Super Duper Vdd stiffening required on 555 timer, what is the best way?u JYyW234a NYyd D 7x mh Cc DGj 567 506H50 EeRr
I am using a 555 timer for a (16bit) frequency sensor/counter.
It works by counting the number of pulses read in the 125ms sample time set by a 555 timer; resets & repeats...
I am using the timer in astable operation.
TH (time pulse high) is the sampling ON signal.
This time is set and trimmed (+/- 5% adjustment range) with a high quality POT.
TL (time pulse low) falling edge initiates a data-latch read --> then a counter reset operation
Right now I have it on a bread board. I am making a PCB for the final design and I want to iron out this problem for the PCB design.
Here is the problem: The measured frequency is not super stable (+/- ~3Hz @ 25kHz) and it takes a while to settle.
I think it is because the sample time is getting effected by the noise on the Vdd rail. I have decoupling caps on all the IC's but it is on a bread board so this can be expected. For the PCB layout I want to insure the 555 timer is on a solid 5v and the DCDC converter output is steady.
Here are some ideas I have on how to do this.
- Use a rail-rail opamp and 4v7 reference to regulate the Timer Vdd @ 4v7
- Use ferrite beads to further decouple the Timer and all the other ICs from each other.
- Use a seperate DCDC converter for the timer.
- Use a linear regulator IC for the Timer Vdd.
Which of these would be the best practice for insuring a constant timer Vdd value?
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8\\$\\begingroup\\$ Maybe you should use a crystal instead. I am actually impressed that it's only +/- 3Hz @ 25 kHz. That's great, considering you're using a 555 timer. \\$\\endgroup\\$ – Harry Svensson 10 hours ago
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\\$\\begingroup\\$ VDD stiffening may not help : it may be some thermal effect (capacitor or the chip itself warming up). indeed, "takes some time to settle" suggests that. The other answers are absolutely correct : if 3Hz in 25kHz isn't good enough, you really want a fundamentally better source (e.g. watch crystal oscillator at 32.768 kHz. \\$\\endgroup\\$ – Brian Drummond 1 hour ago
2 Answers
Your measured short-term stability is about +/-0.01%, which isn't bad for an uncompensated RC timer.
You can improve it by using low temperature coefficient resistors and capacitors in the timing circuit, maybe by bypassing pin 5 to ground, by isolating the circuit thermally and electrically, in the extreme controlling the temperature in an oven, powering it from a battery with an ultra-low noise linear regulator and capacitance multiplier stage, and using opto-isolation on the outputs.
But that's just silly. Use a crystal, they're cheap and orders of magnitude better. For example, a 100kHz crystal, oscillator (74HCU04 + a couple resistors + load caps) and a divide-by-four (eg. a 74HC74). Tolerance (absolute accuracy) of that particular linked crystal is +/-30ppm or about 0.75Hz in 25kHz. Short term stability will be much better again.
There are also programmable oscillator products you can order, there might be one in a useful range for you.
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\\$\\begingroup\\$ "Your measured short-term stability is about +/-0.01%, which isn't bad for an uncompensated RC timer." - How would one compensate it to make it more stable? \\$\\endgroup\\$ – Harry Svensson 9 hours ago
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3\\$\\begingroup\\$ @HarrySvensson You could isolate it from thermal effects like air currents and introduce deliberately temperature sensitive components to compensate for the drift of the capacitor and resistors (and, to a lesser extent, the IC). If crystals (and ceramic resonators) were not so cheap and available, such techniques might make sense. Another method is to use a lookup table driven by temperature, stored in EEPROM to trim some parameter. \\$\\endgroup\\$ – Spehro Pefhany 9 hours ago
I don't think you will ever get the accuracy and stability you want from a 555 timer. The pulse width is determined by the values of resistors and a capacitor, and the values of these elements will change with temperature and over time.
For a precise pulse duration you should be looking at a crystal oscillator with a digital counter to generate the desired pulse.
