Isolating a 4-20 mA transmitter
by Darren Ashby

Recently reader Bruce Cardoza sent in a question about isolating a 4-20 ma transmitter in an application he was working on. I asked for a block diagram of what he needed, and he thoughtfully sent me this very good description:

Darren,

I have read with great interest your articles on product engineering.ı I am a self-employed product development engineer specializing in industrial monitors and controllers (level, flowrate, gas detection, water quality measurements). For some time I have wondered how loop powered isolated 4-20mA transmitters manage to provide the isolation in the power budget available.ı For example: I have a dissolved oxygen sensor, which produces a mV output, which must produce a 4-20mA output.ı Because it is in water with other sensors tied to the same 4-20mA receiving device (same ground), the sensor must be isolated from the 4-20mA output to avoid measurement interference. The scaling and mV to ma output is not a problem.ı

I am thinking of using a PIC micro running at 32kHz + low power ADC, DAC. I cannot come up with a way of powering the sensor input circuitry and isolating the signal to the voltage to current converter within the power budget allowed. As you know, a loop powered 4-20mA transmitter uses the 4mA (x the minimum input voltage for the transmitter) to power itself i.e. the power available for isolation is 4mA x specified min. input voltage for the transmitter. The minimum input voltage is up to the transmitter designer.ı In my case, I can live with 12V.ı This means I have a max. of 4 x 12 = 48mW to power the transmitter. The sensor input opamp + ADC only requires about 500uA at 2.7V or 3V i.e. 1.5mW max.ı Optical isolation for the 2 or 3 wire ADC is more of a problem, but can be done. My greatest problem is producing the isolated power.ı All of the DC/DC converters I have seen require a minimum current in ma i.e. their efficiency is very poor at low current loads.

Any feedback on this would be greatly appreciated.

Bruce Cardoza / Emerald Instruments

WHAT I NEED:

Figure 1: True 2-wire loop-powered Isolated 4-20 ma Transmitter

Notes:

V instrument may be specified to be a minimum of 12V (or greater) to provide enough power to the instrument (max. instrument power = Vinstrument (min.) x 4mA.

The ADC is 12 bits.ı This is shown as part of the micro, but probably will be a separate chip to keep the micro noise out of the conversion.

The micro and DAC may be on either side of the isolation boundary.

The isolation is required since the sensor is in water with several other sensors, which can interfere with each other.

The 3V isolation power may be dropped to 2.5V.

More than one load may be connected in series.

Well Bruce,

One thing jumps out at me right off of the bat. Since you have elected to use a micro with an ADC and a DAC, letıs take advantage of it.

First, put the signal isolation between the micro and the DAC, I suggest a serial DAC. That makes the isolator a simple inexpensive opto-coupler. Companies such as QT sell these for very minimal cost, and passing a digital signal over isolation is much easier than an analog one. In fact, most analog isolators⁽¹⁾ that have any accuracy actually chop the signal and pass it over the isolation digitally. So move the isolation here. You will need to select this part carefully to make sure you donıt exceed your power budget with it.

Figure 2: True 2-wire loop-powered Isolated 4-20 ma Transmitter

Your next problem is getting enough power to operate the conditioning circuitry. As you said, many DC-DC converters have very poor efficiencies at low current draws. I think this has to do with required load on the isolation inductor. You should be able to make a unit that is optimized for your application. I did a little surfing to see what I could find. I found a review on a low current DC/DC TI part at ChipCenter/eChips: http://www.chipcenter.com/power/powp335.htm . I quote from the description, ıPower Save mode keeps efficiency high under low load conditions by changing from PWM to PFM switching schemes.ı You may find an off-the-self solution to your dilemma.

Another possibility is to make your own. By pulsing DC into a small transformer you will get an isolated signal out the other side that you can rectify and use as power. But, your voltage to current converter that is setting your 4-20mA signal must respond faster than the power switching frequency. Otherwise your power supply will contaminate the signal.

A novel idea is to use the 4-20mA current to power a light source that shines on a solar cell to create the power you need. You can get a lot of isolation this way, but I havenıt run the numbers to see if you will get enough current to run your device.

In researching your question stumbled onto www.4-20mA.com. It didnıt help much though.

I will be needing a low current DC/DC supply later this year in one of my own projects. If I come across anything interesting I will let you know.

Darren

Footnote:
HP makes one of these parts, I quote from here: http://www.weidmuller.com.au/interfac/issue_3/inside/item6.htm. ıAmong the latest and greatest new products is an opto-isolated analog-to-digital converter you can assemble from the Hewlett Packard HCPL-7860 "isolated modulator" and the HCPL-x870 "digital interface IC." One of the chips inside the "7860" converts its analog input signal, using a "sigma-delta" scheme, and transmits the resulting digital data across its internal "isolation boundary" to a second chip which communicates via one data and one clock line to the HCPL-x870. The "x870" actually can handle two "7860" devices for a two-channel system. The "x870" can "talk with" a microprocessor via a serial scheme.ıı These type of parts are pretty expensive though, several $$$ compared to about 0.40 cents for the QT part.

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