Current measurement

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Gustav
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Posts: 1
Joined: Sat Jan 23, 2010 1:33 pm

Current measurement

Post by Gustav » Sat May 26, 2018 2:11 pm

Hi

I want to buy an active probe for my picoscopes. I am developing a battery powered device, that needs to have very low sleep mode current consumption. Normally, I would just insert a 10 Ohm resistor in the supply and measure the voltage across, and calculate the current, but there is a x1000 times difference between sleep mode current and operation current. So the resistor needs to be 1 Ohm to not cause too much voltage drop during normal operation.

Does picotech sell an active current probe that has less than 1 Ohm serial resistor, and is able to amplify the voltage by a factor of approximately 1000?

alan
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Posts: 186
Joined: Thu May 25, 2006 8:02 am

Re: Current measurement

Post by alan » Sun May 27, 2018 7:27 am

Hi Gustav,

Could you let us know which PicoScope you have and what is the max current you want to be able to measure from the battery.

To give a quick example using the 1 ohm resistor - lets assume you can cope with a 200mV drop across the resistor at peak current then thats 200mA. I would suggest you monitor this on channel A of your scope set to +-100mV full scale then use the analog offset function available on most of our scopes to get the channel to measure from 0mV to 200mV (assuming you only draw current from the battery in one direction). Your resolution will depend on the resolution of your PicoScope:

8 bits 0.8mA
12 bits 0.05mA
16 bits 3.1uA

With these sort of applications the change in current is often not as fast as you expect (WiFi or similar switches on in microseconds but capacitors in the circuit slow down the rate of current rise) so you can use the resolution enhancement function in the software to gain at least an order of magnitude improvement on the above. If your scope has the feature also enable the bandwidth limit to reduce noise pickup.

Another way of improving things is to also measure the same signal on channel B but set this to a lower range - say 10mV to look at 0 to 20mV so 20mA max that gives you one "coarse" view of the overall current waveform and a second zoomed in one. Combined with resolution enhance you can view the 200mA max signal with sub uA resolution.

To sum up there are three things - first use analog offset to make sure you use the whole input span of the scope range to avoid wasting half, second use resolution enhancement and thirdly use 2 channels set to different ranges.

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