Hello,
before changeing the mulltiplexer to DG4052 on PCB#2 I wanted to see wether the multiplexers have any influence on tempco of the references.
So I started with T.C. measurements.
Unfortunately I recognized that I did not repeat the T.C. measurements of the LS8- parts after I had populated the metal can parts.
Due to the additional heaters on the PCB the measured temperature range on the bottom of the die cast housing has changed even with same environmental temperature profile.
On the LS8 parts the temperature range was 12-45 deg C without population of the metal can parts.
Now it is 18-46 deg C with the metal can parts populated.
And it looks like the effective T.C. relative to the temperature sensor has increased.
So most probably my temperature correction for the LS8 parts was wrong after population of the metal can parts.
Further I recognized that there is still some latch up effect (2 times) after having changed from MAX4052A multiplexers to HEF4052.
I looked at the power supply values at the beginning of the latchup.
And surprise: on entry of the latchup the 15V supply of the LDO (LM2940) increased by 50-60 mV. Whereas the raw value 17V was about constant.
At the same time the voltage appeared more noisy. (Oscillations ?)
I double checked the data sheet and found that I was missing the 22uF capacitor at the output of the LM2940.
There are only 12 pcs 100nF capacitors on the 15 V supply (one for each ADR1399 heater) 2 for the Multiplexers and 2 directly at Output of LM2940 / input of LM2950A.
I tried to measure the oscillations with the scope. But obviously this changed something at the cirquit so no oscillations visible.
I removed the Jumper for the heaters since the "latchup" occured more at higher temperatures = lower heater power.
So in this condition (lower 15V load and 8 capacitors removed) I saw the oscillations 2Vpp with 45 kHz.
A 100uF capacitor which I had on hand fixed the oscillations.
So now I can repeat the T.C. measurements hopefully without any further "latchup" effect.
But I fear the ageing measurements up to now where useless:
Wrong T.C. and one never knows how the unstable 15V supply influenced the measurements.
I am feeling like a bloody beginner.
On the other side: the oscillations are only visible under rare conditions otherwise I would have seen them on cirquit start up.
with best regards
Andreas