I've got a set of pressure transducers and associated displays. Units work on 4-20mA current loop. I need to log the signals. I can put a
250 ohm resistor in series with the display and get a 1-5 volt signal. My problem is that my logger has single-ended inputs with a common ground. Can I make a "differential buffer" with just a single op amp, or will I need to go with a full instrumentation amp setup? I'd also like to get a gain of about 2 to get better resolution from the logger. Ideas/suggestions?
There are some fairly inexpensive handheld DVMs that have isolated RS232 interfaces. Most good benchtop DVMs also have RS232 or USB interfaces, and many of them will datalog, or come with PC datalog software.
If you want to use your existing datalogger, you can buy a commercial
4-20 mA isolator brick. Google 4-20ma isolator
INA117 might work, if you want to do it with parts. Or buy one of the fully isolated amps.
No, they are AC powered. I am measuring the loop current in each loop by inserting a resistor in series with the display (meter). If I use the logger "as-is" (with a common ground), it will short together all of the loops.
Float your 20 mA current sources, and you should be home free. The reason for the current loop is that in situations like yours, it should be as simple and noise-immune as possible. 0 mA means fault, and everything else is just as it should be.
So either make your current sources floating, grounded at the unit, or just design them all with the one common ground.
Can't "float" the power supply. There is only one supply (an open frame "Power One" 2A supply). Let me explain it this way: the displays have two wires, the sensors have two wires, and the power supply has two wires. One wire connects the power supply and all of the displays. One wire connects the power supply and all of the sensors. Each sensor has a wire to each display. I break this last set of wires (between sensor and display) and insert a resistor. I now want to measure the voltage drop across each resistor. I want to use the data logger I have, which has a common analog ground. I don't want to buy a DVM with an output. I don't want to buy isolation modules for each "channel". I just want to use either a simple (741?) op amp circuit or a simple instrument amp to interface (isolate? buffer?) between the resistors and the logger. OK, it's not actually a *logger*, it's a Dataq DI-184 data acquisition system
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but "logger" is easier to spell. And, no, I don't want to buy two DI-158 units at four times the cost ($200 vs $50) to replace the unit I now have.
But thanks to all who bothered to answer. You were > > > I've got a set of pressure transducers and associated displays. =A0Unit=
The problem, then, is you have a single current source and you want TWO different systems (the display and the logger) to measure it. Use a two-output current mirror (op amps and transistors is good, but three matched transistors with emitter resistors is also workable). Putting things in series (like with the sense resistor) can work, but is a bit of effort. An adjustable current offset and a variation of the sense resistor size can get your logger any scaling you desire.
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