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Groner's Low noise measurement amp from Linear Audio vol 3 - spare boards?
Groner's Low noise measurement amp from Linear Audio vol 3 - spare boards?
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Old 26th February 2018, 07:26 PM   #221
jackinnj is offline jackinnj  United States
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Groner's Low noise measurement amp from Linear Audio vol 3 - spare boards?
Quote:
Originally Posted by Audio1Man View Post
Hi jackinnj

What about one more hole to use a metal standoff to ground to a metal plate/can?
Duke
From Linear Tech ApNote 159:
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Old 7th February 2019, 06:03 PM   #222
jan.didden is online now jan.didden  Europe
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Finally got around to measure Samuel's LNA powered by a Silent Switcher with the AP. Confirmed 350pV/RtHz.

I also have a Brookdeal 431 nanovolt preamp, running on batteries, specced at 400pV/RtHz and also confirmed.

Both 60dB gain but the Brookdeal has switchable roll-off at either side of the frequency range and two different input impedances (6k and 15k).

Nice to have some baselines. ;-)

Jan
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Old 22nd February 2019, 07:06 PM   #223
kumori is offline kumori  Europe
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Hello,

First of all:
is there a source for spare pcbs for this amplifier?

I tried to upload the gerber files from nanovolt.ch to the maker-pcb manufacturers oshpark and aisler, but it did not work properly. I always got error messages.

Eventually their auto import feature does not work with complex gerber files, or have to make some adjustments first... Does anybody have experience with manufacturing these pcbs? Are there some recomendations where to go to? I‘m located in Germany. Thanks
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Old 22nd February 2019, 07:52 PM   #224
NicMac is offline NicMac  Italy
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Groner's Low noise measurement amp from Linear Audio vol 3 - spare boards?
Try diyAudio member cwtim01. I got a couple from him not so long ago.
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Old 22nd February 2019, 08:56 PM   #225
gerhard is online now gerhard  Germany
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Quote:
Originally Posted by kumori View Post
Hello,

First of all: is there a source for spare pcbs for this amplifier?

I tried to upload the gerber files from nanovolt.ch to the maker-pcb manufacturers oshpark and aisler, but it did not work properly. I always got error messages.

Eventually their auto import feature does not work with complex gerber files, or have to make some adjustments first... Does anybody have experience with manufacturing these pcbs? Are there some recomendations where to go to? I‘m located in Germany. Thanks
I had some boards recently made at PCBway. That worked out quite nicely. Only problem was German customs in Leipzig who would not believe that the production of 10 boards costs $5 + transport. They gave in when I sent them the URL of PCBway orderering and proposed that for $5 + transport they could have their own shiny circuit boards. It seems they had no interest, but it did the trick. :-)

One of these was a respin of my 220 pV/rtHz preamp. It has now a 2nd input with a AD8620 for browsing with a scope probe; that gives away most of the noise advantage but is convenient and the low pass filter may help. Gain, low pass frequency corners, input selection and protection relays are now controlled with analog switches that are set up by a $2 CPLD that has a opto-coupled SPI-interface to the external world. It is also possible to use mechanical switches instead.

There will be a $5 board respin to remove a bug for the 80 dB gain selection and also an unexpected and intolerable noise increase from 220 pV/rt Hz to 400 pV/rt Hz above 500 KHz. I first had the idea that this was a peaking effect but it wasn't as shown by the network analyzer.

It was just the routing of the input. The 20 op amp inputs were fed by a single U-shaped route and that simply had a rising impedance above 500 KHz. A few Ohms count here. Soldering in half a dozen of wires to connect the left and the right vertical lines of the |_| removed that nearly completely.

Boards of this and the next iteration are/will be available. I don't need all 10 of them. They are based on 10 ADA4898 op amp pairs.( plus some as post amplifiers)

Pics: 1 nV/rt Hz is about at +2 dB (60 Ohm resistor), so the own noise is abt 14 dB below this.
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Last edited by gerhard; 22nd February 2019 at 09:03 PM.
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Old 23rd February 2019, 03:43 AM   #226
kumori is offline kumori  Europe
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Thanks NicMac and Gerhard!
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Old 23rd February 2019, 07:24 PM   #227
jackinnj is offline jackinnj  United States
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Groner's Low noise measurement amp from Linear Audio vol 3 - spare boards?
Osh park - you have to zip the files
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Old 24th February 2019, 01:19 PM   #228
Mike Gergen is offline Mike Gergen  United States
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Gerhard
I'm interested in a board when available.
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Old 24th February 2019, 05:10 PM   #229
gerhard is online now gerhard  Germany
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OK, send me your snail mail address. Cost is a if we should ever meet.

I'd propose the old version with mechanical switches and foil capacitors in the input. The current version has the ado with the CPLD that must be programmed and is transitional with a newer version due in 4 weeks. The main problem with the old version are the many yet still not enough foil capacitors. The new version there has a wet slug tantalum 4700uF / 25V there. That is much better for noise below 50 Hz but comes with a shocking price tag. But, as a first impression, good quality Al electrolytics seem to work. Maybe I was too easily impressed by Jim Williams and the metrology crowd at EEVblog.

The write-up is in < http://www.hoffmann-hochfrequenz.de/downloads/lono.pdf >

Disregard the layout with the ceramic capacitors in the input, although that seems to work the board has never been fabricated. Some more material is in < http://www.hoffmann-hochfrequenz.de/...no_v2_dist.zip >

The necessity of a large input cap is the biggest drawback of bipolar low noise amplifiers. All low voltage noise amplifiers must AC short circuit the input bias resistor through the DUT. The DUT _MUST_ be very low impedance or it would generate so much noise by itself that the use of the amplifier was pointless.

Remember that at the lower -3dB point Rbias and Ccoupling are just equal. That does not mean a lot of shorting. FET amplifiers have an advantage there. 10 Meg will produce even more noise than 10K, but a foil cap is all it takes to short it efficiently.

But then, the FETs have a worse 1/f frequency themselves and the ultra-high gm of the massively paralleled FETs is a stability problem.

BTW. in his article, SG noted a deviation of the gain from the calculated value. That is the influence of the output Z of the op amp that drives the feedback. It happens in simulation also and can be corrected by inserting a VCVS to drive the feedback divider.

cheers,
Gerhard.
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Old 24th February 2019, 06:04 PM   #230
syn08 is offline syn08  Canada
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Quote:
Originally Posted by gerhard View Post
BTW. in his article, SG noted a deviation of the gain from the calculated value. That is the influence of the output Z of the op amp that drives the feedback.It happens in simulation also and can be corrected by inserting a VCVS to drive the feedback divider.
No, to my results, it's mostly the effect of the PCB traces resistance and the feedback resistors tolerances. Please check if the op amp model(s) correctly model the output impedance, not many do.

1cm of 0.5mm wide 1oz. copper trace has about 10mohm. Ignoring any contributions of the feedback resistors tolerance, 10mohm added to the 1ohm grounded feedback resistor leads to a 0.1dB error in the closed loop gain. The effect of adding 10mohm (which is the same order of magnitude as the closed loop op amp output impedance) to the 999ohm feedback resistor is negligible.

If you add the feedback resistors tolerances (I suppose you don't use better than 0.1% tolerances, for cost reasons) the error budget shows you can get an up to 0.3dB error (for a gain of 60dB) with an average (expected) error of 0.2dB. Which is what I got across multiple implementations of various topologies for a LNA, both jfet and bipolar input. I got in one case, by pure chance, 60.07dB gain.

Proof of the fact that the op amp output impedance is negligible compared to a feedback resistor of 999ohm: the closed loop gain variation across the amp closed loop bandwidth (200KHz or more) is very small. The open loop output impedance usually varies significantly with frequency (see attachment), while the loop gain is constant in the closed loop bandwidth. The attached example is for OPA211.
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