Electronic – Isolating an analogue input using ADC/DAC

adcdacdigital-logicplc

I'm looking into isolating an analogue input coming from field equipment and interfacing to a PLC analogue input module. The input will be of the 4-20mA range (1-5V).

There are a few ways to approach this but I am currently pursuing the solution as seen below:

Proposed solution to analogue input isolation

ADC and DAC are being proposed as Microchip MCP3221 and MCP4725 respectively.

A digital isolator would give a smaller footprint than an opto-isolator with its accompanying components and would provide much greater performance. Possible components include TI ISO7420FEDR and Analog Devices ADUM1200ARZ.

Now for the actual question!
Keeping both sides isolated is pretty vital and I'm wondering how this will effect the ADC and DAC linking.

I'm assuming I can send the data line (SDA) through the digital isolator but to synchronise the ADC and DAC they would need a common clock signal. Would sending the clock over the digital isolator be a bad idea? If the digital isolator was many times faster than the clock would it have a negligible effect in terms of delay?

Missing info:

  • Not tied to I2C, SPI would be appropriate too
  • Isolation to a minimum of 2.5kV
  • The ADC will be powered on the 'dirty' side so there is complete isolation, only the data/conversion control signals need to pass through the isolation barrier
  • Resolution should be between 10 and 16 bit, no specific value is required
  • Conversion throughput of around 100ksps would be sufficient

Best Answer

Frankly, SPI interfaces are better suited for isolation than I2C. You can set the master clock to a frequency where the timing works out reliably. I2C is a bidirectional bus which makes isolation a pain.

If you insist on using isolated I2C, AN-913 from Analog Devices is an application note that illustrates how:

Related Topic