What is the voltage measurement sensitivity of an OP-AMP?

[LooseJunkHater]

This is essentially a followup post to https://www.eevblog.com/forum/beginners/why-wont-this-constant-current-circuit-work/

Circuit in question: https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEBmAHAJmgdgGzoRmACzICcpkORKpISCkdApgLRhgBQyWjRcILZOhB9GYHOChQuPEfypyxRahEaQOfWmFwjUIdKj1E9yKNAlrY6ZGHh37kYeg7bqxkKkakLk2-DOIIABKTADOAJahAC4AhgB2AMZMHADuIN4owtoSQtIAZnQ6niB41MW5YNBIzmkGRiYIwu7qaaLgEm256gDmuh68ehlqHL3o6NQZYypgwsO9beiQRvxTUuoATorgyluLepacQfrjAuKMq2w6anJ0a2YIqeA6Ctnb1C1PEsRu-N-StSdiqshhwoihGn02s1JCxMDhiHgEEQZmgiDgsBizFRkAgsDY8Xx0LhIFhwPwACZMPIxACuABsoo8LmdjioLEyTmwcANTtz-lsubx+Cxih93IK+iwQeImsLim1BE4QJTqfSoiw6UxyZIxCU4BwjgqWUarlI+LdrjAHptxRlxeIpGM7BwAA6fErTLIGFBSTi9dhevQBt5rDibDK5DIsBTnewjdISaOegQx6RAA

I've been using Falstad a lot and messing around with this circuit and trying to fix some problems that I've been having with it (mostly related to heat). I thought about using a simple 0.1ohm resistor (which can be found for pretty cheap) as this will dissipate very little heat but it got me thinking; at what *voltages* will an op-amp be unable to amplify a signal (or actually begin to instead amplify noise)? Can most op-amps easily measure a signal voltage of 0.015v (between the inverting/non-inverting input and ground)? What about 0.001v (if I were to say, use a 0.05 ohm current-sense resistor in the circuit, at the BJT emitter)?

Why don't I simply go with a very-low resistance current-sense resistor (such as 0.05ohm) as it'll decrease wasted heat? I understand that at a certain point cost can be a factor, but will it effect the circuit?

Thanks!

[EPAIII]

Perhaps I am missing something here. But WHAT are you trying to do?

If you simply want to light up an LED, a simple, single value resistor in series will do the trick. Say so and I/we can give you the simple design procedure.

If you want to vary the brightness, find that single value resistor for the brightest you want it to be and then add a pot in series with it. With a large enough value on the pot you should be able to turn it completely off. Or pots even come with switches so, "click" and it is off. Again, say so and I/we can give you the simple design procedure.

There are many circuits for powering LEDs available on the internet. Instead of starting from scratch, I suggest you find one that is CLOSE to what is needed as a starting place and work from there.

If you are trying to build something you need to have a clear and precise (with actual numeric values - close anyway) statement of what you want.

Edit: I noticed that the later circuit used a 5 Volt supply instead of 40 Volts so I deleted references to 40 V.

[Andreas]

Hello,

a normal single suply OP like LM358 will have up to 10 mV offset voltage.
So you want at least 100-500 mV at the shunt for reasonable precision.

Precision OP-Amps like LT1013 have less than 1 mV offset.
So you can reduce the voltage drop at the shunt.
But at the same time you have to regard star grounding and voltage drop on the wiring.

with best regards

Andreas

[Jwillis]

Not sure if you noticed. Your LED Max Brightness is set to 600 amps. Try 20m. Change resistor at emitter to 15 Ohms. Should be able to change LED brightness on your circuit.

You can show voltages and current by adding wires in between components then editing the wire. Might help to figure things out.

[Doctorandus_P]

In general,  opamps react to differences of micro volts on their inputs, but all opamps have limitations, and the reason there are so many different opamps on the market is that those limitations are in some different area each time. Generic opamps can be very cheap, but have more severe limitations then more expensive "precision" opamps.
Knowing the limitations of opams and designing your circuit in such a way that those limitations become manageable is a very big part of designing circuits around opamps.

When dealing with current shunt resistors, the first limitation you get into is the input offset voltage of your opamp, and the value of the shunt resistor is a compromise between power dissipation an having enough signal to be able to measure the signal.

If you use an ADC and uC, then you can for example measure the offset of the amplifier, and compensate for that in software.

A very good paper about the limitations of opamps is the "Opamps for Everyone" (slod006b) Application Note.

[LooseJunkHater]

Hello,

a normal single suply OP like LM358 will have up to 10 mV offset voltage.
So you want at least 100-500 mV at the shunt for reasonable precision.

Precision OP-Amps like LT1013 have less than 1 mV offset.
So you can reduce the voltage drop at the shunt.
But at the same time you have to regard star grounding and voltage drop on the wiring.

with best regards

Andreas

Aha, "offset voltage" was the word I was looking for! Thank you!

I didn't actually think about voltage drop along the wiring to be a factor for the measurement and accuracy of the op-amp. However, I don't think it will play a major role in the measurement at a low current of 2A and a current sense resistor of 0.05 ohms (which in itself will have its own inaccuracy and will vary depending on temperature), right? Or am I wrong?

Will minor temperature changes actually greatly effect this circuit and lead to problems maintaining a constant current, if I use a 0.05ohm (or 0.1ohm) resistor?

[MrAl]

This is essentially a followup post to https://www.eevblog.com/forum/beginners/why-wont-this-constant-current-circuit-work/

Circuit in question: https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEBmAHAJmgdgGzoRmACzICcpkORKpISCkdApgLRhgBQyWjRcILZOhB9GYHOChQuPEfypyxRahEaQOfWmFwjUIdKj1E9yKNAlrY6ZGHh37kYeg7bqxkKkakLk2-DOIIABKTADOAJahAC4AhgB2AMZMHADuIN4owtoSQtIAZnQ6niB41MW5YNBIzmkGRiYIwu7qaaLgEm256gDmuh68ehlqHL3o6NQZYypgwsO9beiQRvxTUuoATorgyluLepacQfrjAuKMq2w6anJ0a2YIqeA6Ctnb1C1PEsRu-N-StSdiqshhwoihGn02s1JCxMDhiHgEEQZmgiDgsBizFRkAgsDY8Xx0LhIFhwPwACZMPIxACuABsoo8LmdjioLEyTmwcANTtz-lsubx+Cxih93IK+iwQeImsLim1BE4QJTqfSoiw6UxyZIxCU4BwjgqWUarlI+LdrjAHptxRlxeIpGM7BwAA6fErTLIGFBSTi9dhevQBt5rDibDK5DIsBTnewjdISaOegQx6RAA

I've been using Falstad a lot and messing around with this circuit and trying to fix some problems that I've been having with it (mostly related to heat). I thought about using a simple 0.1ohm resistor (which can be found for pretty cheap) as this will dissipate very little heat but it got me thinking; at what *voltages* will an op-amp be unable to amplify a signal (or actually begin to instead amplify noise)? Can most op-amps easily measure a signal voltage of 0.015v (between the inverting/non-inverting input and ground)? What about 0.001v (if I were to say, use a 0.05 ohm current-sense resistor in the circuit, at the BJT emitter)?

Why don't I simply go with a very-low resistance current-sense resistor (such as 0.05ohm) as it'll decrease wasted heat? I understand that at a certain point cost can be a factor, but will it effect the circuit?

Thanks!

Hi,

There are several limitations to op amps.
There is bandwidth, slew rate, and internal gain to start with.

The internal gain can be high like 100,000, which means if you want a 10v output then the input would have to be no less than 100uv if DC, but in reality it may have to be higher.  If AC, you have to consider how the bandwidth reduces the gain, and depending on the output amplitude you may have to consider the slew rate as well, especially if an AC signal or square wave.  DC is simpler.

The input offset is an equivalent input voltage that gets amplified along with any signal.  If the input offset is 2mv (typical of the LM358) and the gain is 100, the output will be at 0.2 volts.  That could be a problem, but if you are using an ADC to measure the output you can subtract that from the reading as long as the linearity is still ok.
They do make special op amps though that have very tiny input offset specs, and they are referred to as chopper stabilized op amps.  The work good for DC.

A typical value of resistance to sense current is 0.1 Ohms.  0.2 Ohms works a little better though, but it depends on how much current you are going to measure and what the range is, and what is the lowest current you need to measure with good accuracy.  It could be that you would have to add range switching in some cases where you need to measure a very low current and a moderately high current.

The LM358 will work OK with 0.2 Ohms and sort of OK with 0.1 Ohms, but the lower you go the more the input offset will affect the reading.  You can also compensate for the input offset by biasing one of the inputs with a steady DC signal and large value resistor, on the order of 20 megohms.  You may lose a little linearity but maybe not that much.  It's simpler just to subtract the output offset though from your readings.  If you use an analog panel meter you may even be able to adjust the zero so it is a little less than zero, and that way when you connect the op amp it goes to zero with zero input signal.

It's interesting to experiment with these circuits.

They also make dedicated IC chips for measuring current, and hall effect current sensors, and also four terminal current sense resistors made just for that purpose.

[David Hess]

Sensitivity is limited by a combination of open loop gain, drift, and noise.  Offset, and sometime drift, can be calibrated out.  For general purpose parts this will be in the 100 microvolt range, but precision parts can get down to microvolts.

[LooseJunkHater]

Alright so I guess I'll stick with the 0.1ohm resistor and not go any lower. I also really liked @MrAl idea of adding a range switch to allow for more granularity of current selection, so that's what I did! Hopefully this circuit will work in-practice!

https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEBmAHAJmgdgGzoRmACzICcpkORKpISCkdApgLRhgBQyWjRcILZOhB9GYHOChQuPEfypyxRahEaQOfWmFwjUIdKj1E9yKNAlrY6ZGHh37kYeg7aVE1IwVjwds+QYO9hAASkwAzgCWYQAuAIYAdgDGTBwA7iCkEkLgOtnqAGZ0Oh4geNQl2WDQSM7pBkYmCMLG0umi4BLteRwA5h0gJeIZFtJ96OjUmfoT4GDCar2K+pBG-OPUCwBOS8TU7egrUjCcwdPUbDiM6wLaI7yMSGpmCGk5Egq34Mqtb197-LsftcStcpupoigmroli1vCxMI48JAiAcEHhkMgcHoYFgECj0NpcE1UKRAbZGAATJj5WIAVwANtFXtcLlcZkN1G1+KyliwSupxM1uSV2oInCAqTSGdEWPSmBTJN5LupTqLxPdGGwdE8+HQjs8ONsWiwpsaOdM7BwAA6-BSDAwoKScPr2vSDb5bYYoYRTFheZaWvq+j5zAT+9QAZW9N0u0ZNIwgNPpYSYUmc3DZ53Vcezqmk2yG8f6FydDlehaGZosrym2QU3RtteErsdYg4UZYVc1xrBUiTKbTHCAA

 Thank you all for your help!

[MrAl]

Alright so I guess I'll stick with the 0.1ohm resistor and not go any lower. I also really liked @MrAl idea of adding a range switch to allow for more granularity of current selection, so that's what I did! Hopefully this circuit will work in-practice!

https://www.falstad.com/circuit/circuitjs.html?ctz=CQAgjCAMB0l3BWEBmAHAJmgdgGzoRmACzICcpkORKpISCkdApgLRhgBQyWjRcILZOhB9GYHOChQuPEfypyxRahEaQOfWmFwjUIdKj1E9yKNAlrY6ZGHh37kYeg7aVE1IwVjwds+QYO9hAASkwAzgCWYQAuAIYAdgDGTBwA7iCkEkLgOtnqAGZ0Oh4geNQl2WDQSM7pBkYmCMLG0umi4BLteRwA5h0gJeIZFtJ96OjUmfoT4GDCar2K+pBG-OPUCwBOS8TU7egrUjCcwdPUbDiM6wLaI7yMSGpmCGk5Egq34Mqtb197-LsftcStcpupoigmroli1vCxMI48JAiAcEHhkMgcHoYFgECj0NpcE1UKRAbZGAATJj5WIAVwANtFXtcLlcZkN1G1+KyliwSupxM1uSV2oInCAqTSGdEWPSmBTJN5LupTqLxPdGGwdE8+HQjs8ONsWiwpsaOdM7BwAA6-BSDAwoKScPr2vSDb5bYYoYRTFheZaWvq+j5zAT+9QAZW9N0u0ZNIwgNPpYSYUmc3DZ53Vcezqmk2yG8f6FydDlehaGZosrym2QU3RtteErsdYg4UZYVc1xrBUiTKbTHCAA

 Thank you all for your help!

Nice to hear you got it working the way you like.

[Terry Bites]

Fix your sources stablity and lower its bandwith using integrator feedback.
www.analog.com/en/technical-articles/dc-error-characteristics-of-an-op-amp-and-the-effect-on-precision-applications.html
nicely worked out at
https://www.analog.com/en/design-center/interactive-design-tools/opamp-error-budget-calculator.html

Have a look at the LT3085 datasheet (page 19) for a ready baked version of your circuit with very high specs.
$5, on a par with a precsion dual opamp.

[LooseJunkHater]

Fix your sources stablity and lower its bandwith using integrator feedback.
www.analog.com/en/technical-articles/dc-error-characteristics-of-an-op-amp-and-the-effect-on-precision-applications.html
nicely worked out at
https://www.analog.com/en/design-center/interactive-design-tools/opamp-error-budget-calculator.html

Thanks for the links, I'll give them a read!

Have a look at the LT3085 datasheet (page 19) for a ready baked version of your circuit with very high specs.
$5, on a par with a precsion dual opamp.

I did after you suggested it lol. It only supports an input voltage of ~40v but I would like my circuit to be able to work at 60v or 80v. I did try finding alternatives to the LT3085 but couldn't find any which supported the higher voltages that I wanted.

For your attached images, I'm not sure what image2 is showing. Further for image 1, I don't really understand what C1 and R4 are doing? Further there aren't values on most of the components; is it essentially just showing a more optimized template of my circuit?

Edit: I guess it may help for you to know that my available N-FETs are FDP047AN, while my N-BJTs are TIP142.

[radiolistener]

Sensitivity depends on noise figure.

Here is theory for op-amp noise figure calculation:
https://www.ti.com/lit/an/slyt094/slyt094.pdf?ts=1688003447603