#Thanks! And can adding the extra
split gorge
What are you thinking of doing for a more complex power supply? If there are only two wires supplying power to the pico and you have no other connections to the pico, the current in both wires will be equal always.
pastel ridge
I was thinking most power supplies have a smoothing capactor on the output, and since the Pico also has capacitance, there could be a difference.
(That battery has some internal resistance too. If it was an ideal battery, the capacitor would do nothing.)
If the charge on the capacitor is increasing, the current in the + wire is less than the current in the - wire and vice versa.
Actually no because it just comes from the battery? 😅
split gorge
The capacitors on the board supply the current needed when things are switching fast. High frequency current cannot be supplied through wires of significant length. In addition, many voltage regulators are unstable without local capacitors.
The capacitors won't change the fact that the current into the pico + lead will always be equal to the current going out the - lead.
pastel ridge
I was observing current fluctuations in the wires at about 5kHz
split gorge
The current draw of a digital IC is proportional to the number of nodes in it that are toggling and the number of nodes that are toggling in a processor is instruction dependent. You probably have a software loop running at 5kHz. Why are you putting resistors in series with the power supply? Is it just for current measurement?
pastel ridge
It was just for current measurement, for which the answer was "buy a PPK2 if you really need to do that", but after that I wanted to understand what's going on.
And there were two people telling me that having a current sensing resistor on the low side of a digital circuit is always bad so I was trying to get to the bottom of this.
split gorge
It's bad when the ground references of all the digital things you are connecting together are not connected together.
pastel ridge
OK, and in this case all the digital things have their ground references connected together and the resistor was outside of that system.
I tried moving the resistor to the high side and was getting measurements about half the size, but probably the scope just isn't that accurate, so we're back to "buy a PPK2 if you want to do that".
split gorge
Correct. For example, it would be bad if you had I/O on a pico connected to I/O on an arduino if the ground connections of the two boards weren't directly connected together.
When you did that measurement, was everything run off an isolated battery?
pastel ridge
No, actually running from USB power so far. The low side measurements had scope ground on the low side of the resistor, while the high side measurements I used both channels and was sort of trying to subtract them by eye (this software doesn't have that feature).
If I put it on an isolated battery, can scope ground go anywhere?
split gorge
If the circuit is isolated, the scope ground can be connected almost anywhere but, for most scopes, the grounds of all the channels are connected together.
pastel ridge
It's a USB scope so scope ground is pretty much USB ground, for measuring USB devices I tend to not connect it when things aren't that sensitive.
split gorge
The exception is when you have a large piece of equipment and you connect the scope ground to a high frequency signal, the capacitance between the equipment and the earth can cause artifacts.
pastel ridge
So if I connect to VSYS which has a 47uF capacitor right next to it we probably won't notice that?
split gorge
That would be fine.
pastel ridge
I might try it with battery. Thanks for your help!
split gorge
You're welcome. When you tried the high side measurement, did you carefully calibrate what you called zero voltage by disconnecting the load and just measuring across the resistor while it was attached to the supply?
pastel ridge
No I didn't, so if the two channels read a little different that would confuse things