Highside current sensor

[sourcecharge]

After working with IAs, I found that the 4 resistor network of the differential part is a major source of error for any measurement with a wide range of load resistance, and voltage source.

Please see the spread sheet below.

Basically, as the voltage across the shunt gets lower, the error of the IA increases.

So, I decided that I wanted to make a highside current sensor that was not subject to this error.

The result is a differential output using the circuit in the below schematic.

It's basically a low side current sensor, but with a second opamp that isolates the reference ground resistor of the first opamp.

The result is a error subject to only the 2 resistors, with a differential output.

Because the sensor is designed to be measured by a multimeter, in my case, the MS8040, it can measure differential signals.

Yes, this will have some offset, but under normal conditions, I've found that all of this type of current sensors require a zeroing, REL function of the multimeter before the measurement.

I went with the 100mohm shunt again to keep the bandwidth high, but I changed to the lower 10ppm/C type, which was a little more pricey.

I also use a 0.01% matched 9k/1k resistor network.

The input filter is geared to 15khz for CO, and I also added some output filtering too.

I included a paper about the filtering of highside current sensors if you are interested.

I really hope these current sensors will finally be able to accurately measure my bipolar sources with resonant loads using half and full bridge drivers for frequencies up to 12khz.

It's for research purposes.

[langwadt]

why not just use a common INAxxx currentsense amp or a proper diff amp?

[sourcecharge]

I can't tell you how many current sense amplifiers I've looked at, but I know it's alot.

I don't remember why all of them do not fit my needs, but I can tell you I think it might be about isolation.

An IA, or a differential opamp will be subject the the 4 resistor network error problem.

Please see the spread sheet to understand the calculation.

[langwadt]

the point of buying an instrumentation amp or current sense amp is that they come with matched resistors build in

[Someone]

Just use the multimeter on mV range?

[sourcecharge]

the point of buying an instrumentation amp or current sense amp is that they come with matched resistors build in

Seriously?  Have you seen the accuracy they list in their datasheets?

Using four 10kohm resistors in a diff opamp with a 0.01% error gives 10000 +/- 1 ohms, which is the best matched resistors you can get or put inside a chip.

That could be all four of them with the  +/- 1 ohms.

My spread sheet shows a difference of only one resistor with + 0.01 ohms, a 0.0001% error.

Theoretically, it's impossible for them to have the same accuracy as a low side (non inverting) current sensor with 0.01% matched resistors.

If you single out any one of your "common" picks, name it, and the datasheet will show an error of about 0.05% in a narrow ideal measurement.

The truth of it is that when the load or source changes, the error increases as the voltage difference between the inputs of the differential opamp decreases.

[sourcecharge]

Just use the multimeter on mV range?

I'm sorry, I don't understand what you mean by this..

I'm not sure if you are posting in the right thread...

[ledtester]

Just use the multimeter on mV range?

I'm sorry, I don't understand what you mean by this..

I'm not sure if you are posting in the right thread...

It seems that all you are doing is amplifying the voltage across the shunt and then measuring it with a multimeter. Why not just measure the shunt voltage with the multimeter directly?

[sourcecharge]

Well, I'm measuring between 0 to 220mA, and my multimeters are 5.5 digit type.

That would be 22.00mV, vs 220.00mV or 0.10mV vs 1.00mV.

So there is some logic in the simplicity of what you are suggesting, but a loss of significant digits.

I'm trying to keep the shunt as low as I can without decreasing bandwidth so a 1 ohm shunt is not possible.

Buying really expensive volt meters is kinda out of the question, because I would need 2 at least.

[coromonadalix]

gonna hate me,  you're not re-inventing the wheel   your mastech is a 22k count dmm

you have relative measurements if it is what you call differential ?

you have 4 1/2 digits panels meters gives 19999  counts wich is close to your meter  or same for current meters .......   and you just need say 0.1 ohms  shunt, and it gives you a very good precision

and you can find cheapo   dmm's who goes up to 40k 50k or even 60k counts  at "relatively" low prices,  notice the quotes lol

say  some t58d  models on aliexpress


but do you talk about resolution or precision   thats a different thing  if you talk  "digits"

and other member did wrote  you add up errors in your circuit,  you would neeed high precision resistors    ... 

or this for an eample  https://ohm-labs.com/precision-shunts/cs-1-10/

for bipolar   you do have hall effect current sensors,  some are very precise, some brand like Allegro  or others ...  and your voltage losses are near zero

[sourcecharge]

You're right, it's 4.5 digits, not 5.5. My mistake.  Yes, 5.5 digits are usually in the 800 to 2000 dollar range.

My 20k count multimeters are benchtop types, that I can plug into the wall.

I included the MS8040 accuracy specs.

The significant digit that is lost in using no amplifier is a digit that becomes 10x error due the the accuracy of the meters.

For example, the 200mV range has a 0.05% accuracy on DC range, but it has a +6 on the lowest significant digit.

So Using the 100mohm shunt and only the multimeter measuring say 1 mA, it's now +/- 600uV instead of 60uV.

As for the t58d, the "60k" meter you suggest has a 9999 count on the display, (3.5 digits).  Am I missing something?

If you want precision from a multimeter for that extra significant digit, you are going to have to pay through the nose.

I don't have the kind of money for a multimeter.

The 100 mohm shunt I'm using is the best shunt available from digikey, with 10ppm/C and 0.1% error.

Is the shunt you are suggesting any better?

If you have a specific hall effect current sensor that you think is better than a shunt type current sensor, list the model number, maybe post the datasheet.

I've checked "some" hall effect current sensors, but not all I suspect.

The ones I've checked have much less accuracy than the shunt type.

[sourcecharge]

After double checking the shunt you suggested, it is better.

5ppm/C and 0.05%, BUT they look really expensive.

The kind of expensive that requires you to email them to find out the price.

So, kudos for the better shunt, but it's probably out of my price range.

[MariuszD]

Your circuit cannot be called high side current sensing. There is no level shifting here, it's just an amplifier for the meter, the circuit will work at any point in the circuit.

The biggest source of errors in a differential amplifier is resistance asymmetry, which reduces CMRR. The difficulty lies in the fact that a 1% error in the resistor will result more than 1% error of measured value.
There are solutions that are free of this flaw.

[sourcecharge]

Your circuit cannot be called high side current sensing. There is no level shifting here, it's just an amplifier for the meter, the circuit will work at any point in the circuit.

The biggest source of errors in a differential amplifier is resistance asymmetry, which reduces CMRR. The difficulty lies in the fact that a 1% error in the resistor will result more than 1% error of measured value.
There are solutions that are free of this flaw.
(Attachment Link)

[EDIT]

Oh, I'm sorry, I didn't know the definition of the highside current sensor requires a level shift.

It is what it is then.

After quickly checking multiple AI sources about the definition of a highside current sensor, all are in agreement that it does not require a level shift.
[/EDIT]

What you are showing is a non isolated, floating 6V opamp that is powered by the source.

Correct me if I'm wrong, but it looks like an inverting type that uses a mosfet as Rf and also for the "level shift", presumbably for an ADC for a microprocessor.

Although it doesn't have the same problem as a 4 resistor diff opamp, I think the ADC error is going to cancel any increase of accuracy you get from that setup.

Thanks for the suggestion though but I've considered that already.

[MariuszD]

I won't argue about the definition with AI. But what you showed doesn't match what most people understand by this term. Compare with the schematics in Google Images for the query "high-side current sensing"; all of them have a level shift to the local ground. The concept of "high-side" loses its meaning when there is no low side in the circuit. The concept of a "high side multimeter" makes no sense because a multimeter can measure anywhere. The term "high-side" in your post is redundant.

Correct me if I'm wrong, but it looks like an inverting type that uses a mosfet as Rf and also for the "level shift", presumbably for an ADC for a microprocessor.

The circuit does not invert, but regulates the MOSFET current so that the voltages on Rshunt and Rg are equal. So it creates a current proportional to the measured current, and this current is converted into a voltage across Rl. The ratio of Rl to Rg determines the gain.