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ADS8688 Floating channel problem

Other Parts Discussed in Thread: ADS8688

Hi,

       We are working with ADS8688 and facing a problem with floating inputs.

Problem:

          When no inputs/connections is given to the ADC channels(Floating) a potential of around 2.3V is appearing at +ve pin of ADC channel. This voltage is  getting added to the final reading , not necessary as a constant. How do we keep the channels at 0 V in case of disconnected terminals. Below is the circuit diagram. Any inputs are appreciated.

Regards,

Varaprasad.

  • Hello Varaprasad,

    The ADS8688 device incorporates an analog front-end circuit consisting of a Programmable Gain Amplifier (PGA) as shown below. If the inputs to the ADS8688 are kept floating, the output of ~2.2V is expected because the front-end signal conditioning on each analog input channel is biased internally to the appropriate common-mode voltage level for proper operation of internal ADC (refer to section 8.3.4 Programmable Gain Amplifier  on Page 24 of datasheet).  Since this device supports bipolar input signals using a single 5V supply, the internal biasing cannot be done to provide a zero offset at ADC output when inputs are left floating.

    Best Regards,

    Luis

  • Hi,

        Thanks for the input Mr.Luis. We understood the problem. We interfaced the ADC channel to a voltage divider network, but we don't see this ~2.3V adding linearly  to the measured output. So, how do we offset this voltage.

    Regards,

    Varaprasad.

  • Hello Varaprasad,

    The inputs of the ADS8688 input stage consists of the PGA with 1 MOhm input resistors shown as on Fig 51 of the datasheet.  One of the advantages of this input stage is that it offers a 1-MOhm constant resistive input impedance.  This makes the inputs relatively easy to drive (in most cases) without additional driver amplifiers or buffers. Since the internal PGA inputs (after the internal 1MOhm series input resistor), are biased at the required common-mode voltage level, the AINnP terminal has to be driven to zero volts in order for the ADC to produce the zero conversion result; and the device can not generate zero ADC conversion result when the input is left floating.

    One option would be to add circuitry to detect is the sensor/source is connected, and run a diagnostic routine periodically.  Another possible option is to place a pull-down resistor to drive the input to zero volts when the source is disconnected; however, the caveat in this case is that the effective input impedance is no longer 1 MOhm and the sensor/circuit driving the ADS8688 needs to drive this external pull-down resistor.  This option only works if the driving source/circuit has low output impedance and can drive the pull-down resistor without introducing (gain) errors. 

    What kind of driving source/sensor or circuit is connected at the input of the ADS8688?  Please provide the details and/or schematic of your application.  Please also let us know the requirements of the application:  throughput,  number of channels used, DC accuracy, voltage range requirements, etc.

    Best Regards,

    Luis

      

      

  • Hi Luis,

    We have added a voltage follower circuit before ADC pins and it worked. Thank you for the inputs.

    Regards,
    Varaprasad.
  • Hello everyone,

    can i left aux in and aux GND pins floating or connected to ground if i don't use them? In the datasheet is no information about that.

    Best regards,
    Sebastian

  • Hello Sebastian,
    Please connect AUX_GND pin to AGND. AUX_IN has absolute maximum rating as -0.3V to AVDD +0.3V.
    If you do not plan to use this pin, you can pull it down to GND.
    Thanks & Regards,Shridhar.
  • Hi Varaprasad,
    I also met the same problem, could you share the voltage follower circuit to me ? thanks.
    My E-Mail: soccer-wang@ti.com