2

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?

JYelton
  • 34,119
  • 33
  • 145
  • 265
Steve
  • 956
  • 1
  • 19
  • 36
  • Real estate costs might dictate it is more sensible to apply protection at the master ($ per sq inch might be much less) – Andy aka Apr 15 '14 at 07:59
  • Is the upstream device a PLC or something similar? It should already have protected inputs in this case. – Synchrondyne Apr 15 '14 at 09:36
  • 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. – KyranF Apr 15 '14 at 11:49
  • 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. – Spehro Pefhany Apr 15 '14 at 12:13
  • Just to be clear -- you are taking an analog (presumably sensor) signal, digitizing it with the ADC, which is read by the MCU, which then converts it via the DAC to analog to send it via 4-20mA analog loop to a master? – gwideman Apr 16 '14 at 10:21
  • @gwideman . Yes you are correct. I am designing a loop powered temperature transmitter – Steve Apr 17 '14 at 00:58
  • @Andyaka Yes. I agree that it is a lot more sensible to isolate at the master side given the real estate and the limitation on the current consumption. However, it is required specification for the project. – Steve Apr 17 '14 at 01:03
  • @KyranF 4~20mA output is connected to MCU. and ADC is also connected to MCU. Sorry, we haven't even selected parts for the implementation. – Steve Apr 17 '14 at 01:04
  • @PeterK upstream device will be PLC or other DCS system. However, the project requires that the inputs and outputs of the product should be isolated. – Steve Apr 17 '14 at 01:06
  • @SpehroPefhany Power isolation imposes even greater challenge. I can't find way to isolate power efficiently with current consumption limitation of 3.5mA for the whole system with extremely limited real estate. – Steve Apr 17 '14 at 01:10
  • Could a current mirror or coupled inductors be useful here? Will need lots of tuning though – KyranF Apr 17 '14 at 03:35
  • @KyranF I don't understand how current mirroring would be applicable in terms of isolation. Can you explain more? – Steve Apr 17 '14 at 07:14
  • How frequently does the system need to report data? Can the 3mA be used most of the time to charge a battery or capacitor, and that reservoir used to occasionally send data to the master? – gwideman Apr 17 '14 at 11:48
  • 1
    @Jay it is not strictly isolation, so if that is a requirement then dont worry about it. Check this out though! http://m.electronicdesign.com/components/simple-current-sensor-features-galvanic-isolation – KyranF Apr 17 '14 at 12:15

1 Answers1

1

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.

Nick Alexeev
  • 38,181
  • 17
  • 100
  • 237
DaveC
  • 31
  • 2