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ADS1158: Bad Internal Voltage Readings

Part Number: ADS1158

The Ext Ref voltage reading from the ADC is 4.20 (0x326c), on another board the data is 4.11V (0x314f), while on a 3rd board it varies from 4.07V (0x30e6) to 4.08V (0x30e8).   In all cases the the voltage measured across VREFP and VREFN is precisely 4.096V.      

Since the external channel readings are off even more, we suspect an error with the design that could be causing the internal and external readings to be faulty:   

Reading back the config registers, they are all the default power-on values except for:   CONFIG0 and CONFIG1 which we've set to 0x12  and 0x03.   En/disabling the BYPASS (CONFIG0.4) also affects the readings greatly.

What could be causing this issue?

  • Blipton,


    Just to be clear, are you taking the output of the 4.096-V reference and using that as the input to the ADC mux for a measurement?

    If this is the case, I think the problem is with the buffer. The VREFN is used as the negative AIN signal, which will appear at MUXOUTN. This voltage will be -2.5V and will go into the buffer created by U12A. The output of this buffer however will not get to -2.5V and will likely be 10-20mV above the negative supply.

    Op-amps won't drive all the way to the rail and you will likely get to about -2.49V. I would expect that you'd measure something like 4.08V to 4.09V. However, I'd also consider the matching for R73 and R74, which may also add some error. If you're using 1% resistors, this is another concern.

    Regardless, want to consider the voltage from ADCINN to -2.5V. You should also be able to measure the voltage between ADCINP and ADCINN directly.


    Joseph Wu
  • Thanks, the voltage reference I was referring to is the one that can be obtained by setting bit 5 of the SYSRED: SYSTEM READING SELECT REGISTER (Address = 06h).

    Upon closer inspection of the datasheet's Electrical Characteristics, there doesn't seem to be a problem... my readings of 4.10-4.20V  is within the maximum 3% tolerance listed on the table.