Electronics > Metrology

Solartron 7061 noise

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grizewald:
500ppm sounds right and I guess that's why they went for the MC1648 VCO and why the crystal oscillator option isn't a VCXO; having had a good look around, a VCXO with 500ppm pull range doesn't exist!

branadic:
I'm quite curious if you grizwald still have the LTC1052 in the meter without any issues. Have  you changed the zener diodes? I saw the



the other day and for some reason after replacing the ICL7650 by an LTC1052 his meter released the magic smoke. Seems like some problem of the rails at or of the LTC1052.
I wonder, is it related to his specific board revision 706105O5Y of the analog board only that this happened or is this a general problem replacing the old chopper amp?

-branadic-

Kleinstein:
May my guess would be something like an latchup event at the LTC1052 - though I don't know why it should happen.  In the video one of the 15 V supplies was a bit high. With only 7.5 V Zeners the OP would get a nominal 15 V, but with only 2 mA supply current the 7.5 V zeners may have less than 7.5 V drop and thus maybe a supply at the upper limit (18 V limit). The LTC1052 has the specs for +-5 V supply and in the circuit the input and ouput signal is all close to zero. So a +-5 V supply (e.g. 10 V zeners) would be the safer way to run the OP.  The LTC1052 may also be not too happy to work without local decoupling, so it may oscillate and draw a much higher supply current that could increase the heat to the diodes, which may respond with to much supply to the OP while very hot.

A point to check may be the regulator at the supply - maybe oscillation or a poor solderjoint to give spikes with more than +-15 V.

The LTC1052 and ICL7650 are very similar - the main difference is in the noise specs. Even the supply current is very similar.
The LTC1052 is a bit slower, but this should not be a big deal: In the circuit the speed is anyway set by the local FB cap. So the OPs speed would not matter much.

MiDi:
We discussed that already in the ltz1000 thread, where Dr. Frank and branadics version had same issues.
Only the current limiting of the lt1763 saved it from self-destruction after latch-up.
On power rails with higher current limit it is likely the LTC1052 gets damaged.

branadic:
Played a bit with Solatron7061 this weekend and monitored some voltages. I spotted large hum in the readings, so I looked into the oscillator of the PLL and bingo, it was way off. So I brought it back into the range where the PLL was able to lock and readings are much better now. Figure 1 and 2 (100 samples) were taken with filter turned off. Figure 3 shows the effect with filter turned on, reducing the repitition rate in 7 digit mode to about 7.4 s.

-branadic-

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