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Using Digital Pot (DPM) as RTD simulator

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Harry Mughal

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Hi,
em using DPM AD8403 for simulationg RTD's. without excitation, meter shows a contant resistance value but when i excite the DPM by 1mA current then there are big fluctations in my voltage readings quite often. can any1 help me on this?? please its urgent...:sad:
 

barry

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Wait. You're putting a constant 1mA current through the pot and you're measuring the voltage: HOW? You need to be a little more specific, ESPECIALLY if it's so "urgent". Is your current stable? What's the impedance of the thing you're using to measure the voltage? What's the resistance value? How "big" are your fluctuations? How "often" do they occur?
 

Harry Mughal

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Wait. You're putting a constant 1mA current through the pot and you're measuring the voltage: HOW? You need to be a little more specific, ESPECIALLY if it's so "urgent". Is your current stable? What's the impedance of the thing you're using to measure the voltage? What's the resistance value? How "big" are your fluctuations? How "often" do they occur?

Yes, current is constant 1 mA but it comes through mux , for 40msec it remains constt 1mA, then it switches from one channel of mux to other and again remains constt for 40msec. i am measuring voltage via PC based HMI of a Data acquisition system. when i use simple pots , i get no fluctuations but with DPMs there are fluctuaions in temperature from 140C to 165C just as an example. if see the corresponding voltage being fed to ADC of the acquisition sys at oscilloscope i can observe fluctuations in voltage. some times there are no fluctuations with excitation as well but behaviour is quite random to get anythng.
 

Harry Mughal

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yes i am exciting DPM by 1mA current through mux actually, 1mA current continuously for 40ms, then mux switches to another channel hence a pause of smaller duration and again 1mA current for another 40ms nd so on. i am observing temperature on the PC based HMI as well as the voltage of ADC on the oscilloscope. Temperature (just a one case) varies from 140c to 165c, which according to the resistance-temp table corresponds to approx 50ohm. The same fluctuations can be observed at ADC input on scope. However if i don't excite my dpm, it shows a fixed resistance as of expected value. Some times after cold start of my DPM system, there are no fluctuations with excitation as well. but its quite random and i am unnable to point out the problem.
 

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So, you've established that your pots vary with temperature. AT 165 DEGREES C!!!!!

Did you bother to read the data sheet? Like, perhaps, the part where it says the maximum operating temperature is 125 degrees? And what 'resistance-temp table' are you talking about?
 

Harry Mughal

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barry thnx for ur response but i think u didn't get it. this 165 degrees is different from IC operating temp range u just mentioned above. 165 temp means pot is offfering approx 140 ohms resistance. 25 degree variation means pot's resistance value is varying by 6 ohms. nd u know abt RTD's then they have resistance to temperature conversion table. now my problem is that 6 ohm variation in pot's resistance value whenver i excite them by 1mA current source. without excitation . DMM shows fixed resistance. hope u get the point.
 

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My guess is that English is not your native language, so I think there's some translation problems here. I THINK what you're saying is that the resistance is stable, but then you put a current through it AND hook it up to your measurement system, (which you still haven't described) you get varying voltage, right? MAYBE your problem is your measurement system. I asked a number of questions in post #2, but you haven't answered. Have you looked at the voltage with current applied but WITHOUT hooking it up to your undefined measurement system to see if the problem still occurs?
 

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i think em quite clear now. Anyways as i have earlier mentioned abt measurement sys. it is a data acquisition system for multi channel RTDs. 1mA excitation for POT is also being generated by this system. The voltage generated as a result of this excitation is low pass filtered, then amplified, then ADC samples it nd via controller it is sent to the PC for display on GUI. This is my whole Data acquisition system. As mentioned earlier, i am also observing the voltage fed to the ADC on scope. To check the operation of my Data acquisition sytem, i connect simple mechanical POTS and there is no fluctuation in readings. i have ensured stable current source. After every sample voltage reading is changed and my sampling period for one POT/Channel is 1sec. Now all ur queries have been answered so hoping u give me some real solution :)
 

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thnx Garry for ur regard, My problem has been solved :-D
 

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i was considering my measurement system's ground to be isolated from my simulated RTD (DPMs) system. Then just as an experiment, i made both systems ground common, noise got removed nd i was happy at the end of the day but i was shocked by this coz DPM ws only acting as a variable resistor nd i didnt need to do thz for simple Pots. Well, i think its just because of the active nature of DPM as compared to passiveness of simple variable resistor. i don't think there could be any other justification for thz, what 'd u say??
 

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i was considering my measurement system's ground to be isolated from my simulated RTD (DPMs) system. Then just as an experiment, i made both systems ground common, noise got removed nd i was happy at the end of the day but i was shocked by this coz DPM ws only acting as a variable resistor nd i didnt need to do thz for simple Pots. Well, i think its just because of the active nature of DPM as compared to passiveness of simple variable resistor. i don't think there could be any other justification for thz, what 'd u say??

Your logic ground for your pots should be the same potential as the rest of your system ground. This is why posting a schematic would have gone a long way towards getting your question answered quicker.
 

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