same thing happened to me, when I was working for some other project at voltage regulator part. My problem solved when I change battery( power supply) to the circuit.
Previous battery was discharged a bit, and created that problem..
the temperature range i wish is about -4 to 100 Celcius
the thermocouple type i use is a white wire and yellow wire connected together and the outer skin is brown. I think it is a J type thermocouple. I didnt have the data sheet but i have search from website the data sheet.
From my experiment on the thermocouple
after i boil the water and measured, the ouput voltage is only 2.3mV-2.6mV range
i found that the output of the amplifier is not accurate, the output that i expected and i get in reality have alot of deviation espacially when small input voltage is apply. Is there any method to increase the accuracy?
The error will reach about 50% and above when the op-amp input voltage is smaller than 1.2mV.
You don't appear to understand how a thermocouple works or the need for cold junction compensation. For the temperature range that you require a thermocouple is not a good choice. As suggested by Eric, a LM335 would be a much more suitable device.
Have a read of wikipedia's entry. Especially the part where it states, "Thermocouples measure the temperature difference between two points, not absolute temperature."
i know that the thermocouple measure temperature and ouput as voltage, but my problem now is on the amplifier, is it the amplifier cant operate accurately when the input voltage is small?
About the LM335, i have see the data sheet already, but i not sure it will have the same problem as the op-amp i currently use or not.
No it doesn't. It gives you a voltage that is proportional to the difference between the thermocouple and the ambient temperature and so you need someway to measure the ambient temperature. Sounds like catch 22 to me.
If you use an LM35 (not 335) you will be able to feed it directly into the ADC and get a resolution of ½°C. No opamp required. Or, use a one wire device such as the DS18B20 and get accuracy to 1/16th°C.
currently, i have a quesation about the op-amp, is it op-amp cant amplify the input voltage that is small (a few milivolt), i try to input a few milivolt to the noninverting input (i contruct a non-inverting amplifier). But the result i get is not satisfactory. Here i have attach my result.
currently, i have a quesation about the op-amp, is it op-amp cant amplify the input voltage that is small (a few milivolt), i try to input a few milivolt to the noninverting input (i contruct a non-inverting amplifier). But the result i get is not satisfactory. Here i have attach my result.
by using the formular of non-inverting amplifier: Vout = Vin (1+R1/R2), the calculated gain are 101.4069176 and 115.6285714
but what i get from my result is absolutely wrong, it deviate alot from what i want, that is wierd, so i suspect that [the input voltage at the non-inverting terminal cant be too low], but i not sure this point is correct or not.
by using the formular of non-inverting amplifier: Vout = Vin (1+R1/R2), the calculated gain are 101.4069176 and 115.6285714
but what i get from my result is absolutely wrong, it deviate alot from what i want, that is wierd, so i suspect that [the input voltage at the non-inverting terminal cant be too low], but i not sure this point is correct or not.
Is there a problem with posting the circuit diagram,
its pointless telling us that its not working.
I would like to help you solve the problem, so plllllllllllllllllllllzzzzzzzzzzzzz
post the circuit diagram for the op amp... so that we can see what you are doing wrong.
about the zero pot, is it just an add-on circuit of the non-inverting amplifier when tuning the the voltage to 0V or i need to build the 0 pot and tune to 0V before i build the non-inverting amplifier?