Wirign of a Op-Amp MCP6002

Hi, I got myself an MCP6002 to measure a voltage that was too small for the arduino scale. However when I wired the circuit as in the specs sheets, the Vout that I measure is the Value of the Vdd (5volts). Any idea why is not working? Do you think it may be broken? Thanks,

Luis

Unfortunately we need more information. Have you measured the input voltage to the op amp with a meter? How much gain do you have? Give us the values or R1 and R2.

Hi, Thanks for your message.

I am using an R1 of 100 kohms and R2 10 kohmsn for a gain of 101.

I am measuring a voltage drop across a resistance. When it is at is lowest is 0.2 mV (measured with a multimeter).

Still i am getting the maximum voltage .

Thank you,

Luis

I am measuring a voltage drop across a resistance.

If one end of the resistor isn't grounded, you might have a ground problem... So, we might need to see the circuit with the resistor and we would need to know if it shares a common ground with the Arduino.

Typically, you need to totally isolate your "voltmeter", or you need to measure with reference to your "voltmeter" ground. i.e. You can connect the ground/black lead of your multimeter to a + voltage, and the positive/red lead to ground, and you'll simply get a negative reading. That works because the multimeter doesn't share a common ground with whatever you are measuring...

Or if whatever you are measuring shares a ground with your voltmeter, you can measure the voltage with reference to ground... That is, you can measure the voltage "across" the resistor if one end of the resistor is connected to ground. In most Arduino applications, we are measuring with reference to ground.

As a quick check of you amplifier circuit, try connecting Vin to Vref. You should get (about) 0V out of the op-amp.

Another test is to make an approximate 1/200 [u]voltage divider[/u]. Connect it to 5V and you should get about 2.5V out of the amp. (101 x 5/200).

100k and 10k as a noninverting amp is a voltage gain of 11, not 101.

Can you show us a schematic?

That op-amp has an input-offset range of +/-4.5mV, so you won't be able to measure 0.2mV with any accuracy.

Don’t forget that offset voltage gets amplified, too.

polymorph:
100k and 10k as a noninverting amp is a voltage gain of 11, not 101.

Can you show us a schematic?

Hi,

Thank you for your message. You are right about the gain.
I am attaching a schematic.
Thanks,
Luis

I am attaching a schematic

No you are not. That is a physical layout not a schematic. Why are you only powering it with 3V3?

I think the Op Amp can work at 3.3V . or 5 V should be fine too.

I hope this is the one image needed .
Thanks,

Luis

think the Op Amp can work at 3.3V . or 5 V should be fine too.

Yes but if you wired it like you said then there is no way it will output 5V, like you said you saw.
A schematic is what you wired not some page off a data sheet.
Face it you are doing something wrong, we can only help you find that out if you say exactly what you have done and the results you have seen. So far your record on this is not good.

I suspect that your main problem is that the “circuit pulling current” does not have a common ground with the op amp and the Arduino.

Indeed its highly likely that the “circuit pulling current” is at a potential outside the range of the opamp - but
you haven’t provided a schematic drawing or photo of that part of the circuit so we don’t have the information
we need to solve the problem.

Have you measured voltages at the opamp inputs w.r.t. Arduino ground while it is operating? That’s crucial
knowledge here.

dc42:
I suspect that your main problem is that the “circuit pulling current” does not have a common ground with the op amp and the Arduino.

Hi,
Thank you for your message
I connected ground pin of the arduino with the Vss pin (pin 4). I measured 5 Volts when the power supplied is 5V and I measured 3.3V when the voltage supplied is 3.3V from the arduino. Do you think my chip could be damaged?
Thanks,

Luis

Measured that voltage where? We still need a schematic of your actual circuit.

Although for starters, I can see that you have the 5 ohm current sense resistor connected directly between the inverting and noninverting inputs. It should be connected to the noninverting input, and the other side of the resistor to ground.

But the two circuits -must- share a common ground. Do they? What is the circuit pulling current? Is that a high side or low side current sensing resistor?

Hi everybody,

Finally I managed to get the Op-Amp to work thanks to your messages.
I am posting the circuit. I also noticed that when I had the pins inverted from my “circuit pulling current” (VinA- and VinA+) I was sometimes actually dividing the voltage instead of amplifying.

However, It wouldnt work for the whole range, because the current changes and goes above the measuring range of the arduino (5V). I will still need to get a good external ADC. Can you recommend me one? thank you very much,

Luis

No external ADC can convert voltages higher than the power rail.if you want a wider range you must have a higher power rail.

LuisArduino: However, It wouldnt work for the whole range, because the current changes and goes above the measuring range of the arduino (5V). I will still need to get a good external ADC. Can you recommend me one? thank you very much,

Simpler solutions include:

  • use a lower value current sense resistor (so that the voltage drop is no more than 5V @ maximum current);
  • feed the Arduino pin from a voltage divider connected across the current sense resistor.

Can't you generate a schematic from Fritzing? It is very difficult and annoying to try to figure out from a pictorial how you have it wired.