• State Resolved
  • Locked Locked
  • Replies 7 replies
  • Answers 3 answers
  • Subscribers 47 subscribers
  • Views 477 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

AMC1301: AMC1301 input problem.

Ahmet Burma
Prodigy 100 points

Part Number: AMC1301
Other Parts Discussed in Thread: AMC1351

Hi guys.

I want to design isolated voltage to 4-20mA converter. I am using MCU with DAC. I designed the schematic using AMC1301 and XTR11.

AMC.png

DAC value max ==> 3.3V. Circuit if working fine. But i have a little bit problem. The voltage required at the INP input of the AMC is as follows; 

If the DAC is 1V ==> INP = (1/(R63 + R70) * R70) = (1 / 6630) * 510 = 0.07692V

However, I'm measuring a higher voltage, around 0.089, at the AMC's INP input. The resistors have a 0.1% tolerance and are accurate. Any idea why this might be?

Best regards.

  • 0
    TI__Expert 7230 points

    Hi Ahmet,

    Can you provide some more information on your application? 

    This device is intended for current sensing applications, measuring across a low impedance shunt resistor.  The input current of the device is about 30uA, which would cause a ~15mV offset.  You can account for this offset by doing an offset calibration of your system and/or reducing the R63/R70 resistance.

    I would recommend moving to a device with higher input range that is intended for voltage sensing such the AMC0311 or AMC1351.

    Thanks.

  • 0 in reply to Saleh Ahmad
    Prodigy 100 points

    Hi Saleh.

    As I said in my first message, this is an isolated voltage / 4-20mA converter circuit.

    I can't lower the resistor values ​​because the MCU's DAC output doesn't allow it (load > 5k). I understand the effect of bias current. But I have a question. The technical documentation shows a typical bias current of 30uA. When calculating the effect of bias current on voltage, should we use the equivalent resistance of the voltage divider or the resistor connected to ground?


    Equivalent resistance = (R63*R70)/(R63+R70) = 3121200 / 6630 = 470R
    Voltage effect = 30uA * 470R = 14.1mV
    R70 voltage effect = 30uA * 510R = 15.3mV

    If you can clarify this, I can add the offset value as a constant in the program.

    I also think I can use the AMC1351 with the same scheme by just changing the values ​​of a few resistors without revising the PCB.

  • +1 in reply to Ahmet Burma
    TI__Expert 7230 points
    Ahmet Burma said:
    When calculating the effect of bias current on voltage, should we use the equivalent resistance of the voltage divider or the resistor connected to ground?

    It would be with the R70 resistor, since the bias current flows from INP to the device GND1.  To account for this offset, you could do an offset calibration for your system in which you force 0V input and measure.  The 30uA is the typical value, but this value will vary device to device.

    You can also include another resistor on the INN pin to cancel out this bias current.  Please refer to this app note for details on how to do that: https://www.ti.com/lit/an/sbaa350a/sbaa350a.pdf?

    It would be easier to switch to the AMC1351 and have the DAC output feed the AMC device inputs directly.  You could just depopulate R70.

    Thanks.

  • 0 in reply to Saleh Ahmad
    Prodigy 100 points

    Hi Saleh.

    I can't add resistor to INN pin and i don't want to revise PCB. I ordered the AMC1351. I'll keep testing it this way until I get it.

    One more thing. The AMC1301 is running a little warm. Is this normal?

    Thanks for your help.

    Best regards.

  • 0 in reply to Ahmet Burma
    TI__Expert 7230 points

    Hi Ahmet,

    Ahmet Burma said:
    One more thing. The AMC1301 is running a little warm. Is this normal?

    The device shouldn't really be heating up much, is it possible the heating is coming from a nearby source?

    Thanks.

  • 0 in reply to Saleh Ahmad
    Prodigy 100 points

    Hi Saleh.

    I am using 12 AMC1301. 6 isolated input (0-10V input and 4-20mA input) and 6 isolated output (0-10V output and 4-20mA output). All IC's are side by side and there is almost no space between them.

    There's no regulator or similar heat source nearby. After a while of operation (about 60 minutes), IC's reach a temperature of around 40 degrees.

  • 0 in reply to Ahmet Burma
    TI__Expert 7230 points

    Hi Ahmet,

    I think that should probably be fine.

    Thanks.