Jump to content
  • 0

Erroneous Voltage Readings


KevL

Question

I was trying to use the scope to measure the voltage drop across a resistor for a lab exercise and I was getting the strangest readings. I finally figured the problem might be with the AD2 and tried to measure the voltage across a 1.5V AAA battery. The AD2 indicated there was a -19 mV across the terminals of the brand new battery? Is it possible my scope is damaged? I tried using channel 1 and channel 2.

Link to comment
Share on other sites

5 answers to this question

Recommended Posts

Hi @KevL,

When you are measuring across the battery with one of the channels (channel 1 for example), I presume you are using the solid orange wire on the positive side of the battery and the orange wire with the white stripe to measure the negative end of the battery? In the interest of covering all of the bases, is the 30 pin connector connected all of the way into the Analog Discovery 2?

Additionally to clarify, are you just using the Analog Discovery 2 or are you using a BNC adapter? Additionally, if you are using the oscilloscope view (as opposed to the voltmeter view), do you have any attenuation set?

Thanks,
JColvin

Link to comment
Share on other sites

I am just using the Analog Discovery 2 (no BNC adapter). I am using the solid orange wire on the positive side of the battery and the orange with white stripe wire on the negative side.

The 30 pin connector is fully seated in the AD2. I've also tried using individual 2 pin connectors instead of the 30 pin connector and got the same results so I don't think the issue is with the connector.

I was using the oscilloscope view but have switch to the voltmeter view to narrow down the issue. Do you have any other suggestion? Thanks for your quick response.

Link to comment
Share on other sites

Hi @KevL

The Scope inputs are differential but not floating. A common ground connection between the analyzed circuit and device is needed.
Without such the measurements can be wrong or the device and the circuit can be damaged.
https://reference.digilentinc.com/reference/instrumentation/analog-discovery-2/reference-manual#scope

Connecting a floating battery is fine, since this gents balanced through the 1M scope input impedance.

To test if the scope/awg is working you should connect the Scope 1+ (orange) to AWG W1 and Scope 1- (orange/white) to GNG (black) wire, generate and capture some signal.

image.png.766444eac82a078f02edd9f77e2cc76b.png

Link to comment
Share on other sites

I was able to recreate the image you show above following your instructions so maybe all hope is not lost.

I am still getting really strange readings though trying to do my project. I using the AD2 to power a micro-controller with positive V+ set to 3.3V and with the the grounds tied together. In order to get an idea for how much power the microcontroller is consuming I placed a 10 ohm resistor in series with the V+ output from the AD2 and i'm trying to use the scope to measure the voltage drop across that resistor to get an idea of the current being used.

Below is the view I am getting from the scope which doesn't make any sense to me. Why would it be centered around -3.212 V? The voltage drop across the resistor should be only a few mV. The square wave does make sense because the program on the microcontroller is being used to flash an LED at that frequency. Any ideas?

image.thumb.png.183b6bd92213ca8475f7a3eeda695599.png

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...