NE555: pwm jitter

Hi Chris,

Thanks for the reply.
I've checked your comments:
1). The scope is triggering at the upcomming slope of the signal and the trigger level is set properly.
2). The reset pin is connected to Vcc (decoupled with cap to Gnd)
3). Scope is quite new and calibrated

Do you have any other suggestions?

Thanks, Marc

Hi Marc, 

Can you provide a schematic of your circuit? What frequency of operation is the timer configured for?

Best Regards, 

Chris Featherstone

Hi Chris,

Sure. Please find it attached.
The cap we use connected to trigger is between 1 - 3nF
The frequency is important and tuned between 200Hz and 1kHz and then digitalized with dual inverting buffer to a microcontroller.

Reg. Marc

Scan2024-06-13_212919.pdf

Hi Marc, 

This doesn't look like a standard timer configuration so I will need to simulate it to understand it. Below is the standard Astable configuration. Was the goal an Astable configuration? Is it possible that it needs the connection of threshold and trigger as shown below?

-Chris

Hi Chris, 

Thanks for your reply. 2&6 are indeed connected as well.
Rb is tunable so we can tune the output frequency towards the mentioned range.
Is there anything you can add to improve the signal quality?

Reg. Marc

Hey Marc, 

I am currently thinking what can be going on outside of scope settings or configuration issues. If pin 2 and 6 are connected and reset is pulled high there is no reason to see anything but a normal clean square waveform on the output. 

A few things that could help us do a process of elimination would be to 

• Remove the diode connected to the supply
• Replace the potentiometer with a regular resistor
• Remove the output connection to the additional circuitry and just test the output with no load. 

Recreating the waveforms below with your values and show the cap charge and discharge along with the output. On the cap waveform measurements on the scope should show 1/3VDD and 2/3VDD trip points and those should align with the output waveform. A frequency measurement on the scope would be good to see also. The goal would be to isolate to a very simple timer setup and check the functionality. If the device is still showing this behavior it may be good to try a brand new device. 

Best Regards, 

Chris Featherstone

Hi Chris,

Thanks again for the reply.

I checked the items you mentioned but this is not the main issue. The circuitry was fed by a USB hub which seems to have a noisy power. Might need to use a proper DC-DC converter for improving the noise since we want to keep using a USB power supply. Do you have any suggestion or circuit you can recommend?

Hey Marc, 

I am glad the issue was identified. Yes if the supply is noisy or moving, this can create and issue since the 2/3 and 1/3 trip points are created by voltage division from the supply down to the comparator inputs. For the lowest noise supplies you may consider looking at a linear and low dropout regulator or LDO. 

LDO Product list:

https://www.ti.com/power-management/linear-regulators-ldo/products.html

If there are power limitations using an LDO then a DC-DC converter can certainly be an option. 

Below is a link to Ti DC-DC products. 

https://www.ti.com/power-management/dcdc-power-modules/products.html

Depending on what device you would be interested in I can move this thread to our LDO experts or our DC-DC experts for further support on these products. 

Best Regards, 

Chris Featherstone

Hi Chris, 

Many thanks for your reply and good support.
We either power the NE555 seperately from the digital postprocessing using a LDO like TPS73250-EP or if we power all together we have a look for a DC/DC since we then need to power it depending on the setup between 150mA up to 2A max.
In general, would an isolated DC/DC converter have any benefits in our situation?
If required, you can move the thead.

Thanks, Marc

Hey Marc, 

I have moved the thread for support on the DC-DC converter side. I hope this helps resolve the issue in the application circuit. Let me know if I can be of further assistance. 

Best Regards, 

Chris Featherstone

Hi Chris,

Thanks, I'll wait for them to reply.

Another question is regarding temperature. We see the influence of surrounding temp/hum reflecting to the probably the cap and it is changing the output frequency. How would you advise us to compensate for these environmental efffects? Would you know a effective solution? Thanks,

Regards, Marc

Hi Chris, I didn't receive any message from the DC-DC converter side. Can you check again or should I post a new question myself?