Hi,

I am trying to amplify the voltage across a current sensing 0.001 Ohm shunt with a non-inverting op-amp. I then want to display this voltage on a digital meter.

I made a non-inverting op-amp circuit as per the common diagram with a 200 kΩ and a 1.8mΩ resistor. The output voltage does not seem to scale correctly.

The end of the resistor that is connected to ground in the diagrams is connected to the 0 V lead on a 12 V power supply. The -12 V and the + 12 V are connected to the V- and V+ leads on the power supply respectively.

Any idea what I could be doing wrong? I also would like to use the same power supply to power the voltage meter. Is this possible?

 

In this case, a schematic would help enormously. Here is a link with suggestions as to how to make one - http://forum.allaboutcircuits.com/showthread.php?t=28068

 

I've attached the schematic. Can you make some general comments on how a power supply is generally connected to an LM741 Op amp when making a noninverting amp. I was under the impression that you need a split power supply. If this is true and you power it with +12V and -12V, is the output of the amp referenced to the ground of the power supply, or in reference to +12V?

 

If you use a modern opamp with a very high input resistance then its voltage gain is 10 in your circuit.
The old 741 opamp you use has a minimum input resistance of only 300k ohms so the voltage gain in your circuit might be much less than 10.

The output of the opamp is referenced to 0V but it has an input offset voltage that will mess up the output voltage unless it is nulled.

 

Thanks. Can you recommend another op amp? I do not understand the comment about the voltage reference. If I were to use a 12 V power supply and connect V- to the ground and the negative power supply input, would this work?

 

I would use a TL081 opamp in your circuit. It has Fet inputs so its input resistance is almost infinite. Its inputs work at the positive supply voltage so it can sense current in the positive wire (then it does not need a negative power supply polarity) but it cannot sense the negative wire unless it has a negative power supply polarity.

The inputs of most opamps do not work when they are within a few volts of the positive or negative supply. It is called the allowed input common-mode voltage range.

 

I just tried it with the opamp you recommended. It scaled linearly, but not correctly. I can probably fix that by adjusting the resistors that I used.

When I connect the output from the op amp to the digital meter--the meter displays an error. I am powering the meter with the same power supply--so ground on the op amp is negative supply to the meter and common with the lower voltage side of the shunt--any thoughts?

 

I just plotted the input vs output. It is linear but with a 30 mV offset. Is there a way to remove the offset?

 

Hi

I made an non inverting amplifier with a TL081 op amp. I am using a ±12V power supply as the supply to the amp. I have used 200kΩ and 1.8 MΩ resistors. The voltage scaling works well, but there is a 30 mV offset. Is there any easy way to remove the offset?

 

Look to the datasheet on figure 4 and 5 on page 10
http://www.blecken.de/download/ti081.pdf

 

I appreciate the reference, but I honestly don't see how that is helpful.

 

it looks like if you place a pot. there, you would be changing the ratio of the two resistors. That would not be good. The scaling factor is correct, just the zero-offset is wrong. It seems like this should be straight forward. Can you point me to a discussion or reference on making common op amp devices with real world op amps?

 

These threads were directly related, so they are merged.

The typical offset voltage is 3 mv. Keep that in mind while applying a voltage gain of 10. That's where the 30 mv comes from.

Page 8 of the data sheet shows how to connect a pot to null the offset.

it looks like if you place a pot. there, you would be changing the ratio of the two resistors.

Not at all. Look to see what the pot's wiper connects to. You are injecting two currents that will tend to balance the input difference amp and drive the output to zero.

 

If you don't want to add an offset voltage nulling pot to an inexpensive opamp you could buy an expensive opamp that has its input offset voltage nulled at the factory by laser trimming.

 

I recommend that you get a 180K (200K||1.8M) resistor and put it in between your sense resistor monitor point and the positive opamp input. Be sure to disconnect the existing direct connection between the postive input of the opamp and the monitor point on the current sense resistor. This will arrange it so that both of the inputs of the opamp will see the same source impedance. This will tend to minimize the error offset.

hgmjr

 

If you don't want to add an offset voltage nulling pot to an inexpensive opamp you could buy an expensive opamp that has its input offset voltage nulled at the factory by laser trimming.

I think I know understand the source of the confusion. The op amp I am using is a TL082. It does not have the connections for nulling the offset like the TL081. Is it possible to null the offset of the TL082? If not, how would I determine what size potentiometer to use?

 

Your schematic and text clearly showed a lousy old 741 opamp, not a TL082 dual opamp with Fet inputs!
Both opamps need the negative polarity supply in your circuit.
You can fiddle around and add a small trimpot to the inverting input for offset adjustment. Use Ohm's Law to calculate the value of the series resistors for the trimpot.

 

Your schematic and text clearly showed a lousy old 741 opamp, not a TL082 dual opamp with Fet inputs!
Both opamps need the negative polarity supply in your circuit.
You can fiddle around and add a small trimpot to the inverting input for offset adjustment. Use Ohm's Law to calculate the value of the series resistors for the trimpot.

Sorry, I did originally use a 741, but I replaced it with the more modern one as per a posters advice. The circuit works perfectly now accept for the offset. I really don't understand how to determine the appropriate valve for the trimpot.

If I replace the opamp with one that has an offset nut what size pot should I used?

 

Have you tried my suggestion of introducing a 180K ohm resistor between the current sense monitor point and the positive input instead of directly connecting the positive input to the current sense montoring point? This may avoid the need for the nulling.

hgmjr

 

Have you tried my suggestion of introducing a 180K ohm resistor between the current sense monitor point and the positive input instead of directly connecting the positive input to the current sense montoring point? This may avoid the need for the nulling.

hgmjr

But now he is using a Fet-input TL082 dual opamp that has an extremely low input leakage current of typically 30pA (no bias current) for your un-necessary additional resistor. A 741 opamp would need the additional resistor but since its input resistance is so low then its gain will not be accurate.