I have a relaxation oscillator circuit which is supposed to oscillate around 6 MHz. It mostly oscillates in a very stable manner around that frequency.
Sometimes, however, 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:
- On frequent on-off, I can put the circuit in this mode.
- Bringing fingertip near inverting input, not touching just in the air.
- Touching an open ended wire to non-inverting input.
- 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 the non-inverting input with an open ended probe brings it back to 6 MHz
- A power reboot also fixes it.
A few things that I tried to fix this, with no success:
- Putting a 10 pf capacitor between comparator-output <----> comparator-non-inverting input.
- Putting a 200 ohm resistor on comparator-output.