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Original question:

ADS1298 Single-Ended Input Range

ADS131E08S: ADS131E08S Single-Ended Input Range

Mahlet Zewde
Prodigy 195 points
Part Number: ADS131E08S

Hello,

I would like to ask about the single ended input range of the ADS131E08S, if I use a unipolar power supply of Avdd=5V and use the internal Vref=2.4V, and set INN=0 V, what is the allowed voltage range  at the INP?  can it varry varry between -0.3V and 2.4V? taking in to consideration the limit imposed by the Avdd?

What if I use  a bibolar power supplu -2.5 to 2.5 V, and use the internal Vref=4V, set INN=0V,  Is the allowed  voltage range at the INP varry between -2.8V and 2.1V? taking in to consideration the limit imposed by the Avdd?

  • 0
    TI__Mastermind 47595 points

    Hello Mahlet,

    Thanks for your post!

    In the first scenario that you describe AVDD = 5V, AVSS = 0V = INN, Vref = 2.4V. You are correct that it can vary from -0.3V to 2.4V, however I would recommend that you limit the signal to 0 to 2.4V.

    In the second scenario INP can swing from -2.5V to 2.5V, however since you are using a 4V reference you will not be taking advantage of the full scale range.

    It makes more sense to use the 4V reference in the first scenario, giving you an INP range of 0 to 4V and 2.4V reference in the second scenario giving you an INP range of -2.4V to 2.4V. This setup will allow you to take advantage of the full scale range of the device.

    Additionally, all of this is assuming a PGA gain of 1. These input ranges are all divided by the gain since these parameters are the input limits of the ADC.

  • 0 in reply to Alexander Smith
    Prodigy 195 points

    Hi Alexander,

    Thanks again! What I am wondering is how the input common mode range affects the fullscale that is allowed in the INP pin, because for Vref=4V, AVDD=5V, AVSS=0V ,the CM is given as, 2.7<Vcm<2.3 using the equation below.

    Looking at the datasheet page18, it seems that I can not use the INN pin at gnd, for this scenario because for the pseudo diffrential (single ended) mode it should be kept at mid supply, this means 2.5V? Right?

    And from the above picture and explanation, what I understood is  the InxP can only vary 400mVpp around the common mode voltage =2.5V, this seems extremely small value in my opinion, It would be great to hear your explanation regarding this.

    Regards,

    Mahlet

  • 0 in reply to Mahlet Zewde
    TI__Mastermind 47595 points
    Hi Mahlet,

    Sorry, but the pictures didn't come through. Do you mind posting them again so I can see what you're talking about?
  • 0 in reply to Alexander Smith
    Prodigy 195 points

    Hi Alex,

    Sorry for that. here is the question with the attachment again.

    Thanks again! What I am wondering is how the input common mode range affects the fullscale that is allowed in the INP pin, because for Vref=4V, AVDD=5V, AVSS=0V ,the CM is given as, 2.7<Vcm<2.3 using the equation below.

    Looking at the datasheet page18, it seems that I can not use the INN pin at gnd, for this scenario because for the pseudo diffrential (single ended) mode it should be kept at mid supply, this means 2.5V? Right?

    And from the above picture and explanation, what I understood is  the InxP can only vary 400mVpp around the common mode voltage =2.5V, this seems extremely small value in my opinion, It would be great to hear your explanation regarding this. Does this mean INN=2.5V and InP can varry between CM+Vref/2 and CM-Vref/2(0.5V-4.5V)?

    Regards,

    Mahlet

  • 0 in reply to Mahlet Zewde
    TI__Mastermind 47595 points
    Hi Mahlet,

    Thanks for reposting the pictures!

    Your equation is correct and so is your understanding. I apologize for not catching that INN cannot be tied to GND in this configuration with this device.

    In order to take advantage of the full scale, the common mode should be tied to 2.5V, however as your equation points out it is possible to tie the common mode from 2.7>Vcm>2.3. You're right that 400mV is a relatively small amount of variance - but this should be a static value so the input range of INP doesn't have to be constantly re-adjusted to suite the changing common mode voltage.

    In order to set the common mode, I typically see two very large (~10MOhm) resistors connected to the pin (INN), with one resistor connected to AVDD and the other connected to AVSS. This will set the common mode at a static value with low power loss.

    AVDD > 10MOhm > INN(To Device) > 10MOhm > AVSS.

    INP will actually be able to vary from 0 to 5V. This is very confusing and on the docket to be re-written, but from the datasheet:

    "Swing the INxP pin around the common voltage –VREF / gain to VREF / gain and remain within the absolute maximum specifications."

    If CM = 2.5V, Gain =1, and VREF = 4V, from the first part of the statement it means that INP can go from +VREF/Gain+CM (6.5V) to -VREF/Gain+CM (-1.5). However, this is out of the maximum specifications of the input pin. This restricts the 6.5V to -1.5V range back to 5V-0V.

    I should note that it is safer to use 4.5V-0.5V as you indicated, such that if there is an unexpected increase or decrease in voltage, your readings will not be clipped.