btw, at present I am planning to power the whole circuit with the same SMPS.
do you think i should go for another regulator just for the bridge in case i need a 0.5 deg C resolution from my application and the range I am targetting is -20 to 200 deg C?
As far as sensor lead noise is considered, is it a fair idea to have disc capacitors put up around both the lower resistances of the bridge?
I am at present using 1% resistors. Not sure if I may get the TCR type you mentioned. I shall check though.
I have tried to offer the same resistance to both the bridge arms.
The 3-wire RTD in my design may have 3 wire lengths equal to 50-100ft as well. Considering that each lead offers a resistance of Rw.
I have thought that the current through each arm of the bridge would traverse
5v->4k7->100e->Rw->Rw->Gnd thereby offering the bridge balance with whatever length the RTD wire has!
Please correct if I am missing something.
I shall try to experiment as far as putting capacitors is considered.
I had a look at the weblink you sent. Looking at the 3-wire configurations,
"You make this connection" and at the alternative 3-wire config, "Rd (user supplied)"
Figure-4
Can you explain it a bit please?
Yes, other than your calibration resistor (potentiometer) and perhaps a small series resistor associated with it, the other two resistors should be precisely matched with exceptional temperature coefficient. And with the potentiometer calibrated to the RTD at 0 degrees, or with a probe simulator, the potentiometer will balance with the RTD and wire resistance will be accounted for as well.perfect.... i got the explanation. Thanks mate.
So, if my design is now not involving any current sources but the bridge, then the resistors being in cloce match will decide the accuracy of the desing.
Am i right?
I would think not, unless perhaps you want to short protect it some kind of way with a current limiting resistor, perhaps 1kohm of the top of my head. It should have no negative effect either way, except a slight, slight delay in the responsiveness of the probe.[/quote]Other story is the input impedance of my amplifier being very high will not need any componets between the amplifier terminal and the rtd voltmeter probe lead.. right?
What is your feeling about the error that may be there (if it is) in my design due to the lengths of the RTD 3-wires if the length is variable... over the one that the calibration is done for?
ah.. now I got it all, I think.
I shall be calibrating the circuit to the known length and later leave some scope in the processor software if needed.
Let me try it sometime in nxt week with the approach your initial schematic showed. Wish me luck. I am really grateful to you for all the painful efforts you took to stand by me. I shall keep you posted with the design results. I hope you would help in future on this design if need be!
BTW, I had a relook at the IN Amp AD623A datsheet I have. It apears that the input signal is applied across a PNP transistors base-emitter (N-P) junction. Hence, it offers a very high input impedance. My thought.
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