As you all know, if the one shorts a decent multimeter props, it shows 0V if the one is measuring voltage. But, when I short the op amp inputs, I get few micro volts on the output (depends on the op amp and its input offset)! So my question is, how to get zero volt on the output of op amp without effecting the measures value ( I mean if I'm using an op amp to sense current)
Regards,
Tommy
You can use an opamp which has offset pins, like NE5534.
Or calibrate it out in software.
Or us one of the methods described here:
http://www.analog.com/media/en/training-seminars/tutorials/MT-037.pdf
Or use a different opamp with insignificant offset.
Thank you for your reply!
Actually, I tried an OPA277 which has an offset trimmer (and very low input offset which is around 10uV), but, as the output is very low (tens of uV) and swinging a lot, it was so hard to trim it!
Any idea?
Regards,
Tommy
It may happen if its rail to rail performance is not too good, are you powering it from a split supply?
It may happen if its rail to rail performance is not too good, are you powering it from a split supply?
Thank you for your reply!
Yes, it's powered by dual supply. (Split supply using another AD8629 to split 3V3).
Please check the schematic above!
Regards,
Tommy
You may also add a small capacitor in parallel with R3 to limit the bandwidth and make the circuit easier to adjust. Try 1 nF or 10 nF. However, getting down to 1 uV offset range is very difficult as the op amps and the resistors are very sensitive to temperature changes. Have you checked any zero-drift op amps yet? They tend to have very low offset (typ. less than 10 uV down to 2 uV or even better) and you can then use the offset trim shown in figure 6 in the app note mentioned above. Please bare in mind that the trimpot may also drift with temperature.
You may also add a small capacitor in parallel with R3 to limit the bandwidth and make the circuit easier to adjust. Try 1 nF or 10 nF. However, getting down to 1 uV offset range is very difficult as the op amps and the resistors are very sensitive to temperature changes. Have you checked any zero-drift op amps yet? They tend to have very low offset (typ. less than 10 uV down to 2 uV or even better) and you can then use the offset trim shown in figure 6 in the app note mentioned above. Please bare in mind that the trimpot may also drift with temperature.
Thank you for your reply!
I'll make a cup of coffee and straight ahead to my work bench
But dude, AD8629 has typical input offset voltage 1uV and max up to 5uV!
Do I need zero drift op amp?
Regards,
Tommy
If you're hoping to measure down to 10s of uV, the input VGND, output VGND, and R2 VGND need to be physically VERY close together, any u Volt drops in the gnd between them will appear on the output.
You may also add a small capacitor in parallel with R3 to limit the bandwidth and make the circuit easier to adjust. Try 1 nF or 10 nF. However, getting down to 1 uV offset range is very difficult as the op amps and the resistors are very sensitive to temperature changes. Have you checked any zero-drift op amps yet? They tend to have very low offset (typ. less than 10 uV down to 2 uV or even better) and you can then use the offset trim shown in figure 6 in the app note mentioned above. Please bare in mind that the trimpot may also drift with temperature.
Thank you for your reply!
I'll make a cup of coffee and straight ahead to my work bench
But dude, AD8629 has typical input offset voltage 1uV and max up to 5uV!
Do I need zero drift op amp?
Regards,
Tommy
AD8629 is a zero-drift op amp, good. I wasn't reading the thread very carefully, but just noticed that you have tried OPA227 but missed you last post.
The gain of the circuit is 11 (G=1+R3/R2), so the AD8629 offset voltage will be amplified also by that same gain: the 1 uV typical offset voltage will become 11 uV offset voltage.
Adding a 1nF - 10nF capacitor across R3 will limit the bandwidth and reduce the noise somewhat. Using the trimming method shown in the figure 6 of the app note, you may be able to tweak the offset voltage closer to zero. But like I said earlier, the temperature effects in the resistors and trimpot may introduce some drifting as temperature changes, so getting a clean zero output might be a bit frustrating experience.
I think most everyone who has responded missed this part of your initial question:
I get few micro volts on the output (depends on the op amp and its input offset)!
You've got an opamp configured with 11x gain. Even with 1uV of offset, you're going to get 11uV out. Then you get to factor in all of the thermoelectric effects which can easily add a few more uV to the picture. There are applications where this matters, and for those there are ways to minimize these effects. This takes careful design and board layout and similar.
Fortunately current sensing (your stated application) isn't one of them. Typically if you're sensing current you're going to introduce a voltage drop into the sensed circuit on the order of tens of mV, not uV. At 11x gain, this produces an output so large that a few uV isn't going to matter, especially given that you're unlikely to be able to introduce a current sensor (aka resistor) into the circuit with enough absolute accuracy that that 10uV matters.
If you're willing to share a few more of your application details, we might be able to point you in a more acheivable direction. Depending on the exact specifics, sometimes a traditional opamp isn't even the correct solution, especially when sensing currents on higher voltage sources (i.e. more than a few volts).
If you're hoping to measure down to 10s of uV, the input VGND, output VGND, and R2 VGND need to be physically VERY close together, any u Volt drops in the gnd between them will appear on the output.
Thank you for your reply!
Good point to think with!
regards,
Tommy
If you're willing to share a few more of your application details, we might be able to point you in a more acheivable direction. Depending on the exact specifics, sometimes a traditional opamp isn't even the correct solution, especially when sensing currents on higher voltage sources (i.e. more than a few volts).
Thank you for your reply!
I'll start from the beginning then.
The goal is to make (or at least trying to make) an ammeter with range of 5A to 1uA without using several shunt resistors to switch between for every range.
So across just one shunt resistor, voltage drop range is between 500 mV and 0.1uV. The ADC much be 24-bits to handle this wide range. I chose 10 times gain so my new range is 5000mV to 1uV.
As long the shunt resistor is shorting the two op amp inputs without any current going through (without any voltage drop), so the difference between the two terminals must be zero (theoretically) and the ADC must read ZERO on output, which I'm not getting up to now.
Regards,
Tommy
What test equipment are you using to measure the offset ?
What is absolute accuracy you want in measurement, temp range
environment it will be used in ?
Thank you for your reply!
I'm using Voltcraft with minimum resolution 10uVDC, so I could able to get values => 10uV.
I wanna be able to read down to 1uV +/- 0.1uV @ 25 degrees C.
Regards,
Tommy
Adding a 1nF - 10nF capacitor across R3 will limit the bandwidth and reduce the noise somewhat. Using the trimming method shown in the figure 6 of the app note, you may be able to tweak the offset voltage closer to zero. But like I said earlier, the temperature effects in the resistors and trimpot may introduce some drifting as temperature changes, so getting a clean zero output might be a bit frustrating experience.
It rather worked!
But because my multimeter as I mentioned before has 10uV as a minimum resolution, so I'm not sure what does the 000.00 mV on my multimeter screen means
Anyway, I changed my resistors values to 20 and 180R so I got 10 times gain with less noise and bias current of course.
And for the external offset trim I used a 20K resistor and 20K trim-pot and as I said, zeroooo, but I wasn't sure what if it was under 10uV.
Anyway, I'll wait for my 24-bits ADC or I have to buy a new multimeter with more resolution.
OBS! I didn't use any caps because I wasn't sure about the noise value!
OBS! I simulated my circuit on LTspice and got 10uV offset voltage!
Regards,
Tommy
I looked at several voltcraft manuals, they shows 100 uV resolution,
and accuracy in range of +/- .5% to .6%, read (on 200 mV scale) an
accuracy +/- 1000 to +/- 1200 uV.
Regards, Dana.
Adding a 1nF - 10nF capacitor across R3 will limit the bandwidth and reduce the noise somewhat. Using the trimming method shown in the figure 6 of the app note, you may be able to tweak the offset voltage closer to zero. But like I said earlier, the temperature effects in the resistors and trimpot may introduce some drifting as temperature changes, so getting a clean zero output might be a bit frustrating experience.
It rather worked!
But because my multimeter as I mentioned before has 10uV as a minimum resolution, so I'm not sure what does the 000.00 mV on my multimeter screen means
Anyway, I changed my resistors values to 20 and 180R so I got 10 times gain with less noise and bias current of course.
And for the external offset trim I used a 20K resistor and 20K trim-pot and as I said, zeroooo, but I wasn't sure what if it was under 10uV.
Anyway, I'll wait for my 24-bits ADC or I have to buy a new multimeter with more resolution.
OBS! I didn't use any caps because I wasn't sure about the noise value!
OBS! I simulated my circuit on LTspice and got 10uV offset voltage!
Regards,
Tommy
You probably need to increase the feedback resistors in order to reduce the output current, as you are now very close to the maximum specified output current +/-10 mA. I would use something larger values like 200 ohm and 1800 ohm.
The trimpot of 20 kohm is okay, but you now also have to increase the 20 kohm resistor to 200 kohm or more: You can simulate the circuit using different values for the resistor from 200 kohm resistor up to 2 Mohm so that offset trim doesn't affect the gain, but you are still able to adjust the offset +/- 15 uV.
The goal is to make (or at least trying to make) an ammeter with range of 5A to 1uA without using several shunt resistors to switch between for every range.
Measuring 5A down to 1mA is hard enough, so good luck with that.
For measuring a few uV across a current sense resistor a INA226 is going to be hard to beat.
www.ti.com/product/INA226
Measuring 5A down to 1mA is hard enough, so good luck with that.
That helps a lot DUDE
Thank you
BTW, it was between 5A and 1uA, not 1mA, is it still hard enough?
Regards,
Tommy
You probably need to increase the feedback resistors in order to reduce the output current, as you are now very close to the maximum specified output current +/-10 mA. I would use something larger values like 200 ohm and 1800 ohm.
The trimpot of 20 kohm is okay, but you now also have to increase the 20 kohm resistor to 200 kohm or more: You can simulate the circuit using different values for the resistor from 200 kohm resistor up to 2 Mohm so that offset trim doesn't affect the gain, but you are still able to adjust the offset +/- 15 uV.
Isn't so weird 
I ran the simulation, but I changed R1=200, R2=1.8k, R3=10MEG, R8=100k and R9=96k and the simulation gave offset voltage below 1 uV measured from U1 output.
Very interesting project for a beginner!
An Ammeter with 7 digits resolution, made at home!
Using as a reference an instrument having an accuracy of 100 uV in the lowest range, while the requested resolution is 1 uV (accuracy?)!
How the amplifier will output 5V with 1.65V power supply is, of course, unimportant!
The Op-Amp noise figure is at least five times the requested resolution! (Signal 0.1 uV, Noise 0.5 uV typical)
... ...
After that, who cares about thermal voltages, interference...
With a little more effort, a ten or twelve digits instrument is easy to implement!
Very interesting project for a beginner!
An Ammeter with 7 digits resolution, made at home!
Using as a reference an instrument having an accuracy of 100 uV in the lowest range, while the requested resolution is 1 uV (accuracy?)!
How the amplifier will output 5V with 1.65V power supply is, of course, unimportant!
The Op-Amp noise figure is at least five times the requested resolution! (Signal 0.1 uV, Noise 0.5 uV typical)
... ...
After that, who cares about thermal voltages, interference...
With a little more effort, a ten or twelve digits instrument is easy to implement!
Well thank you for your reply first.
I updated the power supply to 5V so it will be +/- 2V5 so the op amp can handle max 2300 mV (which is 2.3 A).
I said at the begging that i'm trying to make this project real.
I'm a beginner, beginners make mistake and I don't feel shame of that.
But, the real shame is, to hang around and read beginners posts and problems and mention their failures in some way as ooooo I AM F%&KING GENIUS!
I bet people like you don't dare to post a question or do anything useful in their life!
Cheers,
Tommy