I am implementing 4~20mA Transmitter that consist of a MCU, a current DAC, and an ADC for measuring an analog signal.

I would like to electrically isolate the analog signal from the loop current so that the higher end control system (i.e master system) is not damaged by some outrageous condition on the lower end (i.e sensor) side.

I've searched various solutions on-line and found that I could either use isolated ADC or digital isolator for I2C or SPI.

However, my device is loop powered, so everything must be done within about 3mA to be safe.

The isolation devices I found on major IC makers normally consume more than 1.6mA per channel.

Anybody have better idea or parts that I could use?

  • \$\begingroup\$ Real estate costs might dictate it is more sensible to apply protection at the master ($ per sq inch might be much less) \$\endgroup\$ – Andy aka Apr 15 '14 at 7:59
  • \$\begingroup\$ Is the upstream device a PLC or something similar? It should already have protected inputs in this case. \$\endgroup\$ – Peter Apr 15 '14 at 9:36
  • \$\begingroup\$ A schematic would really help, not clear if your 4-20ma output is connected to your ADC or what. Would be easier for us to see what does/doesn't need protection or an alternative approach. \$\endgroup\$ – KyranF Apr 15 '14 at 11:49
  • \$\begingroup\$ Signal isolation isn't so big a problem- remember you can have a low duty cycle in most 4~20mA applications, .. 2mA at 5% is only 100uA. Power isolation typically requires a bespoke solution. \$\endgroup\$ – Spehro Pefhany Apr 15 '14 at 12:13
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    \$\begingroup\$ @Jay it is not strictly isolation, so if that is a requirement then dont worry about it. Check this out though! m.electronicdesign.com/components/… \$\endgroup\$ – KyranF Apr 17 '14 at 12:15

The ADuM1441 can be used to implement an isolated SPI interface, and dissipates less than 1 mA at 2 Mbps. Quiescent power dissipation can be lower than 1 uA, making this product ideal for isolated 4-20 mA loop applications.

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