Good that you found the reason. For me it's a complete no go to combine precision circuits and SMPS. However, I suggest reading AN101 by Linear Technology for using switchers and LDOs.
Yes, it was a lesson learned, even for initial operation I will avoid SMPS in future
Ultra precision circuits and SMPS are tough to combine while maintaining the precision, but e.g. keithley did in DMM7510
Thanks for AN101, it was a good bed reading to get an idea of realistic numbers.
Hopefully last OT post, here are the promised details of investigation on LT1763 screw up (follow up of
2375,
2383 &
2388).
The temperature of LT1763 was measured with thermocouple with pcb upside down in free air @15V supply:
room temp: 21.4°C
max @warm up: 30.9°C (dT 9.5°C) - certainly not in equilibrium
min @steady state: 28.5°C (dT 7.1°C)
The reference is GND at power jack for all measurements:
1. Screw up on warm up due to resonance between SMPS and LT1763.
LT1763 dropped from 12V to 10V output with 15V input and ripple of 1Vpp@200kHz.
Same result with 16V supply, so this seems to be an issue, or is it a feature (expected behaviour)?
2. Apparently normal operation at steady state with expected 12V output - input ripple is reduced to 0.76Vpp.
3. Same at steady state with closeup of 12V ripple: 0.76Vpp/0.076Vpp gives attenuation of 10 (20dB), Datasheet states ~30db@200kHz - so it is a bit off.
Interestingly the feedback has 40mVpp ripple, but should have only 1/10th of 12V ripple (7.6mVpp).
There is a lot of current flowing between SMPS and input cap and as the probes are referenced to GND at power jack, there is a bit of voltage drop between GND at power jack and GND at LT1763 output involved.
But luckily we can get the voltage drop due to excessive current from -Ref line, which is ~20mVpp, so ~20mVpp remaining at feedback.
Should be in the ballpark as there was no appropriate probing possible (normal GND leads at probes).
4. Added 0.5m leads to SMPS - resonance has gone and turned into expected ripple for SMPS (down to 1/10th at ~100mVpp).
Output now is 12V on warm up & steady state.
So I had the luck to hit the resonance point of the system with choosen leads
5. For comparison: same with linear PSU