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Original question:

LSF0108: Looking for good IC selection to send PWM signals to motors. LSF doesn't seem to be working.

Receiving 0-5V linear input to 3.3V controller with isolated grounds. (Isolating 24V rail ground from 5V rail ground)

Jigar Patel
Prodigy 85 points
Other Parts Discussed in Thread: AMC1350, AMC3330, TLV9001, ISO122, AMC1200, ISO121

Hello All,

I am willing to use an ideal TI component for receiving 0-5V ADC input to my 3.3V controller. However, my sensor which provides this 0-5V output is powered from 24V rail.

In my entire circuit I have many components like motors, inductive sensors, infrared sensors etc... in 24V rail. And so far I have maintained Ground isolation between 24V rail and 5V rail entirely to avoid unnecessary noise.

Just this one particular sensor where I need to receive 0-5V ADC input to my 3.3V controller is causing me issue because Sensor is using 24V circuit ground and my controller is using 5V circuit ground.

Is there any circuit that can give me this linear isolation with isolated grounds? Could be a linear opto isolator. I am looking for +/- 10mV accuracy. I can do error offset in software if the error is linear though.

Please help if possible. Thank you.

  • 0
    Prodigy 85 points

    Additionally, I would like to mention that, this sensor has capability to provide 4-20mA output as well if we set to. However, question is the same here that to bring sensor output to 3.3V controller with isolated grounds.

  • 0 in reply to Jigar Patel
    TI__Mastermind 47625 points

    Hi Jigar,

    I certainly think we have devices in our portfolio that can provide what you are looking for. 

    A device like the AMC1350 with a +/-5V input would be sufficient. One challenge would be to drop the 24V rail down to 5V to power the high side of the device, VDD1. This device has an input impedance of 1Mohm. https://www.ti.com/product/AMC1350

    Another option would be the AMC3330 with +/-1V 1Gohm input and integrated DC/DC converter. This device would require implementing a resistor divider to drop the 0-5V input to 0-1V, but would not require a power rail on the high side. https://www.ti.com/product/AMC3330

    AMC1311B is the last option I would recommend at this point. This device has a 0-2V 1Gohm input and no internal DC/DC converter, so the resistor divider as well as the power rail would be required. https://www.ti.com/product/AMC1311

    This AMC voltage sensing accuracy excel calculator should help with verifying your +/-10mV error requirement. Please note that I am in the process of updating this calculator to include newer devices in our family, AMC1350 is not currently included. If you are interested in seeing the accuracy calculation for AMC1350, please let me know the configuration settings and I will provide the calculation.  https://www.ti.com/lit/zip/sbar013

    A few questions in regards to the accuracy calculation:

    Is your +/-10mV error requirement referred to the 0-5V input?

    What is the temperature range?

    Worst-case or typical error? 

    All of these devices have a differential output on a common-mode of 1.44V and are designed to interface with a 3.3V controller. A differential to single-ended conversion with an opamp such as TLV9001 near the input of the controller may be necessary. https://www.ti.com/lit/an/sbaa229/sbaa229.pdf

    Please let me know if you have additional questions. 

  • 0 in reply to Alexander Smith
    Prodigy 85 points

    Thank you Smith,

    Here is requested information:
    Is your +/-10mV error requirement referred to the 0-5V input? yes
    What is the temperature range? upto 85 degree C is okay. this circuit will not be tested below 15 degree C and above 65 degree C.
    Worst-case or typical error? +/-20mV could be a worst case error for us.

    So far I like AMC3330 option. I would like to see how much error / accuracy we can get using that IC.

  • 0 in reply to Alexander Smith
    Prodigy 85 points

    I just check stocks. And I found that non of these are available right now to buy! Can we work around some another way?

  • 0 in reply to Jigar Patel
    TI__Mastermind 47625 points

    Hi Jigar,

    10mV with respect to 5V is 0.2% which AMC3330 can achieve. 

    The AMC1311B offers additional increase in accuracy:

    Indeed, there is a stock shortage throughout much of the semiconductor industry. It looks like the only devices we currently have in stock are the AMC1200, ISO122 and ISO121. We continue to build devices as quickly as possible and re-stock as they become available.