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ISO224: isolated amplifier

Ainara Calvo
Prodigy 140 points

Part Number: ISO224
Other Parts Discussed in Thread: AMC1311, AMC1301, AMC1300

Hi,

We are looking for an isolated amplifier with single supply on both sides and a 1-2 gain.

We've found the ISO224, but we have a 0 to 1V input and the result on the output is 0 to 400mV, a very low signal for our application.

Is there any isolated amplifier with the characteriscs we need? it would be for an instrumentation aplication, frecuency doesn't affect us.

We would appreciate your help!

kind regards,

Ainara

  • 0
    TI__Guru 58362 points

    Hi Ainara,

    Thanks for your post and welcome to the forum! 

    The AMC1311 may suit your needs. It has a 0-2V input and a fixed gain of 1. http://www.ti.com/product/AMC1311

    Here is the product list for our other isolated amplifiers: http://www.ti.com/isolation/isolated-amplifiers/products.html

    Most of the devices have either a +/-50mV input or a +/-250mV input for voltage sensing through a resistor divider or current sensing through a shunt resistor. Keep in mind that devices with high input impedance (1Mohm+) are desirable for voltage sensing where devices with low input impedance (~20k-30k) are better for current sensing. 

    Please let me know if you have any questions. 

  • 0 in reply to Alexander Smith
    Prodigy 140 points

    Hi Alex,

    Thanks for your reply.

    After posting my question I continued searching and I came to the same conclusion that the AMC1311 is the one that fits best our proyect when it comes to gain and input range. but, we need the amplifier for current sensing, a 4-20mA loop. The AMC1311 is a high-impedance device, and as you said is better suited for voltage sensing, you think it would still work for our proyect?

    Best regards!

    Ainara

  • 0 in reply to Ainara Calvo
    TI__Guru 58362 points

    Hi Ainara,

    The issue with using a high impedance device for current measurements is that a larger shunt resistor will have to be used, which results in additional power dissipation. If you want to stay with an isolated amplifier, the AMC1300 or AMC1301 would be best in this case. Isolated modulators are another option, but they will have the same input range and gain challenges as the AMC1300 and AMC1301.

    How accurate does your 4-20mA measurement need to be? If you're ok with sacrificing some of the dynamic range the shunt resistor could be sized such that 20mA is equal to +50/+250mV and 4mA is equal to +10/+50mV; for 250mV R = 12.5ohms, for 50mV R = 2.5ohms. In order to get the full dynamic range, an opamp would need to be used on the input, but would introduce additional noise to the signal chain. 

    Using an opamp on the output would tailor the output signal to whatever the MCU needs it to be. An example of this can be seen in this cookbook circuit: http://www.ti.com/lit/an/sbaa317/sbaa317.pdf

  • 0 in reply to Alexander Smith
    Prodigy 140 points

    Hi Alex!

    We need the current measurement to be as accurate as possible, but we prefer to sacrifice the dynamic range in order to not introduce any additional noise to the signal. The output is going into a MCU with a pretty accurate ADC, we hope it'll be enough.

    One more question, both the AMC1300 and AMC1301 are very similar which one would fit our measuring requirements the best?

    Thank you so much for your help. 

    Best regards

    Ainara

  • 0 in reply to Ainara Calvo
    TI__Guru 58362 points

    Hi Ainara,

    Happy to help! 

    To be as accurate as possible, I would recommend the AMC1301 over the AMC1300 as the AMC1301 has better gain error, gain error drift, input offset, and offset drift specifications.

    Both have +/-250mV input with a fixed gain of 8.2.

    The AMC1300 does have a slightly higher isolation working voltage (1500Vrms compared to 1000Vrms). If this specification is important to your application as well, then I would recommend the AMC1300B which has the best of both worlds. The same gain error, gain error drift, input offset, and offset drift specifications as the AMC1301 + the higher isolation working voltage of the AMC1300. 

  • 0 in reply to Alexander Smith
    Prodigy 140 points

    Hi Alex,

    We will go with the AMC1301 then, the isolation is high enough and we need a better gain error.

    Thanks so much again for your help!

    Best regards

    Ainara