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ADS114S06: Resolution

Gerald
Genius 9570 points
Part Number: ADS114S06
Other Parts Discussed in Thread: ADS114S08, ADS124S06

Hi Experts,

Our customer is developing a product using the 16bit ADC, ADS114S06. He is powering the ADC with a 3.3V unipolar supply. It is configured for single-ended input i.e. AIN1 and AINCOM. The input is always positive.
Can you confirm if can they get 16bit resolution for a positive input only? Ideally, they require an ADC code from 0 to 65535 to give them the 16bit resolution for our positive input.

Kind regards,

Gerald

  • +1
    TI__Guru** 113765 points

    Hi Gerald,

    The ADS114S06 always measures differentially.  That is to say AINP relative to AINN.  If AINP is greater than AINN then the measurement range is positive and the output codes range from 0x0000 to 0x7FFF.  If AINP is less than AINN, the output codes range from 0xFFFF to 0x8000.  When AINCOM (which is actually the same as any other input channel) is connected to AGND and the supply is unipolar the measurement range is limited to 15 bits as only positive codes are the result as AINP cannot go below AGND.

    To utilize the full-scale range (all 16 bits) a pseudo-differential measurement can be made by placing the AINN input at something other than AGND.  In this way the measurement can utilize the full-scale range.  The positive range is the AINN input voltage plus the output code conversion result.  And when negative codes are given then the result is the AINN input voltage minus the conversion result.  However consider that nothing is gained from this effort if the reference voltage remains the same.  For the pseudo-differential measurement to be of value the reference input and AINN input should be 1.65V for a 3.3V measurement range.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Genius 9570 points

    Hi Bob,

    Thank you for your detailed response. Customer is now looking at different approaches to fix their problem. If they use the internal 2.5V reference, then is it possible to make a voltage measurement between pins REFP1 and REFN1 using an ADS114S06 ADC?
    Looking at the diagram on page 28 of the data sheet (ads114s06.pdf), it appears that these pins are routed into the input Mux. Note that these two pins are analogous to pins AIN6 and AIN7 if they were using the ADS114S08 ADC.

    We hope to hear your advise. Thank you so much.

    Kind regards,

    Gerald

  • 0 in reply to Gerald
    TI__Guru** 113765 points

    Hi Gerald,

    The main differences between the S06 and S08 is the number of analog input pins available.  For the S08 some of the input pins have dual function capability whereas the same pins have single functions for the S06.  So the answer is no to the ADS114S06 being able to measure a voltage on the pins designated as REFP1 and REFN1 as they are reference only input pins.

    The simplest solution is to use the ADS124S06 which is the 24-bit variant.  This would be a drop-in replacement and would allow 16+ bits in single-ended operation.

    To help in coming up with a good solution, can I get a copy of the schematic and also know the input range and type of sensor measurement being taken?  What is the reference voltage being used?  Is it 3.3V?

    Are they absolutely certain that all 16-bits are required?  If they are using a 3.3V reference, any external noise will drop them to 15-bits anyway if the noise is 100uV or more.  There is often confusion on the number of bits and the actual measurement resolution and what this really means.  In other words, just because an ADC is capable of conversion to 16 bits doesn't necessarily mean that the outcome of the measurement can achieve all 16 bits.   The actual measurement is relative to the reference and a poor reference can add significant noise. 

    The actual value of 1 bit of resolution directly relates to the reference voltage applied.  The value of 1 code is equal to +/-  (or 2 times the value of) VREF/(GAIN*(2^16 - 1).  If the input range can run the supply, then the value of 1 code in the customer use case is 3.3V / 2^15 (with gain of 1 and single-ended) and is 100uV.  The datasheet uses 2.5V for resolution of achieving the noise tables as given in the datasheet changing the value of 1 code to 76uV.  So it is really important that when determining the number of bits, that the correct values are used to determine what that really means.  Personally I prefer using a voltage value of resolution as opposed to number of bits.

    Best regards,

    Bob B