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ADS130E08 custom reference

Johan Larsson44
Prodigy 30 points
Other Parts Discussed in Thread: ADS130E08, ADS131E08, LM4140, REF2025

Hi.

I am investigating using an ADS130E08 in my design.
I have analog input signals of the following ranges:

  • ±50mV (related to system GND)
  • 0-1V
  • -50 to +100mV

I would prefer not to use external amplifying stages due to tolerance requirements, making do with the PGA.
Since I have signals below GND, I will use the bi-polar coupling ±2.5V.

What I am  slightly confused about is the possibility of setting the external reference.

The way I look at it currently is I will be using a gain of 8 for all signals but the 0-1V.
Rendering:

  • ±400mV
  • 0-1V
  • -400mV to 800mV

For this, a reference of ±2.4V is not optimal, due to loss of resolution. I would rather have the possibility to use for example ±1V.

I have a hard time understanding the datasheet regarding this. What is possible to achieve?
As you might guess, I am somewhat junior when it comes to these types of designs :)

Any assistance is deeply appreciated!

Best regards,
Johan

  • 0
    TI__Mastermind 23235 points
    Hello Johan -
    For your configuration, there are some items that will fall outside of what has been tested with the device.

    Given your requirements, we would recommend using a +/-1.5V supply if you want to use a 1V external reference. While using the 1V reference may be OK with the +/-2.5V supply, you may see performance issues (in the form of tones). Not knowing your requirements in more detail, leads us to encourage the usage of the +/-1.5V supply for a 1V reference.

    You could also provide an additional small gain stage in front of the ADC depending on you circuit/requirements.

    Alternatively, if you use the ADS131E08, the increased resolution will help resolve even small signals for larger references. One of the nice features of the higher resolution delta-sigmas in the high dynamic range. This allows for the resolution of small signals as well as large signals without the need to change the gain settings.
  • 0 in reply to Greg Hupp
    Prodigy 30 points

    Hello Greg,

    Thank you for your swift reply!

    I realize I left out some important info regarding the measured signals. In general, they are mostly slow moving signals in the DC or low-frequency range (temperature, pressure etc). Anti-aliasing filters with -3dB points <100Hz will be employed.

    As you mention, ±1.5V supply sounds very reasonable.
    How should I connect VREFN in this case? Most app schematics and also the specs says to tie it to AVSS, which would then be -1.5V.
    Wouldn't this render a reference of -1.5V to +1V, instead of the desired ±1V? Can I generate a -1V reference and tie this to VREFN?

    Regarding the ADS131: we have used it before but are now trying to cut some costs, as we do not really need the 24-bit resolution in our application. 
    We might use it as a fallback in case things do not work out however.

    Regards,
    Johan

  • 0 in reply to Johan Larsson44
    TI__Mastermind 23235 points
    Hello Johan -
    The reference will always be connected between VREFP and VREFN. As you noted, you would want to connect VREFN to AVSS and likewise the reference. So instead of using ground for GND signal of reference, you would want to use AVSS. This will result in a VREF (VREFP-VREFN) of 1.0V. You could use something like LM4140 to generate a 1.024V reference. So if you measured VREFP with respect to ground, you will probably get something around -0.5V.
  • 0
    TI__Mastermind 23235 points
    Hello Johan -
    Is there a reason that you don't want to use a reference IC (for example REF2025 or similar)? Using a zener diode and resistor divider will work, but will probably not be a very good reference. The voltage will vary with the zener and resistor tolerance/ratio of the divider and the noise content will probably be much higher than just a small reference IC and a single buffer.
  • 0 in reply to Greg Hupp
    Prodigy 30 points

    Hello Greg, 

    The reason is mainly one of cost + I have some lust for experimentation. I imagine that if the voltage would vary with the zener during drift there is also the possibility to more frequently calibrate the chip using the high accuracy TESTP, where I would have a REF3221 or something similar of high accuracy. Any "static" tolerances are not an issue since these can be calibrated easily.

    Is it noise components from the 3V3 (should actually be 12V, error in schematic) suppling the zener reference that could be the main issue? I realize I should add some filter components to this stage.

    I could also just add the option of using a better reference (such as REF2025).

    Is driving the chip with ±1.8V an issue?