Siganl Conversion
Siganl Conversion
(OP)
I use a PLC with 0-10v analog inputs. My devices (pressure transducers) have a 4-20 ma output. To convert, I put a 500 ohm resistor inline with my signal to generate a voltage drop. This results in a signal of 2-10v proportional to the ma signal. This works fine for my application, but I am cutting off some of my resolution.
Now the question...
Shouldn't I be able to use an OP AMP instead of the resistor workaround to generate a true 0-10v signal from my ma device? It seems as if I should be able to set an OP AMP up as a difference amp with my device on one input and a constant 4 ma siganl on the other. Since the output of the OP AMP should then be proportional to the DIFFERENCE between the two signals, I should get 0 - 10 v output (with an appropriately sized feedback resistor).
I should, but I can't get this to work in my mock-ups. Any ideas? Is there a better way to do this?
Thanks,
g
Now the question...
Shouldn't I be able to use an OP AMP instead of the resistor workaround to generate a true 0-10v signal from my ma device? It seems as if I should be able to set an OP AMP up as a difference amp with my device on one input and a constant 4 ma siganl on the other. Since the output of the OP AMP should then be proportional to the DIFFERENCE between the two signals, I should get 0 - 10 v output (with an appropriately sized feedback resistor).
I should, but I can't get this to work in my mock-ups. Any ideas? Is there a better way to do this?
Thanks,
g





RE: Siganl Conversion
Connect its + input to the 500 Ohm, the -
input to +2 V reference.
<nbucska@pcperipherals.com>
RE: Siganl Conversion
RE: Siganl Conversion
RE: Siganl Conversion
Good point, but generally pressure (PID) loops, if he has one, are fairly slow compared with the speed of an analog convertor. To get positive feedback would require the reversal of polarity not just adding an amplifier. But your right, you do have to watch the polarity.
I personally would rather get rid of extra devices, just for reliability sake. That is why a direct connection from the transducer to the I/O is the best.
RE: Siganl Conversion
E. James Angelo, Jr., Electronics: BJTs, FETs, and Microcircuits, McGraw-Hill Book Co., 1969
Fig. 13-3 Graphical analysis of the Amplifier in Fig. 13-2. Essentially, an odd number of amplifier stages creates the stable amplifier with overall feedback and an even number of stages leads to the bistable device. However, by the proper design of two stage amplifier feedback, it can be the stable two-stage amplifier.
RE: Siganl Conversion