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I have a relaxation oscillator circuit, which is supposed to oscillate around 6 MHz, and mostly it oscillates in a very stable manner around it.

However sometimes it starts to oscillate at 25 MHz, which is the fastest oscillation available for this comparator (TLV3202) given its propagation delay is 40 ns. Some cases where this happens are:

  1. on frequent on-off, I can put the circuit in this mode
  2. bringing fingertip near inverting input, not touching just in air.
  3. touching an open ended wire to non-inverting input.
  4. also during normal handling of running circuit we have seen this. As this circuit will be used for further capacitive sensing, we are in trouble.

A few observations:

  • touching non-inverting input with an open ended probe bring it back to 6 MHz
  • or power reboot also fixes it.

Circuit diagram:

schematic

PCB layout:

PCB layout

A few things that I tried to fix this, with no success:

  1. putting a 10 pf capacitor between comparator-output <----> comparator-non-inverting input.
  2. putting a 200 ohm resistor on comparator-output.
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    \$\begingroup\$ Check your title! 25 mHz (millihertz) is not 40ns (nanosecond period) ;) \$\endgroup\$ – carloc May 8 at 7:45
  • \$\begingroup\$ fixed,its Mega only \$\endgroup\$ – neeraj May 8 at 8:26
  • \$\begingroup\$ Supply decoupling? Supply voltage? It's "MHz" and not "Mhz". \$\endgroup\$ – Andy aka May 8 at 8:30
  • \$\begingroup\$ Supply Voltage is 3.3v from an LDO ( mic5205) , decoupling caps are 0.1UF + 10UF. BTW I have tried to even add a third large electrolytic 100UF. \$\endgroup\$ – neeraj May 8 at 9:44
  • \$\begingroup\$ Partial schematics like this may hide problems...where else does pin 2 (inverting input) go? These RC oscillators often run with period approx. = time constant. Yours seems to run faster, at a higher frequency. Perhaps you should join C13 & R11 at a common ground point. \$\endgroup\$ – glen_geek May 8 at 13:41
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From the symptoms ---- vulnerable to various pieces of metal near the pins 1,2,3 --- this seems like Electric Field feedback, thru the air.

Thus 2 steps to a cure

(1) capture some of the electric field coupling from output to non-inverting input; these are pins 1 and pins 2. Perform this capturing, by drilling 2 or 3 or 4 holes down thru the GND plane, and install some vertical metal pins on both sides of the feedback resistor. These grounded bits of metal serve as RETURN_PATHS for the Efield.

Your PCB, which looks good, by the way, has direct coupling between pins 1 and 2, INSIDE THE PCB, under pins 1 and 2, inside the FE-4 dielectric. NOthing to be done about that, unless you could bring the Grounded foil to be closer to those pins, which would capture more of the Efield underneath the solder-pads.

(2) add a small capacitor (5--10pF) across the +feedback resistor R13

(2a) reduce the 4.7K ohm resistors (3 of them) to 1Kohm

And where is the timing capacitor?

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  • \$\begingroup\$ trace coming out of pad towards lower right corner has two capacitor in parallel. One is visible on lower right side another capacitor is formed by a cylnderical capacitor. These 2 in parallel are my timing capacitor. Let me try (2) and (2a) both. Didn't understand "bring the Grounded foil to be closer to those pins" \$\endgroup\$ – neeraj May 8 at 21:51
  • \$\begingroup\$ BTW earlier I added a 10pf cap between (+ve) and (-ve) and that stopped all 25Mhz oscillation, I must admit I don't know the theory behind this, but it was the only suggestion mentioned in tlv3202 datasheet regarding oscillation issues. I have already tried (2a) , just remembered. 10 pf between output and (+ve), it didn't help. I was able to put oscillator in 25MHz freq. which feedback resistor you mentioned in (1) +ve or -ve ?? \$\endgroup\$ – neeraj May 8 at 22:01
  • \$\begingroup\$ As I wrote, there is coupling thru the air. To reduce that, have small pieces of metal that are grounded, over the Vout, the Vin+, the Vin-. \$\endgroup\$ – analogsystemsrf May 8 at 22:52

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