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| vonorfasyexela:
Hi guys! This is my first post here so please pardon mistakes etc. I have a device which can measure the resistance of a conductor using 3-wire measurement. To check that is works correctly we need a some kind of resistance imitator. For simplicity let's say that the device drives 1mA current into this imitator and measure the voltage across it's terminals. Then it calculates the resistance dividing voltage into 1 mA. I have developed this schematic as a resistor imitator: But when I power up this schematic (and NOT pushing any measure current between OUT and ground) I got this: Zoom in: So the gate voltage increases over time until it reaches around 4.2V. Then we can see some oscillation. This process repeats with some more or less stable period. Maybe someone can explain why gate voltage changes this way? If you need some extra info about the schematic etc please ask. |
| Zero999:
The circuit won't work. The op-amp will attempt to adjust its output voltage so that both inputs are the same voltage, which is 2.2*5(68+2.2) = 0.157V in this case. Why not just use a variable resistor? |
| SiliconWizard:
Yep. I didn't quite get where the "terminals" of this "virtual resistor" (which it's not) would be either? |
| 001:
U muddle with resistor vs load conception |
| SiliconWizard:
Couple thoughts: - Is your measurement device supposed to do precision measurements? If so, checking it would be best done with a series of precision resistors. Possibly a precision decade resistor box, or something. - If you don't care much about accuracy, but only want to do a rough check, using a variable resistor is obviously much simpler. - If you thought about implementing an active resistor just to be able to control the resistor value from a voltage/programmatically, you could just use a digital potentiometer instead (just check its specs carefully, especially voltage range.) I doubt you'll do better accuracy-wise implementing an "active resistor". - If you're still interested in implementing "active resistors", just look that term up in your favorite search engine. The most basic active resistor can be implemented with just one MOSFET. Will that accurately mimic a true resistor? Hardly. Will it be easily tunable? Not so much. But it's often used in CMOS design, as they take up a lot less area than "true" resistors. You'll also find more sophisticated implementations, but you'll have a hard time getting any to be even as accurate as a simple digital pot. |
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