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Regarding ADS1248 VBIAS

SIVARAMA KUMAR PASUPULETI
Prodigy 160 points
Other Parts Discussed in Thread: ADS1248, PGA280

Hai,

I am using ADS1248 for +/-10V (Differential)  input range sensing. As we have only +5V supply available we are operating ADC in Unipolar Mode.

My Question is

1. Whether we can use the ADC  with unipolar supply for this differential  input range or not?

2. If not, can we activate VBIAS voltage source on -ve input terminal so that the negative input present at the ADC input terminals will be in Common Mode range Of ADC?

 

 

  • 0
    TI__Guru* 92655 points
    Sivarama,


    The ADC cannot measure a signal that is ±10V. That is too large for the device and may cause damage. Depending on what you are measuring, you may try to use a precision voltage divider or some sort of of gain of 1/2 stage to get the signal to within the input range.

    I don't recommend the VBIAS signal as a negative input for the ADC unless it is used for a thermocouple. The VBIAS isn't meant to be used to sink or source any significant current which may be present depending on the sensor. Also, this signal may be noisy which may also cause errors in the measurement. Using VBIAS for the thermocouple, this noise is common-mode and should not add direct error to the measurement.

    If you absolutely need a negative input terminal, I would consider using VREFOUT. It is less noisy and is capable of sourcing more current. However, there is a load regulation specification and you may still have some effect changes in the reference.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 160 points

    Hai Joseph,

                          Thanks for your responce. Here for +/- 10v input range we are using a potential devider of Gain 0.09911.

    Input voltage                      Gain stage (potential Divider)                              Input at ADC

          -10V                                                             0.09911                                                   -0.9911V

              0V                                                             0.09911                                                   0V    

            10V                                                             0.09911                                                  0.9911V                                                              

    Q1) Whether this negative voltage will be processed by ADC or NOT (ADC - Unipolar supply)?

           If the ADC will not Process the negative voltage with unipolar supply operation, then how can we make it ?

           We have 5V supply available. Because of Cost constraints we are not using bi-polar supply. So please suggest me how to make it  with Unipolar supply?

           Below is the one of the reply that we got from the Ti Technical person regarding +/-10V input.

    +/-10V input

    Activate bias either on VF4

    Use internal reference of the   device

     In our application AIN0 pin of ADC is positive and AIN1 pin of ADC is negative. So we are activating VBIAS.

    Q2) According to the Common-Mode range input formula of ADS1248 , What is VIN  and What is Gain?

    Formula:                              Min                                             Max

                                 AVSS+0.1+(VIN*GAIN/2)      AVDD-0.1-(VIN*GAIN/2)          

    Q3) What is the purpose of VBIAS? (if there are any documents please provide the link).  

  • 0 in reply to SIVARAMA KUMAR PASUPULETI
    TI__Guru* 92655 points
    Sivarama,


    I'll answer these questions in the same order they were asked.

    1. The ADC can measure the voltages given, but it depends not just on the magnitude of the input voltage (AINP-AINN) but also the what the absolute value of each input is as well. Both AINP and AINN must be within the common-mode input range of AVSS+0.1+(VIN*GAIN/2) to AVDD-0.1-(VIN*GAIN/2).

    For example the ADC can measure a -0.9911V signal if the the AINN voltage is 2.5V and the AINP voltage is at 1.5089V. However, you can't measure it if the AINN is at 0V and AINP is at -0.9911V.

    There is a post that discusses common-mode input range and it includes a set of Powerpoint slides:

    e2e.ti.com/.../1383202

    It explains the limitations of the PGA and how that affects the common-mode input range.

    If you are having problems translating the ±10V to something that is inside the common-mode input range, I'll need more information about what it is that you're measuring. What is the application? What sort source impedance do you have from the sensor? Normally, something like the PGA280 would be an ideal bridge to set the input to the proper level. However, without higher supply rails, you need to use some sort of simple difference amplifier. In that case, you will have to overcome resistor matching for gain error and you are also limited by the input offset of the amplifier.

    2. The gain in the equation is the PGA gain. If you look at the discussion in the forum post that I gave the link to, you can see how the PGA affects the common-mode input range.

    3. The VBIAS was designed to be used to set a bias voltage for an unbiased thermocouple. Generally, a thermocouple requires a setting to establish the DC operating point. The VBIAS is used to set the negative terminal of the thermocouple.

    I mentioned this in another recent post, but I wouldn't use the VBIAS as the negative input for anything other than a thermocouple. The VBIAS isn't meant to be used to sink or source any significant current which may be present depending on the sensor. Also, this signal may be noisy which may also cause errors in the measurement. Using VBIAS for the thermocouple, this noise is common-mode and should not add direct error to the measurement.


    Joseph Wu