I developed a board and I like to measure the power for each block. Power can be a few micro amps to a few hundred mAs. (Depending on the block). I have zero ohm resistors at the input of each block so I can insert a DMM. I don't have the budget for a nice Scope Current probe. What would be a good way to measure individual currents on each block? (I tried to connect the DMM in between but it changes the total load on that block it is not as effective. Shunt resistor is also not preferred since at low currents, I need to put a high value resistor and that impacts the circuits).
If the method could provide measurements up to 10KHz, it would be great.
Are there low cost home-grown measurement probes that are sold that I can use?
Best Answer
This is a headache that anyone who has tried to make such measurements will recognize instantly. Multimeters tend to have excessively high shunt resistors for the purpose (to make them cheaper, they drop lots of voltage (like 200mV full scale)- good for their circuit, not so good for yours), so they tend to unduly affect the circuit being measured.
A solution is to use a much lower shunt resistance and a very low offset voltage so-called "zero drift" op-amp amplifier with a high gain (such as 100). A 10 ohm resistor will drop 10uV/uA, so a gain of 100 would give you 1mV/uA and you could read it out on your multimeter (200mV range). If your load is 200uA, that's only 2mV of drop.
As it turns out, this market gap has been noticed, and a product was developed which more than meets your specified requirements. The schematic is published so you could buy the parts and build it, or simply buy the finished product.
If you choose to build it, I suggest putting the battery powered gain-of-100 amplifier into a box and using an external shunt customized for each test (assuming such tests are going to be occasional). A low-drift amplifier is useful for other kinds of testing such as thermocouple and load cell testing.