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ADS114S06B: ADS114S06B

marcelo freitas
Prodigy 40 points
Part Number: ADS114S06B

Hi, I'm using this ADC to convert two kind of signals, 0-60mVcc / 0-10Vcc, please see attached schematic.

I need to measure 0-60mV on AIN0 channel and 0-10Vcc on AIN2 channel.

Problem found: When I apply a signal higher then 2 or 3 Vcc on my input signal, reading AIN2 channel, I have wrong values.
When I open R7, I have the right values.

But I'm using 10K(R7) exactly to protect over current on AIN0 channel.

Can you help with this issue?

Thanks!

  • 0
    TI__Mastermind 44670 points

    Hi Marcelo,

    The issue has to do with one (unselected) analog input being over-driven and affecting the measurement result on the selected input. There is a note about this on page 23 of the datasheet.

    To overcome this limitation you would either need to separate the mV and V inputs to avoid overdriving AIN0, or provide some additional protection to the AIN0 input.

    Even though you are not significantly overdriving the AIN0, because of the current limiting resistor, the internal ESD diodes on the AIN0 pin are forward biased and sinking enough current to affect the measurement on AIN2. Unfortunately, I do not know if further liming the input current on AIN0 would resolve the issue; however, there would be no harm with replacing the R7 resistor with a 100k resistor to see if this provides better results. If it does, then you might be able to utilize one of the ADC's GPIO pins to control a MOSFET switch in place of R7, to significantly limit the input current when performing the 0-10V measurement. Otherwise, you may need to consider an external clamping circuit to shunt the input current to ground, instead of through the ADC ESD diodes. A low-leakage TVS diode that clamps around 3.3V (see https://www.digikey.com/short/pmz4mr) might make sense to add between R7 and AIN0.

     

  • 0 in reply to Christopher Hall
    TI__Mastermind 22815 points

    Hi Marcelo,

    not related to your actual problem, but following two additional comments:

    1. The power-supply decoupling capacitor on AVDD needs to be at least 330nF as mentioned in the Pin Functions table in the datasheet.
    2. As you are performing single-ended measurements using a unipolar analog supply, you will have to bypass the internal PGA (if you have not already done so anyway). Means set the PGA_EN[1:0] bit to 00b.

    Regards,

  • 0 in reply to Christopher Hall
    Prodigy 40 points

    Hi Chris,

    Thanks by your soon reply.

    I've tried R7 =100K before, with no sucess.

    Now I'm trying something to clamp the signal of AIN0 channel. I used zener, diodes, but as you mentioned, the leakage current modifies my input signal, mainly with thermal changes.

    Do you have another suggestion?

    Thanks in advance!

  • 0 in reply to Joachim Wuerker
    Prodigy 40 points

    Hi Joachim,

    I appreciate your comments! 
    I'll change the capacitor, and the configuration of PGA was right.

    Thanks!

  • 0 in reply to marcelo freitas
    TI__Mastermind 44670 points

    Hi Marcelo,

    I recommended a TVS diode since they typically have lower leakage currents. However, it can be a challenge to find TVS diodes with reverse breakdown voltages around 3.3V, and from what I found from a quick search they seem to have higher leakage currents than TVS diodes with higher breakdown voltages.

    Alternatively, if you can control or detect which type of input is being used, then I think a MOSFET switch could be a good option to protect the AIN0 input. I mentioned replacing R7 with a series switch in my last post, but you could also have a shunting switch to ground after R7 that directs current through the switch instead of through the ADC's ESD diodes.