I have an application that uses 3 displacement sensors with analog current output from 0 to 20mA. I have a real time DAQ system that only measures voltage from 0 to 10V. I have read the previous response to Pradheep's question and it seems like the current values are too high to get the kind of precision that I need. I was wondering if I could use the resistor in series method and if there was some trick that could help me develop this setup? Any help at all would be appreciated, because I've never had to measure currents like this before so I'm a little beside myself trying to figure this one out...Also if the resistor "shunt" is placed in series what kind of accuracy can be achieved?
If you put the current into an opamp inverting input, with a 500 ohm feedback resistor, and the non-inverting input grounded, it will convert 0-20mA to 0 to -10V, and the output will be stiff even if it's loaded a little. You'd have to invert the output to +10 V. If the opamp needs to be protected from voltages, you can use a series resistor with diodes to plus and minus supply on the opamp input to limit voltage excursion, but you'd have to make sure the current source has enough compliance to handle the extra resistance. The source will only need enough voltage compliance to handle the drop across that protective resistance.You could also just use a 500 ohm resistor, if the DAQ system has high input impedance. Using just a 500 ohm resistor, the current source would have to have the compliance to produce the 10V.
Look before you leap, John -- there are a few potential gotchas here.
First, not all op amps are capable of sourcing/sinking 20mA (and that has to be a min., not typ. on the data sheet).
Second, you really have to look carefully at your system before you just plug it in. Many sensors that output a 4-20mA (or 0-20mA) current operate on another supply voltage, and have a simplified internal circuit like thios (view in fixed font or M$ Notepad):
If your sensor common is connected to DAC GND, you may have an issue connecting the external power supply (which may exceed the DAC power supply or the input common mode voltage) to the DAC input. And if you connect the negative sides of more than one sensor together, it won't work.
Another difficulty is power-up sequencing. If your sensor supply comes up before the DAC's, you may get latchup when the DAC turns on. Not pretty.
For current output sensors, you typically place a 250 ohm (0-5V) or 500 ohm (0-10V) load resistor at the receiving end of your signal, and then use a diff amp to get the input voltage to the DAC. That's how it's done.
That's a good point.You could put a transistor buffer on the opamp output, or, in a one-direction application like this, you could hang a resistor to the minus supply, pulling 10mA out. Then the opamp could switch between sinking and sourcing 10 mA.
Do you think using a series resistor with diodes to prevent excursion more than V+ plus .7 or V- or ground minus 0.7 would prevent that problem? With a hookup like that, wouldn't an early start of the sensor supply at worst just try to power up your circuit?
That seems like the best solution if you have 24V available from the sensors.
Hi, John. Depending on Eric's application, everything you suggested
*could* work -- it all depends on the specifics of his application. Many sensors with current output are self-powered, and you just take the current output wire referenced to it's COM wire for the 4-20mA output. Some require the mA output be referenced to the V+ side (view in fixed font or M$ Notepad):
Some have built-in isolated floating power supplies, and just have two output wires -- you can tie either side to the PC common without worry. And many aren't self-powered at all -- you apply a minimum voltage between the two wires, and the sensor powers from that, with total current draw guaranteed to be 4-20mA, like this:
This won't work for a 0-20mA output (they do exist), so it's more likely it's one of the other types of outputs unless the OP mis-described his app.
The difficulty comes when interfacing to a PC or other DAQ system where you have fixed voltages referenced to PC ground. It's trivially easy to smoke a board input, and I've gotten a lot more cautious over the years about this particular problem, in part because you occasionally have to supply a solution before you know all the details (much like Eric's post).
For the two wire solution shown in Fig. 2 above, if the +15V of the DAC board is sufficient, he'd need no input protection at all. Many sensors require more than 15V, though. With a higher power supply voltage (say, +24V), Eric should just be able to put a 12V zener across the 250 ohm resistor for protection.
But using a combination of voltage dividers at the input, followed by a diff amp with enough gain to compensate for the voltage divider, is good as a universal input which can accept common mode voltages quite a bit above the power supply rails.
The concept I mentioned above is expressed as a one-IC solution from Analog Devices. The AD629 is a high common mode voltage diff amp which has laser trimming to achieve minimum offset and gain error. For a few bucks per channel, he can have a universal front end that can handle common mode voltages of up to +/-270V with the +/-15V supply commonly available on DAQ boards. The low end version of the AD629 specs 1mV max offset and 0.05% max gain error, which is a small price to pay:
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Eric can also implement the high common mode voltage diff amp with discrete components and standard op amps, just as shown in the suggested wiring diagram on the data sheet. Resistor matching is a bit of a pain though, especially with multiple channels. If the OP wants to go that route (been there, done that, got the tee-shirt), having a factory cal cycle and/or autocal cycles can help with offset and gain non-linearities. And after all, the OP didn't specify accuracy or precision here -- he was just asking. And I don't remember needing more than 12 or 13 bit resolution on any position or displacement sensor output signal -- actually, 10 or even 9 bits (0.1% or 0.2%) would almost always have been sufficient.
If the OP wants to look at the wide variety of current sense solutions, he might want to look at Linear App Note 105, the Current Sense Circuit Collection.
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Punch in AN105 on search, and right-click "Save Target As..." (long .pdf file alert). They don't cheat too much toward their product, and one of those solutions should be right for the OP.
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