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I thought as much, I ordered some 1% resistors with the LMC662's.ljcox said:Oh, I meant to add that you should use 1% resistors.
ljcox said:R4 is necessary in order to make it a diff amp with minimal thermal drift.
R6 could be eliminated by choosing R1, R2 & R3 such that their Thevenin resistance is = 10k.
This leads to my next point. The circuit posted by Sig239 overlooked the need to make the Thevenin resistances on the + and - inputs equal. And I missed it also.
So I've drawn the attachment to do this and to use a 100k feedback resistor rather than 50k as I suggested.
The reason the Thevenin resistances on the + and - inputs must be equal is to minimise the thermal drift that would otherwise occur due to changes in the input bias currents when the IC temperature changes. However, this is less of an issue if the op amp has FET inputs.
Chris_P said:I got a bit sidetracked and I have only just now managed to build and test the circuit Len kindly designed for me. A couple of things are confusing me. Originally I planned on using a voltage divider to give me the reference voltage of 2.532V for the inverting input. My voltage divider had 5V - 9.75K - 10K - Gnd which should give 2.532V. But on Len's design the voltage divider is 5V - 10K - 9.75K - GND with an extra variable 200 in series with the 9.75K, but this only gives me 2.468V - 2.494V. Is there a particular reason for this?
Chris_P said:While testing the circuit I am finding that the temp readings are 7 - 8 degrees C too high. Even swapping the resistors in the voltage divider around doesn't change this much. I am trying to get my brain around the circuit and how it works but I am struggling. Would varying the reference voltage to the inverting input be a way of calibrating the circuit? Are the resistances used for the voltage divider (10K and 9.75K) important to making the Thevenin resistances on the + and - inputs equal? Can I replace that part of the circuit with a multiturn trimpot and if so, what value would I need? I have a 10K one, would I need a 20K one instead?
Yes, you are right, it is approx 4.98V.eng1 said:The supply voltage might not be 5 V exactly. You can add another resistor between the trimmer and ground.
I was hoping to get away with just 2 wires to the sensor. The sensor is going to be external and I was thinking of using an RCA socket and mount the LM335 on one end of an audio cable. I'll have to do some experimenting, thanks.eng1 said:You can use the multiturn trimmer to calibrate the LM335 instead. That's what I usually do.
Leftyretro said:It's common in instrumentation measurement applications to design in more gain then your calculated amount, say X10 instead of X5. Then have the amplifier output go to a 20 turn trim pot and adjust and take the desired output from the wiper. That way you have two calibration adjustments, a 'zero' adjustment pot that drives the inverted input and a 'span' adjustment to set the range. That way your circuitry will not be so dependent on having 'perfect value' feedback resistors. Anyway that's the way the process control industry does it frequently....
Good luck
Chris_P said:Me again, its taken me a while to make a waterproof LM335 sensor and get another digital thermometer to make some comparisons. Readings are pretty close but I think I would like to try these recommendations above. I have modified the drawing Len did for me and attached it to show what I have and what I think Leftyretro is suggesting.
What I am not sure on, (more stupid questions) what resistors do I need to change to change the gain from 5X to 10X? And what would be a suggested value for the 20 turn trimpot on the output?
If this circuit is correct how would I calibrate it properly? ie how would I set the "zero adjustment" pot and the "range pot"?
Just wondering if anyone has an idea how I do what gootee has suggested. I need to generate a DC voltage from 2.532 to 3.532V to calibrate my thermometer.gootee said:For calibration, someone else will probably have the answer. But conceptually, if you had a signal generator that could produce a waveform with your 1V p-p amplitude, and also provide a DC offset so that the signal generator's output was covering the same range that your temperature sensor would cover, then you could just connect the signal generator in place of your sensor, connect your circuit's output to an oscilloscope, and adjust the pots until the output was covering 0 to 5 Volts. Actually, such a signal generator could be easily and cheaply built, with another opamp or two (and in many other ways).
seveprim said:Instead of trying to get the LM335 to do the task why not search for a better temp IC !!!!
Note you cant get 0.1 c accuracy from a device that is not that accuarate ie +-3 degrees if you want the accuaracy got to go a K type thermocouple probe and something like AD595 then you can get what ya want ( i have code and schematics but I was measuring 600 c and higher !
Leftyretro said:Well actually thermocouples are not all that accurate either. Purity of metals used and quality of construction make only the higher cost lab quality TCs capable of decent accuracy but no where near +/- .01C. TCs strengths are in their wide temp range, ruggedness and lower costs.
RTD sensors are considered much more accurate then TCs but even then +/- 0.1C is probably not obtainable except for selected high cost lab types. In many practical applications good repeatability in a sensor is all that is required and doesn't carry the costs of extreme accuracy specifications.
Lefty
Lefty
seveprim said:I am afraid you are wrong on the thermocouple they produce a known voltage per degree the typical value for a K type is 40.44 micro volts/°C if they use impure metals that voltage would not match the specification for a K type as such metal purity is very consistant I have used these devices along with AD595 and can very easily get 0.1 c reading and yes I have had it checked with calibrated meters.
The AD595 converts the 40.44micro volts per degree to a 10millivolt/°C depending on the analog to digital type you will be able to read fractional values of a degree. The errors come into play are caused by chips(AD595 and the ADC) itself not the K type probe( two well specified metal types welded together under controled enviorments) but I conceede there could be manufacturers who make bad ones!
Any chip errors can usually be sorted with a lookup table to correct it.