HP 3245A Universal Source - 1.5ppm jump in output

[branadic]

Maybe I have missed it, but was R400 adjusted for operation with ADR1399?

-branadic-

[bastl_r]

What effect should the reduction have?
The +5V before CR400 is far too low anyway. 

[branadic]

Oh, I've read +15 V first.
Even with an LM399 that doesn't make sense and shouldn't work, unless that low potential is way lower than zero.

-branadic-

[Kleinstein]

The CR400 and R400 part is only for start up. During normal operation CR400 is blocking. No real need to adjust R400 as the exact value is non critical. The main point is avoiding that the OP-amp goes negative, which an OP07 with no negative supply can not do anyway.

[branadic]

Right, but R401 does need adjustment to provide 3 mA for ADR1399 (11.6 V - 6.95 V)/3.5k = 1.3 mA, which is not enough for it.

-branadic-

[IanJ]

Right, but R401 does need adjustment to provide 3 mA for ADR1399 (11.6 V - 6.95 V)/3.5k = 1.3 mA, which is not enough for it.

-branadic-

Yes, R401 was adjusted to set 3mA zener current when the 1399 was fitted.

Ian

[ch_scr]

I have seen 0805 "square block" smd resistors get "jumpy" after manhandling them a bit too much upon "desolder & put back". The MELF types seem more robust, bigger contact surface to the solder-able ends. Is the ADR1399 jumpy when you *now* measure it?

[Kleinstein]

Yes resistor can become "jumpy" and show extra 1/f type noise. This is not just a function of the form factor and soldering. Also normal THT resistors with careful sodering can show this. Not all resistors are equal in this respect and it is not so easy to predict which are good and which are a bit more noisy.
As an example I have found more noise in PTF56 type resistors (250 mW THT) than in some 0805 size SMDs (Susumu RR).

The HP3245A circuit for the 10 V range is however made in a way that only few resistors really contribute. The main point are 3 resistors inside the RP100 array. R400 is contributing too little to expect a noticible effect - unless the resistor is really defective or has a cold solder joint.
The rare jumps make it hard to say anthing definitive - it could be an odd reference.