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Sawtooth signal when powering two components from same sourc
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Sawtooth signal when powering two components from same sourc

by alexdr5398 on Mon Oct 07, 2019 1:48 pm

I am getting this weird sawtooth signal when powering two components from the same 3.3 V power source.

This is what the signal looks like.

sawtooth.PNG
sawtooth.PNG (13.97 KiB) Viewed 155 times




I am using the Trinket M0 to collect data from the ADS1220 ADC. Right now, i am using a 9V battery regulated to 3.3V to power both components. The reason I don't just power everything through USB is because the 9V battery provides a much more stable power source than the USB, so the signal is much less noisy. Before I had the ADS1220 power from the 3.3V and the trinket powered from the 5V USB connection, but i would prefer they both be powered from the same source, so I disconnected the 5V source to the trinket and connected the 3.3V source to the BAT pin.

After I did this, I get the sawtooth signal that you see above.

When I measure the voltage coming from the 3V pin on the trinket I can see it moving up and down in sync with the signal above, ranging from 3.14V to 3.16V, When I power the trinket with an external power supply, the sawtooth goes away and I get a steady signal.

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Re: Sawtooth signal when powering two components from same s

by adafruit_support_bill on Mon Oct 07, 2019 2:13 pm

Right now, i am using a 9V battery regulated to 3.3V to power both components . . .
. . . and connected the 3.3V source to the BAT pin.

The on-board regulator on the Trinket needs a supply voltage at least a few tenths of a volt above 3.3v in order to provide stable regulation. If you have a stable, regulated 3.3v supply, you can connect it directly to the 3.3v pin - bypassing the on-board regulator.

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Re: Sawtooth signal when powering two components from same s

by alexdr5398 on Mon Oct 07, 2019 2:18 pm

I will try that thank you.

Is the 3.3V pin the one labelled '3V'? In the data sheet it says that that one is a 3.3V output. Is it fine to use it as the power supply input?

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Re: Sawtooth signal when powering two components from same s

by adafruit_support_bill on Mon Oct 07, 2019 2:23 pm

Yes. It is the one labeled 3v. As long as your supply is regulated, it is OK to feed power via the 3v pin.

However, you should disconnect your external 3.3v supply when connected to a USB power source. Otherwise the 2 regulators will be battling for control.

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Re: Sawtooth signal when powering two components from same s

by alexdr5398 on Mon Oct 07, 2019 2:29 pm

I connected the 3.3V source to the 3V pin and the sawtooth signal still persists.

Also the USB power will always be disconnected since I only want to use the USB cable for communication with my computer. I did this by removing the diode D2, as suggested in this thread: viewtopic.php?f=52&t=157160.

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Re: Sawtooth signal when powering two components from same s

by adafruit_support_bill on Mon Oct 07, 2019 2:54 pm

Sounds like your external regulator is the issue then.

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Re: Sawtooth signal when powering two components from same s

by alexdr5398 on Mon Oct 07, 2019 2:56 pm

Do you have any idea why that would be causing an issue?

When I have them powered from difference sources the signal is perfect. Only when I have them both connected to the 3.3V source do I get the sawtooth signal.

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Re: Sawtooth signal when powering two components from same s

by adafruit_support_bill on Mon Oct 07, 2019 3:01 pm

What is the other component you are powering with the 9v cell?

If you post some photos showing your whole setup, we might find some clues there.

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Re: Sawtooth signal when powering two components from same s

by alexdr5398 on Mon Oct 07, 2019 3:25 pm

I can't add a photo because the file is too large.

But here a simplified diagram which should have all of the important parts.

Scan from 2019-10-07 03_27_57 PM.pdf
(865.92 KiB) Downloaded 5 times


The only thing I change in this diagram which causes the problem is changed the power supply on the trinket.

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Re: Sawtooth signal when powering two components from same s

by adafruit_support_bill on Mon Oct 07, 2019 6:39 pm

You might want to review the app notes in the data sheet for your regulator chip. In particular, section 9.2.1.2.1 which states:
A 1-µF (or greater) capacitor is required between the output and ground for stability at output voltages of 5 V or
higher. At lower output voltages, more capacitance is required (2.2 µF or more is recommended for 3-V and
3.3-V versions). Without this capacitor the device oscillates.

http://www.ti.com/lit/ds/symlink/lp2951-n.pdf

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Re: Sawtooth signal when powering two components from same s

by alexdr5398 on Tue Oct 08, 2019 9:27 am

Yes, I was aware of that.

I tried the higher capacitance while I was designing the circuit but I found that the smaller capacitors gave a less noisy signal.

Anyway, I tried attaching both 10 uF and 1 uF capacitors to the voltage regulator and the saw tooth signal is still there.

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Re: Sawtooth signal when powering two components from same s

by adafruit_support_bill on Tue Oct 08, 2019 10:51 am

Have you tried powering the Trinket via BAT+ and powering your ADC from the Trinket 3.3v rail?

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Re: Sawtooth signal when powering two components from same s

by alexdr5398 on Tue Oct 08, 2019 3:24 pm

I tried that and it did not fix the issue.


I decided to go with a somewhat band-aid solution and just use two 9V batteries as separate sources with one controlled using a transistor (with the base connected to the first) so that I can still turn them on and off using the same switch.

Thanks for your help!

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Re: Sawtooth signal when powering two components from same s

by adafruit_support_bill on Tue Oct 08, 2019 4:21 pm

Good to hear that you found a solution. Thanks for the update.

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