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There appears to be several options out there when it comes to isolating analog signals. This topic has been addressed on this website in a few forums as well. The problem for someone such as myself... I'm not convinced some of the solutions discussed apply to my application.

It would seem that the favorite is an isolation amplifier. But the output would be differential? I guess my hang-up is the output and does that work with my micro. The micro is single ended input. But also my inputs into the amplifier inputs. Can the geniuses here take a look at what I'm thinking and comment? Feel free to insult my intelligence if you so choose, just don't insult my mom.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The second U1 and U2 are actually U3 and U4. Sorry about that. \$\endgroup\$
    – D4ILYD0SE
    Feb 27, 2019 at 17:09
  • \$\begingroup\$ Please provide a link to the datasheet for the AMC1100. Also, please specify the voltage levels of your analog inputs. \$\endgroup\$ Feb 27, 2019 at 17:09
  • \$\begingroup\$ [AMC1100](www.ti.com/lit/ds/symlink/amc1100.pdf) The input voltages can range from 0-28, but I am thinking I will divide those down. \$\endgroup\$
    – D4ILYD0SE
    Feb 27, 2019 at 17:13

2 Answers 2

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Please connect VINN to GND1 on these amplifiers as per manufacturer recommendation. These chips do get stuck if you go beyond common mode specs due to transient or ESD.

But the output would be differential? I guess my hang-up is the output and does that work with my micro. The micro is single ended input.

You can draw up a schematic with amplifiers do convert differential to common mode. This will add a lot more uncertainties in your circuit. In offset, (thermal) drift and noise.

Or you use two ADC channels, and subtract in software.


TI also sells a single-ended input isolation amplifier, the AMC1311.

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  • \$\begingroup\$ The AN_REF is that GND but traveled through 100+ FT of "stuff." Would I want to use a 1M OHM resistor between the two? Also, I shall look into the AMC1311, thank you. \$\endgroup\$
    – D4ILYD0SE
    Feb 27, 2019 at 17:21
  • \$\begingroup\$ @D4ILYD0SE no resistor. Hard short VINN to GND, or the low resistance (12 ohm) input filter in the datasheet. \$\endgroup\$
    – Jeroen3
    Feb 27, 2019 at 18:31
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Analog isolation amplifiers are often expensive, power hungry and have mediocre initial accuracy that only gets worse with temperature changes and aging. The market offerings have improved in the last few years, but even so I would only go with an analog isolation amplifier as a last resort. Prefer instead, if possible, putting the microcontroller on the other side of the isolation barrier and provide it with isolated power and data connectivity. Maybe even use two micros if needed. Or see if your objective can be achieved in amother way (e.g. a closed loop controller with some form of isolator in the control or error path rather than in the sensing or reference path).

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  • \$\begingroup\$ But that's the issue. To get the analog to the Micro, the analog has to travel across the isolation barrier somehow. I'm told a linear amplifier is perfectly capable of this due to its high impedance. \$\endgroup\$
    – D4ILYD0SE
    Mar 4, 2019 at 21:50
  • \$\begingroup\$ @D4ilyd0se I was suggesting putting the micro on the same side of the isolation barrier as the analog. Then isolate the micro's power and digital data as necessary. Doesn't work in all situations, depends on what the overall system needs. \$\endgroup\$ Mar 4, 2019 at 23:54

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