op-amp output to 16F876A

[Chandu06450]

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..

 

[ericgibbs]

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

Hi,
Look at the LM335Z datasheet

 

[Benjermin]

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.

 

[Pommie]

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."

Mike.

 

[Benjermin]

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.

 

[Pommie]

i know that the thermocouple measure temperature and ouput as voltage,

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.

Mike.

 

[ericgibbs]

Morning Mike,

The basic circuit for the LM35 only covers +2C thru +150C.

IIUC the OP wants to go down to -4C.
This can easily be done, but it will require some changes to the LM35/PIC interface.

The LM335 type is in Kelvin, so a little program maths is required.

 

[Benjermin]

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.

 

[ericgibbs]

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.

hi,
I dont see any results.?
Please post a clear drawing showing the thermocouple and the amp with the supply voltages.

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Give it a go now.

 

[Benjermin]

here is the reuslt

 

[ericgibbs]

here is the reuslt

hi,
Looking at your results you only have a nominal gain of about 77.?

Please post your circuit.

 

[Benjermin]

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.

 

[ericgibbs]

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.

 

[Benjermin]

here is the amplifier circuit

 

[ericgibbs]

here is the amplifier circuit

Hi,
Plotted your 1st resusts.
The slope is OK, nice and linear, BUT the OPA has a -mV error offset.
You will need to add a zero adjust pot circuit.

How did you measure the mV inputs.?

EDIT:
Check the output of the OPA with the inputs shorted, post what you measure.

EDIT2: added a second plot, shows the error more clearly.

 

[Benjermin]

You will need to add a zero adjust pot circuit.

how to connect the zero adjust pot circuit? can show me the schematic of the circuit?

 

[ericgibbs]

how to connect the zero adjust pot circuit? can show me the schematic of the circuit?

hi Ben,

The usual method is a 10K multi turn pot about 10K.
Connect one end of the pot to +V via 47K resistor, the other end of the pot to -V via a 47K.

The wiper of the pot is connected to the INV inp of the OPA via a 10K resistor.

Disconnect the thermocouple, connect the NI inp to 0V via a 1K0, adjust the pot slowly for 0V out from the amp.

Both halves of the dual amp require this zero pot.

 

[Benjermin]

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?

 

[Benjermin]

Check the output of the OPA with the inputs shorted, post what you measure.

when the input is shorted, the output is fluctuated from 33mV to 36.4mV

 

[audioguru]

The output fluctuates because the lousy old MC1458 dual opamp is noisy. You should use a more modern low noise opamp.