the internal reference in the reference mux selection and turn on the internal reference. There should be a capacitor between REFOUT and REFCOM of 1uF or larger. Also, REFCOM should connect to AVSS (AGND). The reference settings are shown on page 59 of the ADS114S06 datasheet.

    Best regards,
    Bob B
  • The reference is set to internal as described above:

    aux = B00001010; // bypass both reference buffers; 2.5V internal ref; internal ref always on

    Anything wrong there ?

    There is a cap between REFOUT and REFCOM of 1uF just as figure 94 of "ADS114S0xB Low-Power, Low-Noise, Highly Integrated, 6- and 12-Channel, 4-kSPS, 16-Bit, Delta-Sigma ADC With PGA and Voltage Reference" document depicts. Same goes for REFCOM, connected to AVSS (AGND).

    Oh, and, not sure if there is much difference between the two, but my part is an ADS114S06B (i.e. ADS114S06BIPBSR). I have noticed there are different datasheets for this one and the one you have linked to. Anyway, I have already seen that register for reference settings and written to it as above.

    What else should I do ? Am I correctly using the AIN p/n of the PGA for single ended unipolar signal measurements ?

    Thank you
  • Hi Catalin,

    I just mistyped when I responded earlier as I was looking at the ADS114S06B datasheet. The devices are very similar, with some differences mainly in the electrical specifications.

    I was able to get the correct response with AVDD at 5V, AINP at +3.3V and AINCOM with VBIAS turned on. When applying a 0V input, the device will most likely see negative full-scale. It would be better to use REFOUT instead of VBIAS as this will allow for a more stable input and quieter result. But in the end VBIAS should work, and you should get a result corresponding to 3.3-2.5V.

    You might want to double check and measure the input voltages are correct relative to AGND by measuring with a voltmeter. You should be able to measure 2.5V at AINCOM, and 3.3V at AIN5. Also, read back the register settings following a write to make sure you have the correct settings applied.

    Best regards,
    Bob B
  • Of course... I was writing incorrectly to the device. I am getting old.... :|