Trouble measuring uA on EX330 Multimeter

[purfield]

Hi everyone,
I'm in the middle of developing my very first electronics project (woohoo!).  It's a small Bluetooth LE device, and I'm at the stage where I'm optimizing my code for maximum battery life.  I'm currently taking measurements off of a development board.

I'm using an Extech EX330 multimeter based on Dave's recommendation and the fact that it has a uA setting. 

When I measure current on the mA setting, I get ~0.05 mA measurement, which is about what I expect.  The value changes a bit depending on device activity, but ranges from .02mA to .07mA.  I've also measured voltage drop over a 10-ohm resistor in series with the power source and get .2mV to .7mV, so the measurements agree.

However, when I set the EX330 to uA mode, the meter just gives me a constant reading of about 1070uA, and my device loses power.  Power source is a CR2032 coin cell battery.

I can't figure out why this is happening, since I'm not changing anything between measurements other than turning the knob on the multimeter.  Any ideas?  I'm a big time newbie so go easy on me! 

Also, I'd like to thank everyone for everything I've learned by lurking on this forum and watching Dave's youtube videos.  It's all been incredibly helpful!

[IanB]

That's pretty odd.

I think I would try to do a separate validation test of the meter to see if it's working properly.

The meter has a 400 µA range I believe, so you could try to measure a current of about 200 µA. Get a 47k resistor and a 9 V Battery and measure the current through the resistor. It should be approximately 9/47000 = 190 µA.

Also make sure your CR2032 is fresh. Measure the voltage across it when it is powering the circuit and check that it is 3 V or so.

[Lightages]

Could it be the burden voltage of the meter on microamp range? If you have another meter, measure the resistance of the EX330 in the microamp range.

[Wytnucls]

I looked at burden voltage too, but such an increase in current measurement would only make sense if the circuit impedance dropped dramatically with the loss of voltage across it.
The shunt resistance on uA range should be at least a couple of hundred Ohms.

[purfield]

Thanks for the replies everyone!   

My hypothesis is that the problem is burden voltage.  Although the average current consumption of the circuit is low (50uA), the peak current is quite high (maybe around 15mA for 2ms).  So I suspect there is a large burden voltage drop from my peak current which is rendering my circuit inoperative. 

Does that sound reasonable?

[IanB]

Yes.

I just checked one of my multimeters and the resistance between the input terminals on the µA range is 100 ohms. At 15 mA this would give a voltage drop of 0.015 x 100 = 1.5 V. If your battery is 3 V the supply voltage is going to be halved.

You could probably compensate for this by putting a large capacitor across the power supply terminals of your device (say 100 µF). Then put the meter between the battery and the capacitor. The capacitor will supply the current pulses as needed and the meter will measure the average current used to recharge the capacitor.

[Wytnucls]

You could check that your meter is working correctly by measuring a small current feeding a 50K resistor at 5 volts, for instance. The current shunt resistance won't make a difference with that circuit and you should measure in this case 100 uA.