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.

ADS8860: ADS8860: ADS8860 / ADS8861 Minimum reading

Part Number: ADS8860
Other Parts Discussed in Thread: REF6250, LM4120, REF6050, ADS8861, OPA376, THS4551, LM7705, OPA320


I wonder if anyone can help me. I am using the ADS8860 ADC and some of the boards I have had manufacturered are exhibiting a minimum reading that I can't seem to get rid of.

I have some boards that when presented with GND as the input read 0, but others that read 30-40. I have tried adding additional decoupling to the reference and the AVDD and DVDD lines but nothing seems to make any difference. The board has a switch to select between a 5V LM4120 and REF6250 reference, but adding decoupling after the switch makes no difference to the digital reading output by the ADC.

Any thoughts would be much appreciated.


  • Hi Chris,

    Looking at the electrical characteristics table of the ADS8860 (datasheet p3), the expected offset error of the ADS8860 (single-ended) device is typical ±1mV and ±4mV maximum. Since you are using the REF6250/LM4120 (5V reference), the LSB weight is 76.3µV. The offset of the ADS8860 is ±13 LSBs typical, ±52 LSBs maximum. Also, any offset of the driving circuit could contribute on the overall offset.  

    Note, the offsets of the driving circuitry and ADS8860 could be positive or negative, so in order to measure the offset on the ADS8860 single-ended device, you would need to bias the input at a voltage a few 100 millivolts higher than GND. Also, if the driver amplifier/circuit uses unipolar supply, the output swing of the amplifier may be limited to within a few 10’s of millivolts to GND.

    -       Can you please provide a schematic showing the ADS8860, REF6050/LM4120 circuit, driver amplifier and RC kickback filter, supply connections and bypass capacitors?

    -       What operational amplifier and RC kickback filter is used to drive the ADS8860?

    -       What is the maximum sampling rate required in the application?

    The title of the post mentions the ADS8861 (fully-differential version) as well. Are you also using the ADS8861? If you are , please provide a schematic of this circuit as well.

    Thank you and Kind Regards,


  • Hi Luis,

    Many thanks for the detailed reply. I had forgotton about the offset voltage in the ADCs, doh! The ADC schematic is:

    The LM4120 schematic is:

    and the ref6250 schematic is:

    The driving op-amp is the OPA376 in non-inverting configuration.

    For the differential version the circuits are identical, really identical except AIN_N goes to the diff signal N instead of ground and the driving amplifier is the THS4551.


  • Sorry, the images didn't work, here they are:

  • Hi Chris,

    The output swing of the OPA376 amplifier is limited to a typical of 10-20mV  from GND (rail supply); therefore you will not be able to reliably measure signals at GND. The great majority of amplifiers with unipolar supplies will have the output swing limitation to the rail supply (GND).  Please see OPA376 datasheet spec below.

    A possible solution is to use the LM7705 Low-Noise Negative Bias generator (LM7705).  This device produces a regulated output voltage of -0.232.  You can use the device to generate the (V-) supply of the OPA376, and in this manner, the output swing of the amplifier will not be limited to GND.

    Please note, the OPA376 is a GBW 5.5MHz BW amplifier, and may not be able to completely charge the internal sample-and-hold  of the ADS8860 and settle within the acquisition period when operating the ADS8860 SAR ADC at full- throughput of 1-MSPS.  The circuit may only settle at slower sampling rates.  

    A possible suggestion is to use the OPA320 (GBW=20MHz) if you require to use the ADC at full throughput of 1-MSPS.  

    For a detailed tutorial on how to design/select the amplifier, and RC kickback filter driving the SAR ADC, please check the TI  Precision LAB series:

    Thank you,

    Best Regards,