Electronics > Metrology

3458A Worklog

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MiDi:

--- Quote from: Dr. Frank on January 20, 2025, 08:00:59 pm ---Have you already replaced the noisy OpAmps for the +/- 12V and +5V references at U180, like Chuck Beuning did, or wanted to do?
Others already claimed, that w/p replacement, the lesser noise of the ADR1000 would not become effective.

Have you observed what happens with your specific ADR1000, when it's switched off for several days, or weeks?

I have observed that its reference voltage needs 1..2 weeks to recover to its old, powered-on state.
When sitting idle, my specimen showed a temporary shift of up to +5ppm after re-powering.

--- End quote ---

Replacement of ref gain op amps are next steps, first want to get data with A9 replacement only.
The ADR1000 is only a week on until now, will do further tests after it has settled (TC, on/off hysteresis, ...) - so in 3-4 years maybe  :-DD.



--- Quote from: Kleinstein on January 20, 2025, 08:06:03 pm ---The capacitor C3 shown in the ADR1000 ref. circuit can make quite some difference for the noise in the 100 kHz range.

The higher frequency (e.g. half the run-up modulation frequency or some 160 kHz) reference noise can modulate down to near zero. The run-up alternates between using the positive and negative reference to get a zero. The switching effectively acts like an RF mixer, muliplying the ref. noise with the modulation pattern (as +-1).  There is some gain from the 50K/40 K resistor ratio. At least the 7 V to +-12 V gain is largely filtered, otherwise the ref. noise could be even worse.  In my estimate for the noise contributions the mixed down ref. noise is about on par with the integrator noise (mainyl the LT1001). There is quite some uncertainty here as the 100 kHz range noise for the LTZ1000 is not in the datasheet and details of the ref. circuit can have an effect from filtering to an added noise peak.

Filtering out the part about some 50 kHz does not take much. I think this is a part that is missed in the design, not just for the 3458. It is a bit tricky part that is easy to miss. It's also a slightly odd part that gets less when the voltage gets higher. As a complication the frequencies involved would also change.

--- End quote ---

Indeed, C3 adds additional LPF.
Thought about replacing op amps in the integrator/slope amp to decrease noise, any suggestions?
At least LT1001 in integrator could benefit from lower (current & voltage) noise upgrade.
Additional LPF of +-12/5V ref voltages could be another thing to try.

Kleinstein:
For the amplifiers I would consider the OPA205.
U110 should be effective with a noise gain of 2.25. Here the current noise is also relative important.
U165 should be effective with a noise gain of 1.25.
U160 should be effective like the reference.
U151 (for the +5 V ref. level) should be non critical and hardly worth a change.
In the integrator (U110) one may want to increase R112 because of the higher bandwidth.

The slope amplifier is a bit noisy, but still good enough. The quantization noise should still be a little higher. It would mainly effect the short integration times, like < 1 ms and hardly noticable at 1 PLC.
A logical position for a low pass filter is at the input of U160.

MiDi:
The KiCad project files and documentation for A9 ADR1000 are released: https://github.com/EleDes/3458A_A9_REF_PCB_KICAD




Specs of realized A9 (trimmed R415/R2 for low unheated TC for oven setpoint):
Tset59Iz10.51E-3Vz6.693419R415/R212600VQ2be0.5147VR4140.5256Enpp 0.1-10Hz330E-9STD 28E-9 (DIY LFLNA-80 1054Z 20MHz BWL, 2MSPS, 24Mpts, mean of 13 Samples)Enpp 0.1-10Hz258E-9STD 23E-9 (DIY LFLNA-80 1054Z 20MHz BWL, 1kSPS, 12kpts, mean of 14 Samples)En 1Hz17E-9(DIY LFLNA-80 3458A 1V, 1NPLC, AZ off, fast)
For comparison: typical LTZ1000A noise @3.6mA is ~840nVpp and ~60nV @1Hz (from DS)

         


Big thanks to xdevs, reps discord, the metrology meetings and the other voltnuts that shared their experience, published their designs and contributed to development.

MiDi:
3458A ADR1000 vs LTZ#3 drift, two short off times had very little effect on ADR1000 (March 2nd and 3rd):



The TC of A9 ADR1000 inside 3458A is around -80ppb/K vs. interpolated CAL175 (from hourly ACAL):

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