ELA
Senior Member
- Occupation
- Electrical Test Engineer
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ELA:
The transducer almost certainly has internal amplification and thus I expect an AC small signal output impedance is very close to zero from the negative feedback of the amplifier.
There are limitations by the available drive current. The AD524 can drive a 2 k load to +/-10 V.
The wiring from the amplifier may have a higher impedance than the amplifier itself.
If internally the output impedance was 100 ohms with no feed back, the gain was 1,000,000, and 100% negative feedback, then output impedance is about 0.000,1 ohm. A lower gain amplifier and it is about 0.001 ohm.
See following references:
http://www.electro-tech-online.com/general-electronics-chat/35701-op-amp-cct-output-impedance.html
http://zebu.uoregon.edu/~rayfrey/431/notes9.pdf
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Gar,
I understand basic op-amp theory and the concept that an op-amp output may be considered to be low impedance.
I am talking real world relative circuit impedances. Most 0-10V circuits I have worked with are relatively low current (relatively speaking - high impedance).
If you are only driving a low current in a cable then signal to noise ratio can be low. The suggestion was to raise the signal level in relation to the noise being induced.
Consider that you may be able to drive 2k minimum but in reality you are only driving 1-10Meg (scope input impedance). As I am sure you know this is a very noise sensitive situation.
Consider that an analog voltage output module may be able to drive
10v/2k =5 ma absolute maximum.
When the output is at 1V it is only sourcing 0.5ma. This is not a real large current level in a noisy environment.
Yet is is a lot better than when you are not terminating the cable and using only the O'scope input impedance.
My suggestion was to terminate the transducer in as low of an impedance as possible.
Even if a device may be able to source a relatively higher current does not mean that it is doing so in the circuit in question. It depends upon the circuits termination. That is what was talking about.
I am not talking about absolute op-amp theory.
Consider that many analog (0-10V) input modules have input impedances >1Meg ohm.
I have had situations where someone mistakenly designed a 0-10V drive signal to control devices in a noisy industrial environment. Control was erratic.
Replacing the transducer with a 4-20ma output solved the problem.
Granted that there may be other ways to address low signal to noise issues but 0-10V analog voltage modules are not the best choice for a noisy industrial environment.
The point was that many 0-10V voltage signal circuits are way more suceptable to noise than current loops. This is especially true if the voltage output is terminated in a high impedance. Thus, once again, the suggestion to terminate in a lower impedance,