USB signal noise

frohr

Well-known member
Hi,
I send data from adxl via ads1256 and teensy 4.0 to PC via USB. I have some issue about noise. If I connect it to PC I have OK noise. But if I connect to tablet or notebook, the noise is 10x higher. Same code, same wiring, same usb cable, same python app, very different noise. All powered from USB cable.
See pictures:

PC noise:
PC-noise.jpg


Tablet noise:
tablet-noise.jpg

Any idea why?
Thanks
 
This is probably a grounding issue, or stable power source. ADC's are quite picky about this.
 
ads1256 seems to require external reference voltage, is that correct? If so, how are you providing this voltage? Also what is your adxl accelerometer's model number? There are many accelerators starting with adxl. If it happens to be a ratiometric one, then its supply voltage will affect its readout.
 
as i see it needs a 2.5v reference voltage. By using for example a LM236-2.5 you can get this stable from the 5v usb source.
Most breakout boards are stabilized, but indeed as liudr asks, it good to know how you connect or what you use for the ads1256 too.
 
(btw: I just messed up my mcp3208 one minute ago by failing to ground my opamp input buffer, so 12v was creating a carnage inside the ADC :).
 
I made simple pcb where is inserted Teensy 4.0 and ADS1256. Then I have wires to connector and to ADXL1005 eval board.
ADS has 2.5V reference pin - I do not use it, I think it is somehov internaly (I hope).
Anyway, If I connect it to the PC, it is ok. Then just reconnect usb to the laptop or tablet (everything is in the same positon) and noise is 10x higher.
Maybe I can try power tensy from battery?

ADXL:
https://www.analog.com/media/en/technical-documentation/data-sheets/adxl1005.pdf

ADS:
https://www.banggood.com/ADS1256-High-precision-ADC-Module-Conversion-24-bit-8-channel-p-1335941.html?imageAb=1&cur_warehouse=CN&a=1654521786.5673&DCC=CZ&currency=CZK&a=1654521800.0024&akmClientCountry=QA

Board:
pcb.JPG

Analyzer:
analyzer.jpg
 
Just a blind guess - a common problem when connecting 2 or more powered sensitive analog devices is called "ground loop". Usually a quick way to troubleshoot involves powering all but 1 device from batteries or "floating" power supplies and eliminating all possible connections which might connect it to ground of other devices. Sometimes this isn't practical if you have a lot of connections, as pretty much any cable includes a ground connection.

If the USB connection (and very likely its ground) is causing the trouble, and you can't find any other solution (low-pass filtering +5V USB power has little or no improvement), a USB isolator might solve it. The bad news is USB isolators are only cheap for 12 MBit speed. If you need 480 Mbit, they become much more expensive.

Here are 2 USB isolators I have personally verified work with Teensy 4.

https://hifimediy.com/product/intona-hi-speed-isolator-1kv/

https://hifimediy.com/product/usb-isolator/

Both of these are limited to providing about 200 mA to Teensy. If you've added circuitry which brings the total current above 200 mA, you'll also need a separate isolated (or "floating") power supply for Teensy.
 
But why is it ok with PC and not ok with notebook? Another grounding of USB in those devices? But: I will start to use my device without USB cable - could you recommend me how to send array of 30000 floats bus Bluetooth? And of course it will be powered by battery 3.7V with step up converter to 5V. This solution is ok?
Thank you for help.
 
Your PC probably has better (true) grounding and filtering, also for you usb port, than a notebook.
Like Paul said: you can first try to add low-pass filters. Also for example try bypass capacitors to filter out the unwanted AC noise signals from your 5v DC input.
It's not uncommon that this happens in tablets or notebooks. I think you should not rely on external factors in your product. Every notebook or pc can have different issues in power supply.
If this has no effect, perhaps an external supply and decoupling/isolation could fix your noise issues.

http://www.learningaboutelectronics.com/Articles/What-is-a-bypass-capacitor.html
 
Your PC probably has better (true) grounding and filtering, also for you usb port, than a notebook.
Like Paul said: you can first try to add low-pass filters. Also for example try bypass capacitors to filter out the unwanted AC noise signals from your 5v DC input.
It's not uncommon that this happens in tablets or notebooks. I think you should not rely on external factors in your product. Every notebook or pc can have different issues in power supply.
If this has no effect, perhaps an external supply and decoupling/isolation could fix your noise issues.

http://www.learningaboutelectronics.com/Articles/What-is-a-bypass-capacitor.html



If I undestand well (not sure) I have to add capacitor and resistor between power of ADS / ADXL and GND = Capacitor will be connected between 5V and GND. Is that correct?

wiring.JPG
 
Back
Top