I have a problem getting opamp to work as I want. What I want is that input voltage of 0 to 20 mV is transferred to 0 to 2 V but when input difference is 8.3 mV I get output of 640 mV (gain is 640/8.3= around 77) but not around 100
I use LM358.
Another test is while input 5 mV output is 300 mV (gain =60)
INPUT IS USED FROM THERMOCOUPLE from soldering hand
The LM358 is not the way to go ..
You will always have problems with low output voltages ..
For this application you should consider a rail-to-rail (at least output) opamp ..
Examples:
LMV358M, TLV2252, ..
None of these opamps exhibits feature (rail-to-rail output swing) that is required in your application ..
If you want to operate with single supply (0-5V) you need an opamp with rail-to-rail output swing, or another option is to use the LM358 and supply it of symmetrical power supply, for example (-5V 0V +5V) ..
GOOGLE using the following key-words:
OPAMP + RAIL-TO-RAIL
Another technique would be to shift the reference (shunt resistor currently connected to common) to 1.0V This would lift the opamp output off the bottom rail. Too much gain and you hit the other rail though.
Ok I got AD 623AN from a friend.
I used 413 Ohm resistor to set gain 242 and it works good on low signal but I saw it's top output is 1.17 V (I need 5 V top) so when reaching 20 mV input, output is 1.17 V
Ref pin is tied to ground.
Look at this: ( I need output from 0V to 5 V) what should I connect to ref pin in order to get it
First of all, in the above example the output voltage range should be 1.211V to 3.077V ..
In your case:
(-) input and Vref (it provides a direct means of injecting a precise offset to the output) should be connected to 0V ..
(+) input 0-20mV ..
R(gain) ≈ 401.6Ω ..
The the output range should be 0V to 5V ..
use ad620 instrumentation amplifier.i used it for 10 to 500 gain in different applications.i used it for 1mV to 30mV input voltage.and its working great.
Can you exactly tell me how did you connect it? Every pin
I connected it by picture above (measuring thermocouple's voltage) execpt ref was at GND and without capacitor
Added after 2 hours 44 minutes:
I tried setting gain to 2 and I put 1V and GND ( to +IN and -IN ) and I got 2 V output (ref and -In grounded).
Hmmmm it works but not with gain of 1000 for example I it won't go above 1.15 V. It does amplify signal to 1.15 V but when output reaches 1.15 V it stops for some reason?
Added after 2 hours 49 minutes:
Got it solved .... had to apply aditional Vcm voltage