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.

ADS8691: ADS8691: if could i set the ads8691 external reference voltage as the default mode ? thus i see the ads8691 datesheet show it is inside reference as default,but i want use outside reference as fefault when power on the device ?

Part Number: ADS8691
Other Parts Discussed in Thread: ADS8900B, TIDA-01333, TIDA-00764, OPA192

ADS8691: if could i set the ads8691 external reference voltage as the default mode ? thus  i see the ads8691 datesheet show it is inside reference as default,but i want use outside reference as fefault when power on the device ?

  • Part Number: ADS8691

    ADS8691 : when the ads8691 input pin  impending,the output have a DC2.5V VBB, how could i eliminat it ?

  • Hello Andrew,

    Thanks for your query on E2E forum.

    As the information you see in ADS8691 datasheet, the internal reference is enabled by default when the device is powered up, but the external voltage reference can be used by programming INTREF_DIS bit of RANGE_SEL_REG register to logic 1 to disable the internal reference source after the device is powered up.

    When your ADS8691 device's input is floating, the conversion codes relevant to 2.5V DC for +/-3Vref input range will be observed because this ADC integrates an analog front-end circuit consisting of a Programmable Gain Amplifier (PGA) and the front-end signal conditioning is biased internally to the appropriate common-mode voltage level for proper operation of internal ADC. When the input is not floating or there is an available signal, this bias voltage will not be observed on output.

    There are a couple of solutions can be used if do not want to see this voltage for this particular situation:

    1.  Use a front-end amplifier which is a best solution because the amplifier’s output can drive the ADC close to 0V when the Input is floating.

    2. Use a pull-down resistor (Rp, an example as below), it will have a divided voltage for ADC input combining with ADC's 1Mohm constant input impedance, the smaller value of pull-down resistor, the lower divided voltage and code when input is floating.

    3. If multi-channel is used in your system, a simultaneous-sampling differential input  ADC can be used, please let me know if this is your application.

    Thanks.


    Best regards

    Dale

  • 1. front-end amplifier would be better choice
    2. pull-down may case other problem
    3. simulaneous-sampling is always needed, ADS8900B is my first choice, use ADS8691 just because it's simple, only one power need, if add front-end amplifier, additional two power need

    i'm reading TIDA-00764 & TIDA-01333, a mini different is 374 ohm resistor VS 0 ohm between AIN_GND and GND, why?

    OPA192's DC Out Impedance as high as 3k ohm! That's another problem.
  • Hi Andrew,

    ADS8900 is a single channel ADC, it's not simulaneous-sampling ADC. The reason I asked you is, if multi-channels are needed on your board and no need for ch-ch isolation, we will have a differential version of ADS8588S soon which is an 8-ch input ADC, we can provide a input floating solution without op-amp and pull down resistor to you based on this device.

    Balanced input RC filter is recommended for this ADS8691, please refer to ADS8681EVM's user guide, the resistor may not be 0ohm and can be selected with the expected cut-off frequency of RC filter.

    Thanks&regards

    Dale