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I am testing how low I can get the power consumption of the RedBear BLE Nano, which is a development board for the Nordic NRF51822 System on Chip that contains an ARM micrcocontroller, and BLE 4.0 mdoule.

I have produced some software that should have turned off all of the subsystems in the processor, and put the device into a sleep mode that is interrupted by a timer periodically.

This seems to work, I am powering the module form a bench supply and I have a multi-meter in series. When I have my finger touching the metal case that shields the Raytac module which is on the BLE Nano, the multimeter reads a constand 20uA, which is great. Then happily comes out of sleep mode every 10 seconds, flashes an LED and goes back to sleep, again at 20uA.

If I do not touch the metal shield, then I see this 20uA value climb to ~330uA very slowly (takes about 1-2 minutes), then descend down to ~200uA, where it seems to stabilise.

My bench supply only tells me the current draw in milliamps, so it's registering zero the whole time.

I have no idea why there is a difference when I am not touching the shielding, and whether the module is really only consuming ~20uA or closer to 200uA.

If anyone can explain which current draw is likely to be the actual one, and why there is a difference, that would be great.

Thanks

Steve
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  • IS your meter on AC measurement? – Andy aka Sep 29 '16 at 15:01
  • Just double checked, and nope, it's all set to DC – Steve Sep 29 '16 at 15:05
  • What is the device consumption in active operational state, compared to sleep state? – Ale..chenski Sep 29 '16 at 15:09
  • Do you have any FM Radio Stations (or TV) in 5-miles proximity? – Ale..chenski Sep 29 '16 at 15:15
  • Since you influence the current with your finger there might be an external rf incomming signal. For a test make a small Faraday cage. Feed the unit with a battery and put everything including the meter in the cage. This way you can start to eliminate some causes – Decapod Sep 29 '16 at 15:29
  • Are all pins defined in a manner that the documentation says is lowest power mode. This classically is : All inputs pulled high or low with internal or external pullup/downs, all outputs set to levels matching OC V on loads. BUT in some modern cases documentation gives other options. || An open cct input can do much as you report. – Russell McMahon Sep 29 '16 at 15:36
  • Does that mean that it's more likely to be only consuming 20uA, and the rest of what I'm seeing is environmental? – Steve Sep 29 '16 at 15:39
  • @Steve, it means that to measure a current in 20uA range some qualification must be involved. I did ask my questions for a reason. You did not answer. – Ale..chenski Sep 29 '16 at 15:49
  • Apologies, depending on what it's doing, the active current has quite a variation, anywhere between 300uA and 1.5mA, excluding external peripherals, such as LEDs. – Steve Sep 29 '16 at 15:57
  • Also, yes, there are radio transmitters within a few miles – Steve Sep 29 '16 at 15:58
  • Then you are in trouble with FM radio around you. Also, don't use auto-ranging current DMM. Use shunt resistor and Milli-voltmeter instead. – Ale..chenski Sep 29 '16 at 17:13
  • The multimeter is not auto-ranging. Can you explain the effect that the FM radio is having on module, and why it results in the effect I am seeing? – Steve Sep 29 '16 at 17:32

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Stray EMI gets rectified from parasitic diodes on DMM and stored in stray capacitance. Common mode high Z EM fields can be in many volts range and on an unbalanced measurement ( low z ground, high Z signal ) result in the CM noise becoming a differential signal due to poor CMRR ( CM rejection ratio)

Solutions include raising CM impedance with a large Ferrite CM choke or BALUN around probe wire pair (clamshell or torroid) or grounding CM source of noise (shield)

Essentially this is measurement error but illustrates how Sensor amplifiers can also make measurement errors from loop antenna effects of long leads.

Tony Stewart EE75
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  • Can you expand your acronyms for me please? – Steve Sep 29 '16 at 15:38
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    Sure , which ones? or refer to here and enter acronym and save in your bookmarks http://acronyms.thefreedictionary.com/balun EM ( electromagnetic) Z = impedance, CMRR Common mode rejection ratio ( op amp spec) – Tony Stewart EE75 Sep 29 '16 at 15:43