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ISO721

Hii my name is aied ,a hardware engineer 

i am designing a circuit for voltage measuring  purpose , my input comes from CT (current transformer ) with values of 0 to 100mA .and i use an opamp to transform the current to voltage 

so the input to my device is current not voltage !!

i need to isolate my device from the environment and i came across your isolation IC products 

and my question is,  if i have an current input as i described above  ( that is not a digital communication line  ) what iso isolation chip i can use ?

a will also say that after converting the current to voltage i will get 3.3v DC max using the op-amp 

but i need to put the op amp after the isolation chip 

in other words do you have a chip that can deliver current from input pin to output pin that is also isolated ?

i can use the op-amp circuit to convert the current to voltage and then connect it to the isolation chip but then i will run the risk of my op-amp circuit is not protected .

i will be very thankful for your help 

 

  • Hello Aied,

    Thanks for your post and welcome to the forum!

    For current sensing applications we typically recommend measuring across a shunt resistor using a device with low input impedance. An example implementation can be seen in figure 49. of the AMC1301 datasheet.

    We have both isolated amplifiers and isolated modulators.

    Some examples of isolated amplifiers: AMC1100, AMC1200, AMC1301.
    Isolated modulators: AMC1203, AMC1204, AMC1303, AMC1304, AMC1305, AMC1306.

    The AMC1303/4/5/6 have multiple versions, either supporting a +/- 50mV input or +/- 250mV input and CMOS or LVDS outputs.

    Best regards,

    Alex Smith
    Applications Engineer | Precision Delta-Sigma Converters

    Check out our helpful resources:
    TI Precision Data Converters | TI Precision Labs - ADCs | Analog Engineer's Calculator Data Converters Learning Center | Selection Guide

  • In reply to Alexander Smith:

    hii alex

    thanks for your answer 

    i looked up the datasheet for the AMC chip family 

    and i have a question

    it seem that the AMC family is suitable for my application because they have the isolation that i need , but i didn't understand   how to use them 

    i have an input current with a sine wave shape with an amplitude that can range  from 0 to 100mA and frequency of 50 hz .

    if i feed this current to a 30 ohm resistor i will get a sine wave voltage with an amplitude of 3v=100mA X 30ohm with a frequency of 50hz.

    i want to transmit this sine wave with an amplitude of 3 volts and frequency of 50hz  from input of the AMC isolation amplifier to the output ,cause i need to convert this sine wave voltage trough an ADC converter on the other side of the AMC isolation amplifier  

    from what i saw in order to keep linearity  that the AMC1300 can accept 50mv between the input pins , but i want to transmit a 3 volt sine signal through the AMS isolation amplifier   

    how can i do that ?-

    again i be happy for your help if you can shed some light on how to do this application .

  • In reply to AIED BEBAR:

    Hi Aied,

    Happy to help!

    There are a couple of sections in the datasheets that should help, analog input, typical applications, and detailed design procedure.

    Isolated amplifiers are typically used in higher resolution solutions. You may need an additional operational amplifier on the output of the isolated amplifier to shift the common mode and adjust the gain of the signal to fit the full-scale range of your ADC. Is your ADC single-ended or differential?

    The isolated modulators are great for directly interfacing with a microcontroller's digital filter input. This solution will cut down on the number of devices in the signal chain, but will most likely have lower resolution.

    You are on the right track! You will need to use a smaller resistor (Rshunt) in order to get the signal into the input range of the AMC devices.

    Rshunt = Vin/current = 50mV/100mA = 0.5Ohm.

    This TI design goes through a detailed design procedure and should help quite a bit: www.ti.com/.../slau521.pdf
    Additionally, taking a look at the 4 ADC cookbook circuits for current sensing may be helpful: www.ti.com/.../slyy137.pdf

    Best regards,

    Alex Smith
    Applications Engineer | Precision Delta-Sigma Converters

    Check out our helpful resources:
    TI Precision Data Converters | TI Precision Labs - ADCs | Analog Engineer's Calculator Data Converters Learning Center | Selection Guide

  • In reply to Alexander Smith:

    hii alex , good evening its evening at our time zone :) maybe morning at yours
    i think i got my answer from your detailed design procedure
    i will keep working! i have a few AMC1200 chips in hand i will build circuit them next week and test it out ,if a will have more questions i will get back tom you thanks again .