This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS1120 full 16 bit resolution in single ended mode?

Other Parts Discussed in Thread: REF2041, ADS1120, ADS1220, ADS1246

I have problems understanding differential ADC, and principle of its voltage references. Lets assume that ADS1120 REFP is connected to 4.096V and REFN is connected to GND (0V). What is the voltage that defines code area where conversion result is 0x0000? Is it the one connected to the REFN? When measuring unipolar voltage, it will never go below 0V, and thus we will loose one bit of resolution. Would it be possible to use dual output reference like REF2041 and to connect 4.096V output to REFP and 2.048 output to REFN, and thus achieve 16bit resolution in single ended mode?

  • Hi Milan,

    The reference for the ADS1120 has a differential input, but the reference does not work in the way you have mentioned.  It is the analog input itself that determines the output code.  The input measures differentially with the positive codes resulting from AINP being a greater voltage than the AINN voltage.  If AINN is greater than AINP then the output codes are negative.  You are correct that you will lose one bit if you make single-ended measurement where AINN is fixed to AGND.

    You have some options.  One is to use the ADS1220 which gives you an overall increase in resolution.  A second option would be to set AINN to a voltage in the middle between the sensor full-scale and AGND.  For example, if the AVDD is 5V, you can set the reference voltage and AINN to 2.5V. In this case all codes will be positive for voltages at AINP above 2.5V, and negative codes for all voltages below 2.5V.  2.5V will result in 0 code.  This also assumes the PGA is bypassed, and the gain is set to 1.

    Best regards,

    Bob B

  • Thank you for your answer Bob.


    I have just two more follow up question about method you mentioned later in the text - connect RefVoltage to AINN and REFP, bypass PGA and set gain to 1. Is this method limited to AVDD=2xRefVoltage, or can I for example use 2.048V reference connected to AINN and REFP, with AVDD being 5V? Is this applicable to ADS1220 and ADS1246/7/8?


    Thanks in advance,

    Milan Laketic

  • Hi Milan,

    Your described method is basically a pseudo-differential measurement.  The full-scale range is (+/-VREF)/PGA for the devices mentioned or 2*VREF if the gain is set to 1.  The absolute input is set by AVDD, so you cannot apply a voltage to any input that is greater than AVDD+0.3V. You can certainly apply less voltage until AVSS-0.3V.  So you can apply any voltage between those two points.  The reference voltage determines when the output code is full-scale, either positive or negative.  If you apply 2.048V to AINN, and 4.095V to AINP you will see an output code close to positive full-scale. 4.096 to 5V applied to AINP you will see full-scale.  Going in the opposite direction if AINP is 0V, you will see negative full-scale.  This assumes the ADS1120 or ADS1220 when the PGA is bypassed.

    The consideration for the ADS124x devices is with respect to any common-mode restriction.  You can avoid the common-mode restriction by bypassing the PGA on the ADS1120, and ADS1220.  However, the ADS124x devices do not have a PGA bypass option and are required to observe the restriction.  This means you cannot connect either AINN or AINP to the less than AVSS+0.1V or greater than AVDD-0.1V without a measurement error.  With the given scenario for the ADS124x devices with AVDD at 5V, you can set AINN at 2.048V and measure the full positive range, but not the negative range because you cannot set the input to 0.1V or below on AINP without seeing an error.  If you apply a slightly negative bias (-0.1V or lower) to the AVSS, then you can measure to 0V as long as the maximum restriction of 5.5V is not exceeded for AVDD-AVSS.

    Best regards,

    Bob B

  • Bob, thank you again for the thorough clarification.

    Is this restriction on ADS124X parts prevents accurate measurement just below AVSS+0.1V or entire negative code range below (for example) 2.048V is with errors?

    ADS124X would be used for multi channel 4-20mA sensor acquisition, using low side 100 Ohm shunt and high common mode op-amp in differential configuration with x2 amplification. In normal operating conditions voltage on input wouldn't go bellow ~800mV, so if only problem is the region below AVSS+0.1V, not entire negative code range, this restriction won't be much of the problem.

    Thanks again,
    Milan Laketic
  • Hi Milan,

    For the ADS124x devices you can accurately measure all negative codes up until the common-mode restriction is reached.  Given the scenario you discussed this method should work well for you.

    Best regards,

    Bob B