Electronics > Metrology

Valhalla 2701c schematics + firmware V3

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Electrole:
Inspired by the observations with the OPA134 made by Le_Bassiste I measured the noise of the Valhalla 2701C when using different op-amps for IC8 in the low-pass filter: LF356N (used by Valhalla), and these following substitutes: AD743JN, AD711AQ, OPA277PA, LT1677IN8 and LT1793ACN8. These were selected among single op-amps with low current noise I had readily available. I did not have the OPA134 available.

The noise in 10 kHz bandwidth was measured with NI USB-4431 with EG&G model 113 (set to 100 x gain) as pre-amplifier, with the 2701C set to 10 V in its 20 V range. I found that the measured noise was essentially the same for all op-amp used in this test, around 29.2 to 29.8 mV, with no solid correlation to the op-amp used.

The wide-band noise (about 20 MHz) was measured with Rohde & Schwarz URE, with the 2701C set to 10 V in 20 V range. This was also essentially the same for all op-amps used in this test, about 1.36 mV - 1.41 mV (which is higher than my previous measurements as the mains related noise using this setup depends on what else is connected to my mains power strip!) Again, not really any correlation to the op-amp used in this test.

The interesting part, however, is the low-frequency noise. At low frequencies the 1/f noise performance of the op-amp used for IC8 makes a difference. I measured the low-frequency noise with an EM Electronics model N1a nanovoltmeter (via a filter between the 2701C and the N1a to suppress noise above 10 Hz, and common-mode noise), with the 2701C set to 10 µV in the 2 V range. The voltage from the N1a was digitized by a Fluke 8842A at its medium measurement rate. An FFT was made on filtered data in LabVIEW. The LF356N used by Valhalla gives the highest noise of the op-amps used in this test. The following summarizes the findings, expressed as the standard deviation, and the observed peak-to-peak voltage, for two different frequency spans:

LF356N
Std. dev. = 305 nV / 2.33 µVpp 0.01 Hz - 1 Hz
Std. dev. = 269 nV / 2.31 µVpp 0.1 Hz - 1 Hz

AD743JN
Std. dev. = 199 nV / 1.77 µVpp 0.01 Hz - 1 Hz
Std. dev. = 162 nV / 1.48 µVpp 0.1 Hz - 1 Hz

AD711AQ
Std. dev. = 217 nV / 1.79 µVpp 0.01 Hz - 1 Hz
Std. dev. = 184 nV / 1.41 µVpp 0.1 Hz - 1 Hz

OPA277PA
Std. dev. = 204 nV / 1.51 µVpp 0.01 Hz - 1 Hz
Std. dev. = 163 nV / 1.23 µVpp 0.1 Hz - 1 Hz

LT1677IN8
Std. dev. = 257 nV / 2.11 µVpp 0.01 Hz - 1 Hz
Std. dev. = 222 nV / 1.82 µVpp 0.1 Hz - 1 Hz

LT1793ACN8
Std. dev. = 249 nV / 1.91 µVpp 0.01 Hz - 1 Hz
Std. dev. = 200 nV / 1.58 µVpp 0.1 Hz - 1 Hz

Based on this, I think it would sense to replace the LF356 with AD743, AD711 or OPA277. Other op-amps may be work equally well, the question is where the 1/f noise kicks in and that the op-amp is fine with the actual impedance level of the filter. Some low-noise op-amps with high current noise will not produce good results.

The OPA134 has low current noise, but its 1/f voltage noise starts to creep up below 100 Hz, so it would be interesting to find the noise at low frequencies.
Le_Bassiste: What was the frequency span of your measurements when you observed the improvement compared to LF356N?

RaymondMack:
Thanks for taking those measurements Electrole! I have some OPA277PA sitting in a drawer, should be an easy swap. Though maybe I should put a gold plated collet socket for comparison tests. I have an EG&G 5113 preamp and an Agilent 89410A VSA that can do FFT up to 10MHz for comparison. Probably not the best setup, but it should work.

Unfortunately further progress from me is halted since I killed one of the HV MOSFETs :palm: while probing around with my DMM. I believe my unit has an issue with the smaller transformer. With the 2x 20nF capacitor removed I can hear audible buzzing from the small transformer used to power the mainboard. When the bypass capacitor is installed the buzzing greatly quiets down, but doesn't completely go away. I need to put one of my wideband current probes on the capacitor to see what the ground current looks like.

When probing with my DMM4050 in AC volts (approx. 1 megohm AC coupled to earth ground) the compliance light in the current source would start flashing/dimly glow and random relays would start switching on and off. The 2701C uses opto isolators with the digital side connected to earth ground, so I don't think it has to do to the digital side. It must be one of the protection mechanisms on the analog side getting triggered. This would happen when trying to measure the AC common mode at node "L" on C211 as well as transformer nodes E9 and E10 on the edge of the PCB. In particular, the HV MOSFET died while probing E9, which caused the relays and LEDs to start randomly clicking/light up. It seemed like an oscillation. After disconnecting the DMM, relay RLJ clicked on and off about once per second |O. If I remember correctly, I had disconnected the 2x 20nF capacitors and everything from the front panel terminals. So the only current path would be leakage / arcing through the transformer itself.

While killing my 2701C I did discover that the primary source of my CM noise is not related to the switching supply. I believe that the bypass capacitor should be located at node "L" but while probing I had the issues discussed above. Looking at the schematic, I don't see how I killed the MOSFET or why the entire analog circuit goes haywire when probing E9, E10 or "L". Node "L" is connected to earth ground by the 2x 20nF when set to normal polarity through relays RLD and RLG. This is why the noise goes up when the polarity is reversed: node "L" is disconnected from the bypass capacitors and the CM signal passes through the HI lead of my 3400B to earth ground. Moving the bypass capacitor closer to "L" should--in theory--help with my noise issue as it shortens the path length and ignores the polarity reversal relays.

I should have some replacement MOSFETs here soon. There may be a few more parts that need to be replaced, as I haven't done an exhaustive search for bad parts yet. Thankfully the analog side is fairly simple, so it shouldn't be too hard to get it working again.

RaymondMack:
I finished repairing the 2701C today. All that had failed was a single MTP1N95 MOSFET. That said, I decided to replace all five with FQP2N90 from onsemi so the threshold voltages of each part would match. The FQP2N90 are a reasonably close match for the MTP1N95 and seem to work fine so far.

While checking the calibrator over, I noticed that some of the buzzing near the transformers appears to come from C201, a 1kV 10nF Y5U ceramic part. I'm going to try swapping it out tomorrow and see if I can reduce the audible noise. While I'm at it, I'll check for ringing and slew rate.

After that, I'll add in a collet socket for IC8 and run some tests with the stock LF356 vs an OPA277PA. I couldn't find any axial 2uF polypropylene capacitors to replace C6, C7 and C8 but I do have some 1uF parts that I can solder in parallel on the underside of the board for testing. If they help, I'll order some proper replacements.

I just noticed that this is the same filter topology as the active filter module used in the Fluke 8505/6A. If all goes well here, I may try replacing the LM301 with OPA277PA in my 8506A's active filter after I finish fixing the current shunt module.

Le_Bassiste:

--- Quote from: Electrole on August 08, 2021, 12:52:12 pm ---
Le_Bassiste: What was the frequency span of your measurements when you observed the improvement compared to LF356N?

--- End quote ---
unfortunately, i don't have such nifty gear as yours in my possession. i'm directly measuring the 10 V output of the 2701C in 2-wire mode with a keithley 2002 set to 10 NPLC, no FILT, and let it collect 200 samples. when testing the OPA134, that set of samples had SDEV = 2.85 µV.
for comparison, an LTZ1000 measured in the same manner directly at the output (Dr. Frank's design) gives SDEV = 1.0 µV. that's about as good as it gets with my setup. a FLUKE 343A comes out at SDEV = 1.5 µV with above settings.

for wide-band noise measurement, i'm using the 2002 simply in ACRMS mode, 200 mV, no FILT. baseline of the 2002 is about 30 µVRMS. the 2701C @ 10 VDC gives approx. 0.87 mVRMS after warm-up. it doesn't have Y-caps on the LO SENSE (yet), but i inserted 50 µH toroidal inductors on each of the four front output wires, and a 470 pF cap across + and - output terminals. it wasn't equipped with any EMI measures when i got it, and had about 2 mVRMS before the modification. again, a 343A would read just about 50 µV with the same settings.


RaymondMack:
I got busy with work and a few other projects so I haven't had much time to work on the 2701C until last night. I replaced C201 with a more modern ceramic part and the audible buzzing associated with the old capacitor went away. I then held a short length of flexible plastic tubing up to my ear to safely "probe" around and find the remaining source of the buzzing, and it seems that aside from normal transformer hum on the small transformer, the switching transformer is the primary culprit. I may pour some epoxy in the large gap between the laminations and windings on each side of the transformer, since that is where my probing identified as the primary source of the buzzing. For some reason I thought the smaller transformer was louder than it is :o.

I was going to probe the switcher but it turns out I didn’t have any alkaline AA batteries and my NiMh batteries were slightly too fat to fit in my Yokogawa 701921 differential probe. So measurements / further adjustment of the switcher circuit will have to wait until I can remember to buy some more batteries.

As for other news, I did some simulation in Tina-TI and think that something like the OPA189 is actually a better replacement than the OPA277. Simulations of the OPA134 were disappointing, as it was worse than the LF356. It is interesting to note that the noise of the LF356 peaks at 5Hz with the default filter capacitors and can be tamed somewhat by lowering C8 to 0.5uF and increasing C6,7 to 2uF and C4 to 1uF. From the simulation results, I think I'll just increase C6,7 to 2uF and use an OPA189. I still want to do comparison testing with the OPA277 and LF356 for validation. It could be that the switching noise of the PSU swamps out the noise of the voltage reference circuit.

Step response testing shows that the settling time increases by about 72% when C6,7 = 2uF. With the OPA189 (or similar AZ opamp) C6,7 could be lowered somewhat and still see a measurable improvement in reference noise without excessively increasing the calibrator's settling time.

(Note: All macro models except for the LF356 were downloaded from TI. So these should be fairly representative of real noise performance in the given filter circuit. Color Key: Maroon = OPA134, Blue = OPA189, Gold = OPA277, Green = LF356.)

Also, I believe the LTC1052 is the better replacement for the ILC7650s. It either matches or beats the ILC7650s in most DC parameters and notably has both lower current noise and voltage noise. The current noise of the LTC1052 at 10Hz is 0.6 fA/sqrt(Hz) vs. 10 fA/sqrt(Hz)  (or 0.01 pA/sqrt(Hz) as per the datasheet) and the voltage noise from DC to 10 Hz is 1.5uV verses 2uV. So it promises to be an all-around improvement over the ILC7650s. The TLC2652A has better long term stability and Vos temp co but has worse voltage noise and the current noise is only specified for 4 fA/sqrt(Hz) at 1kHz making me believe it is higher than even the ICL7650s at low frequencies.

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