The datasheet only seems to mention a "typical 120ppm" drift over 1000 hour timespan at 25C which isn't
very conclusive (A year is 8000 hours or so). Typical doesn't mean much in a datasheet, maximum
drift rating would be more useful.
Have you calibrated your multimeter recently too? Why are you only quoting 2 significant figures?
Your device is D-class accuracy (+/-1.0%) so you would expect it to read between 4.06 and 4.14V
in the first place.
I measure with the same multimeter not calibrated since last time. The measurement from today is 4.21 voltage.
I measure 4-20mA sensors with the arduino over a 250ohm resistors. and i use the lm4040 as a voltage reference on one analog pin. And today when i was measuring i never got the maximum value of the sensor. So i put the arduino with a powersupply and measured voltage and current over the resistor. I had 5 voltage over the resistor at 20 mA and 1023 counts on A5 pin but still not max pressure value.
then i measured the voltage over A1 and saw that i had 4.21 voltage. i then changed 4100 to 4210 and got the max sensor value when i had 1023 adc counts.
the code is like this
Cal_sensor = analogRead (A1);
Cal_value = 4100 / Cal_sensor;
sensorvalue = analogRead (A5);
// voltage to pressure 0 - 1,6 bar: y=kx+m. K = 0,4, m = -0,4
pressure = ((sensorvalue * Cal_value) / 1000) * 0,4 - 0,4;
So im pretty shure that i have 4.21 volt over the lm4040. And last time i did this calibration a year ago i had 4,1 volgate
Arduino has an inbuild reference voltage that might be more stable than what you have now.
Dump the reference part.
Replace the 250ohm resistor for a 51ohm/1%/metalfilm resistor.
Put this in the void setup() of your code: analogReference(INTERNAL);
Read the analogue pin normally.
Pre-reads and smoothing can further increase accuracy and stability.
Leo..
I can't use this method becouse i have a "LCD keypad shield" on my arduino. and the different button gives from 0-5 volt to A0 pin depending on what what button is pressed. so i need to be able to read 0-5 volt on A0.
Thats why i have my LM4040 on A1 and use this as reference, but now im not sure if i can trust my reference on A1.
Thanks Leo
Sorry i change my post above when I realized why I doesnt use the internal reference, Its becouse of my LCD keypad shield. so i need to be able to havr 0-5 volt readings on the arduino.
The question about my drifting lm4040 is still a mystery to me, and unfortunately I can't use the internal ref
Hi,
Do you have bypass capacitors connected to the LM4040, and is one of them an electrolytic?
If so have you checked or replaced it?
The TI apps circuits suggest a 10uF electro across it.
However the MAXIM spec says it doesn't need one.
sebb898:
Sorry i change my post above when I realized why I doesnt use the internal reference, Its becouse of my LCD keypad shield.
Understood.
If you have experience in smd soldering you could replace the top resistor in the keypad chain.
And recalibrate the keys.
Maybe easier to fix the problem with the voltage ref.
leo..
Things do fail.
I have replaced hundreds of drifting 33volt reference chips in TVs in the seventies and eighties.
Just replace the LM4040, and take it from there.
Leo..