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peter64

uCurrent Gold Measurements Of Sensors When Using Digital Discovery Protocol Mode

Question

Hello, 

I recently purchased a digital discovery and I was hoping to use it to test various sensors.

It works fine for sending requests to sensors and making simple scripts.

However I can't seem to use it to measure current flowing from the VIO pin to the sensor in conjunction with the uCurrent Gold I own.

http://www.eevblog.com/product/ucurrentgold/

Every time I attempt to do it it triggers over-current protection.

Could you explain why this happens and if there is any way I could use my uCurrent Gold with the Digital Discovery to accurately measure current flowing from VIO to the sensor?

I expect there may be no way to do what I want to do, so just understanding why it isn't possible would be satisfying.

Thanks,

Peter

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On 10/13/2017 at 7:02 PM, attila said:

Hi @peter64

The VIO output is limited to 100mA and high capacitive loads might cause overcurrent. 
Try connecting your circuit to VIO before enabling the output.

Hey attila,

It' didn't make a difference if I connect it to VIO before or after I enable.

I did get it to measure a current by putting it in the ground path instead of vio.

Unfortunately the value that was measured was totally incorrect (1.8 A or there abouts).

When I connect my regular multi-meter in series it gives accurate current measurements.

I'm just curious why I can't connect the uCurrent Gold in series without either triggering over current protection or getting current readings that are totally wrong.

Any ideas you might have would be appreciated. I'm not sure if this is a high capacitive load or not, below is a link to the schematic.

http://www.eevblog.com/files/uCurrentRev5schematic.pdf

Thanks,

Peter

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@peter64, please:

1) post a photo of your entire setup that clearly shows all connections

2) post a photo of the switch settings on your uCurrent Gold

3) indicate the range of resistances your sensor is expected to produce

- Chris

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Hey clf,

I just had another try and I think I found the issue.

The arduino I was using to monitor the uCurrent gold outputs was wired to the same PC via USB.

This appears to have caused some kind of ground loop where current would flow through one us device and out the other (I suspect it's related to the battery in the uCurrent gold but if you have a better explanation I would love to know the exact reason this happened).

The uCurrentGold Current input was wired in series with the VIO from the diligent to the sensor a MPU9250.

The arduino was wired up with A0 going to uCurrent Gold Voltage Out + and GND going to uCurrent Gold Voltage Out -.

Then it was connected to the same PC via a regular USB cable.

 

The solution was to get two notebook pc's fully disconnect them from ground (and each other, network cables, kvm, etc).

Then I could monitor the uCurrent gold output with my arduino whilst still using the digilent to control the sensors.

The other alterative was to connect my battery powered multimeter to the uCurrent gold (but I wanted to use the arduino to collect the high frequency current usage waveforms).

I was considering buying a real usb oscilliscope (or perhaps the analog discovery) but I don't know if the ADC inputs of the analogue discovery will suffer from the same issue.

Infact I don't know if I can run the analog discovery and digital discovery on the same pc at the same time (I expect not).

Anyways I have a solution for the time being (thanks for your offer of help, it encouraged me to have another try and find the issue).

Peter

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After searching the forums a little more about ground loops I came upon the following post.

I'm wondering if something like the following would solve my issue and if it would be compatible.

Perhaps it would even allow one to use a digital discovery and analogy discovery on the same pc connected to the same circuit?

Any thoughts?

https://hifimediy.com/high-speed-usb-isolator-480Mbps

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Hi @peter64

The uCurrent has low resistance (10m,10,10k) between it +IN and -IN. The -IN is directly connected to its -OUT. In your setup this is at VIO level relative to device/PC ground. The +OUT is at the uCurrent amplification level above -OUT (VIO) level. 
In you setup you basically have shorted the VIO through uCurrent (+IN ~Resistor~ -IN = -OUT) to ground (arduino=PC=AD).

Yes, you can use multiple devices connected to the same PC (Analog Discovery, Digital Discovery...). Each can be controlled by separate instances of WaveForms application.

The Digital Discovery can measure the VIO voltage and current at about 6mV and 0.6mA resolution. Isn't this suffice for your measurements?

To measure the uCurrent output you need a floating voltmeter (battery powered or isolated) or a differential measurement between +OUT and -OUT. For this you could use the Analog Discovery differential scope inputs.
Alternatively you could measure with arduino the voltage difference at +OUT and -OUT relative to ground, but such measurement might be inadequately inaccurate.

For accuracy you should take in account that the scope inputs have 1M resistance that will sink a little current, so you will have (voltage at -IN) /1M (1-3uA) current offset in the measurement.
Connecting the uCurrent -IN to VIO you will measure negative current values but the measurement sink current won't cause offset in the current measurement.

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Hey Attila,

Thanks so much for the detailed and comprehensive reply! It hadn't even occurred to me that the -IN was connected to the -OUT (which I was tying to ground through the USB)

I'm going to give it some more thought, but you've provided me with a number of options. I think either an analog discovery or usb2 isolator will be in order.

Also I didn't realize the digital discovery measured the vio current that is really cool. Unfortunately 0.6mA is not enough resolution in this case as I'm measuring around 4-20 uA.

 

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Hey Attila,

Sorry to ask a slightly unrelated question (I can create a new forum post for it if you want) but is it possible to increase the Scope record view to more than 10 million samples when operating at 1-2 Mhz?

I was considering geting an analog discovery and using it to record 1-2 minutes of 1-2 Mhz sample data. (this is currently what I'm doing with my arduino).

The demo AD2 in Waveforms appears to have a limit of 10 million samples in record mode in Scope view though.

Base on my understanding record mode is just buffering data through the 2x8K scope buffer directly to usb (so I'm guessing the 10 million sample limit is imposed purely by the Waveforms package).

Is there anyway to override or change this max value ? Or would I need to directly interface with the waveforms SDK to run in a continous record mode for 1-2 minutes?

Thanks for any help you can provide.

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Hi @peter64

The 10M sample limit is imposed by the WaveForms application due to data processing measurements, fft... and drawing.
For custom application/script (WF SDK) the record length is not limited.

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