DIY Active Current Probe

[Navarro]

Hi Everyone,

Does anyone know any current DIY probe like N2820A from Keysight? I'm thinking about developing one like that for better knowledge of project current consumption as I'm developing a lot of  battery based devices but doesn't have a oscilloscope for a N2820A. I have already googled about it and didn't found any DIY projects like that.

What do you guys think about it? I want opinions about this project.

Best Regards
Felipe

[joeqsmith]

Shunt with an LNA, sure you could pulls something off. Fixed gain and shunt, lower BW would make it a breeze. Maybe.   I found a link, saw the shunt and thought easy enough.  Not a lot of thought from my side... 

[elecman14]

A fixed shunt as joeqsmith says might be the way to go. Check out https://www.eevblog.com/forum/beginners/how-to-measure-current-on-an-oscilloscope/

[David Hess]

So it uses a current shunt with a limited +/-12 volt common mode range which makes sense for a +/-15 volt powered device and it has a bandwidth up to 3 MHz; there is nothing special about that.

Keysight gives no common mode rejection specification so it is probably pretty horrible; my guess is about 36 dB and they probably intend this for measuring supply voltages which are not shifting in level.  That makes designing an alternative even easier.

When making these types of measurement, I usually just build the current monitoring circuit with an oscilloscope friendly output into the device.

[Navarro]

I'm thinking about a INA110 (2.5Mhz with gain 10) + OPA197 for another 10 gain or 100 gain for a second low-current channel. With a 0R1 shunt that I already have.

My ideia is measuring a max of 2A with this probe for debugging my embedded circuits. I wanna characterize my consumption for better battery estimating.

[David Hess]

I'm thinking about a INA110 (2.5Mhz with gain 10) + OPA197 for another 10 gain or 100 gain for a second low-current channel. With a 0R1 shunt that I already have.

I think the low common mode rejection at high frequencies of the INA110 will limit performance but there are not really any better choices unless a different design is used.  It will certainly work and it is simple enough to try just to get an idea of what kind of performance is available.

My idea is measuring a max of 2A with this probe for debugging my embedded circuits. I wanna characterize my consumption for better battery estimating.

But with what kind of resolution or noise performance?

[Navarro]

David,

There is only one project here that we are going  to use this 2A. Others RFM69HCW, some LoRa and Zigbee radios and others consumes only 200~300mA at máx. I want to be able to get around 1uA~10uA resolution. 

Here is a preview of the schematic: https://github.com/PY1CX/Oscilloscope-Current-Probe/blob/master/NE-CurrentSense.pdf (I know that R11 and R12 are with the wrong value)

Kind Regards,
Felipe Navarro

[Navarro]

My little beast inside a Hammond 1590B box.

[joeqsmith]

If you add the buffered option, it's a trace cut vs not populating a zero ohm?   
Are you really needing much more than a basic meter with data logging for what you are trying to do? 
Looking good.

[Navarro]

Joe, I added a 0R resistor but didn't uploaded the new version yet.

Yes, I need more than a basic meter that does data logging, because with the oscilloscope I can see fast sleeps and wakeups of the embedded application. I was thinking about the possibilities of adding a 16bit SAR with 1MSPS ADC in the project (for data-logging via USB + calc of power consumption) but for now, I'm sticking as a Oscilloscope probe.

Best Regards
Felipe Navarro

[Navarro]

"Big Nice Changes" https://github.com/PY1CX/Oscilloscope-Current-Probe

- Removed the X1 Output buffer optional
- Lowered the resistor network of the X10 to a lower value and added a follower with the second opamp for higher Johnson noise perfomace.
- Added a filter network in the input but I'm going to try to stick without it.

[Kleinstein]

To get a 1-10 µA resolution, one should more think about a 1 Ohms shunt. Otherwise DC offsets would be rather high.

[guenthert]

I wonder whether one wouldn't get better results for power (or rather energy) measurements of CPU based low-energy projects, where the current might change several orders of magnitude and is almost all the time very small, by using a converter with logarithmic response.