non inverting opamp used for low side current measurement has got too high gain

[wanna_eat_1kg_of_elecons]

Hello,
i did build a non inverting opamp like this one:

https://upload.wikimedia.org/wikipedia/commons/6/66/Operational_amplifier_noninverting.svg


Between the V_in and the ground is a 0.01Ω Shunt.
Rf is 156k and Rg is 10k

I expect:
A gain of 16.6
because the formula is Gain = 1+ Rf/rg = 1+156/10 = 16.6


But:
when i have got a voltage of 47mV on the shunt i get a output voltage from the opamp of 0.99V.
When i have got voltage of 94.6mV on the shunt i get a output voltage of 2.01V...

This means the gain is always about 21. :-(

What i did was ad a resistor between the + input of the opamp and the high end of the shunt. This did not improve a thing. I red somewhere this is necessary is this true?

I measured the resistors the values i measured were:

9.999 k for the Rg and 154.826 K for the Rf resistor this way i should get a 16.5 gain and not a gain of 21....

I dont need a opamp with a accuracy of 1% but 5% would be nice.

The question is what is going wrong here? I know i can use trimpots but for me this is not a option.

edit: forgot to tell i uase a LM324 and the voltage is 6.2V the low voltage is 0 volts

 

[ericgibbs]

whats the output from the LM324 when you have 0V on the non invert input, ie connected to 0V.?

 

[wanna_eat_1kg_of_elecons]

I shorted the shunt and then i get a V_out of -3.6mV. Is this a answer to your question?

 

[ericgibbs]

I shorted the shunt and then i get a V_out of -3.6mV. Is this a answer to your question?

hi,
Just checking that the low output limit of 5mV typical and 20mV max was within specification.

Do you have an output load of approx 10K on the output.??

 

[wanna_eat_1kg_of_elecons]

I have got a 7k5 resistor connected between the output and the + of the next opamp within the same dip package the next opmap is used as a comparator there is nothing else connected to the + of the next opamp only the 7k5.

+ = non inverting input

 

[wanna_eat_1kg_of_elecons]

I also measured the voltage on the non inverting input of the text opamp which is connected trough the 7k5 to the output of the non inverting opamp this voltage is about -0.9 mV...

where did you get the 20mV? i can only find input offset voltage in the datasheet which could be 9 mV max. (page 5).

 

[ericgibbs]

I have got a 7k5 resistor connected between the output and the + of the next opamp within the same dip package the next opmap is used as a comparator there is nothing else connected to the + of the next opamp only the 7k5.

+ = non inverting input

Keep the 7k5 in place and connect a 10K resistor from the LM324 output to 0V.

 

[wanna_eat_1kg_of_elecons]

i shall connect a 10k resistor from the output to gnd. And tell you the results. Can you please explain the reason to do this so i can learn and understand?

 

[wanna_eat_1kg_of_elecons]

Yes Yes!! you are great!! now i have got a gain 16.59 /me bakes ericgibbs a chocolate cake happy happy me!! Is it always necessary to pull down the output of the opamp ? I am also using a comparator with hysteresis do i need to pull down that one also?

 

[ericgibbs]

Yes Yes!! you are great!! now i have got a gain 16.59 /me bakes ericgibbs a chocolate cake happy happy me!! Is it always necessary to pull down the output of the opamp ? I am also using a comparator with hysteresis do i need to pull down that one also?

Its just that the output specification of the LM324 is given with a 10K load.

Which type of comparator are you using.??

BTW: make that Dark chocolate , not Milk,,

 

[wanna_eat_1kg_of_elecons]

OK i found that they use a 10k load describing the "Output voltage Swing"is that the part you used? I am using the same LM324 as a comparator, don't need speed and accuracy of 5% is more than enough.

Well the best chocolate cake is the cake with one pound (1/2 kg) of dark chocolate 8 eggs split the white and yellow, mix the white part till it gets hard trow in the melted chocolate, and almost no flower ) nom nom nom.... but that is a bit off topic must have the real recipe somewhere .... but it was something like that

 

[ericgibbs]

hi,
For a simple comparator a LM324 should be OK, no pull up or down required.

 

[audioguru]

Please could somebody translate the SVG file and post the schematic as a PNG file?

 

[wanna_eat_1kg_of_elecons]

What would be realistic values for the gain of a non inverting opamp like this (connected to a shunt and with a lm324)? I thought It would be better to increase the gain and lower the power rating of the shunt so not use a 0.01R(10mOhm) shunt but use a 0.005R (5mOhm)shunt and make the gain 33 and not 16.6.

 

[ericgibbs]

Please could somebody translate the SVG file and post the schematic as a PNG file?

hi,
Its a bit small.

 

[Blueteeth]

What would be realistic values for the gain of a non inverting opamp like this (connected to a shunt and with a lm324)? I thought It would be better to increase the gain and lower the power rating of the shunt so not use a 0.01R(10mOhm) shunt but use a 0.005R (5mOhm)shunt and make the gain 33 and not 16.6.

Note, all opamps have input offset voltages, which are amplified by your gain. The use of a smaller current sense resistor is appealing (lower power dissipation = smaller resistor) but even with precision resistors setting the gain, the lower the voltage across the sense resistor, the more prominent the offset error will become. Eric - jump in if I'm wrong here.. as you know from my thread on current sensing, opamps aren't my strong point.

You can always calibrate it after building with a smaller value trimmer to zero it and check it against a known precision current. That, to me at least, sounds more appealing than buying a more expensive opemp with a small offset, when only one of the opamps in the package needs this precision.