Low noise, high bias voltage on picoAmp TIA's input, howto?

On Mon, 24 May 2021 19:59:20 +1000, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>On 21/05/2021 22:08, timo.k...@ibtk.de wrote:
>> Dear all,
>> 
>> at first I want to thank all the experienced guys, Phil, Winfield, Gerhard... who share their valuable knowledge. That helps a lot! Thank you!
>> I'm working at a Helmholtz facility in Germany/Dresden on equipment used in fundamental research.
>> 
>> To measure e- and ion beam currents down to sub-pA I built up Fameio, a module including TIA (9 decadic ranges), diffamp, ADC/DAC, supplies, bias -200V to +200V:
>> https://ibtk.de/project/hzdr/Fameio-presentation/20210521_Fameio.html
>> The variable 2mA current limited bias, which has to sit on the measuring tip (in relation to GND), is a deadly for the TIA input in the case of Triax cabling errors (shorts).
>> The TIA is very sensitive to bias noise, if the bias node is not perfectly AC-coupled to the front-ends OPA supply. As well as any differences between its +in and Guard_ext couple into the measured signal due to the cable capacitance.
>> For lowering the noise I will throw out the iseg modules and use a Royer.
>> 
>> Question:
>> I'd like to have a more robust bias-solution.
>> I thought about photocouplers to feed the DC voltage in series to the TIA tip directly, it's open voltage is nice (30V) but the current is way too low (Isc=4uA).
>> It could be possible to use an DCDC instead the same way, but it may be challenging due to the needs of isolation, screening and guarding.
>> 
>
>I don't think connecting the bias voltage in series with the input pin 
>is very practical - the voltage source would have to be very small 
>physically in order to avoid excessive capacitance. If a 
>transformer-based DC-DC converter were used, it would also be hard to 
>get the inter-winding capacitance small enough to not suffer from 
>interference from the AC voltage on the windings, even with quite 
>careful screening. A photovoltaic device (perhaps driving a step-up 
>autotransformer) might be ok, if it is small enough, but I think your 
>existing approach is better.
>
>Anyway, I don't think that putting the voltage source in series with the 
>input terminal has any advantage for the robustness against damage when 
>the input is short-circuited, as the bias will still appear at the 
>amplifier input. To make it more robust, I think you need to add more 
>series resistance (perhaps with some measures to stop that from causing 
>degraded performance) and/or add more protection diodes (which should be 
>chosen carefully and operated at zero bias).
>
> From what I can see, it looks like you have floated the entire 
>front-end at the bias voltage, including the shield cans (which would 
>minimise collection of current from ions generated in the air inside the 
>shield can). This is what I would have done. I presume that the power 
>supplies for the op-amps are provided by DC-DC converters with their 
>outputs referenced to the bias voltage. I could not find any diagram of 
>the front-end that included the isolated power supplies, and the power 
>pins of the op-amps seem to be hidden so I can't really tell what you 
>have connected them to.
>
>It seems to me that you would need an instrumentation amplifier to shift 
>the signal to be ground-referenced. I guess U7A is doing that, but I 
>don't know what part number U7A is as it isn't labelled. If it is a 
>difference amplifier then I can't understand why you connect the 
>feedback resistor to its output (ground-referenced) rather than to the 
>output of U5A.
>
>I do wonder whether it is worth considering replacing the switchable 
>feedback resistors by a p-n junction (a diode-connected transistor, JL 
>and PH could suggest one with low leakage and good log-conformance) and 
>then digitising the result of that (as well as a replica diode at the 
>same temperature and known current). That way you get a voltage 
>proportional to kt/q log the current ratio. Whilst I would not expect 
>much better than 5% accuracy that way, it might still be better than the 
>tolerance of a 1 Teraohm resistor, and removes the need for 
>auto-ranging, relays etc.
>
>
>
>

If you ever need a really low noise floating voltage or current
source, consider a PV photodiode. Good for maybe 9 volts and 10s of
uA.

The next best is maybe the Murata dc/dc converters, but still pretty
noisy.

https://www.dropbox.com/s/0e9lzg2fcbhsd04/Murata_7.jpg?raw=1

I needed about a watt there, but it adds a lot of jitter, so I use the
off-the-board option.



-- 

John Larkin      Highland Technology, Inc

The best designs are necessarily accidental.


  

Am 24.05.21 um 17:47 schrieb jlarkin@highlandsniptechnology.com:

> 
> If you ever need a really low noise floating voltage or current
> source, consider a PV photodiode. Good for maybe 9 volts and 10s of
> uA. 

Yes, I wanted to propose a small flock of
<  https://www.digikey.de/products/de?keywords=VO1263    >
or similar.
I have used similar ones for BF862 or CE3520K3 bias.

Alternatively rising the frequency of the DC/DC above
the frequency band of interest, where it can easily be filtered.

At 400 mW you can probably do great things with
LC step-up networks or these Macom wideband transformers.

Cheers, Gerhard

On Mon, 24 May 2021 18:10:48 +0200, Gerhard Hoffmann <dk4xp@arcor.de>
wrote:

>Am 24.05.21 um 17:47 schrieb jlarkin@highlandsniptechnology.com:
>
>> 
>> If you ever need a really low noise floating voltage or current
>> source, consider a PV photodiode. Good for maybe 9 volts and 10s of
>> uA. 
>
>Yes, I wanted to propose a small flock of
><  https://www.digikey.de/products/de?keywords=VO1263    >
>or similar.
>I have used similar ones for BF862 or CE3520K3 bias.
>
>Alternatively rising the frequency of the DC/DC above
>the frequency band of interest, where it can easily be filtered.
>
>At 400 mW you can probably do great things with
>LC step-up networks or these Macom wideband transformers.
>
>Cheers, Gerhard

We have used audio-type transformers, driven by sine waves, as fairly
quiet floating power supplies. Low frequency gives less coupled AC
current and no spikes.

I think I've seen that done in SMU's, with a custom shielded
transformer.

On 25/05/2021 00:10, timo.k...@ibtk.de wrote:
>> I have to do something like this, I would use two cascaded toroidal
>> transformers...
> That sounds very interesting.
> Are you sure, that toroidals are best-suited with respect to their stray field? Years ago I read anywhere, they're not, I believe from Bruno Putzeys.
> So the best classD-Amp modules (as far as I know) have RM-cores instead of toroidals. See here (lower left):
> https://ibtk.de/project/amplifier/Power-Amp/Fotos/20191004_Power-Amp_4xNC252MP_open_IMG_2149_1k.jpg  
> The cap coupling could be worse, don't know...
> 

I meant to write "if I had to do something like this". I haven't though 
about it enough yet though. I agree that RM cores might be just as good 
or better than toroids, and I now realise that making one winding have 
only a couple of turns and operate at very low, balanced voltage is of 
limited benefit unless the other winding also has only a couple of turns 
with little voltage between them. (Or, if it has more turns with more 
voltage between parts of it, it will need excellent screening against 
capacitive coupling.)

It is strange that as far as I know there isn't any off-the-shelf DC-DC 
converter for instrumentation, with a well screened transformer and 
quiet drive circuit.

Bill Sloman
24.05.2021, 17:11:16 (gestern) 
> Toroidal transformers have no stray field, if they are non-progressively wound.

Didn't here about that, so I have to learn a bit.

> https://www.amazon.com/Coaxial-AC-Bridges-B-Kibble/dp/0852743890

$900 and up? Hmmm...
Beginning from p.101 it's of practical interest. Thank you!
All that stuff is important, if the "DCDC" powers the bias, which is inserted before the TIA.
If I can place it on the bottom of the PCB and let drive bias the preamp reference similar to the recent situation, the radiated field is of much less concern, at least I hope that.

jla...@highlandsniptechnology.com
24.05.2021, 17:48:05 (gestern) 
> If you ever need a really low noise floating voltage or current
> source, consider a PV photodiode. Good for maybe 9 volts and 10s of uA.

Gerhard Hoffmann
24.05.2021, 18:10:54 (gestern) 
> Yes, I wanted to propose a small flock of...VO1263...

Thanks, looks a lot better than the Toshiba TLP3924.

> The next best is maybe the Murata dc/dc converters, but still pretty
> noisy.

I'm a little bit done with DCDC modules. Most of them do not specify no load in currents. So I was surprised about the ~8W power consumption of the Fameio module, containing 5 DCDCs. The TDKs consume ~30mA @24V each!
I tested a LT8302 flyback with EFD15, it's Iq was ~2mA, the efficiency >80%, which is okay. Additionally it produces all needed voltages from _one_ flyback, except the bias. Because there are linear regs afterwards, this seems to be the better solution.

If I could drive the Baxandall sine converter to efficiencies significantly above 50%, this one could produce all voltages.

> Alternatively rising the frequency of the DC/DC above
> the frequency band of interest, where it can easily be filtered.

That's already the case: 100kHz power supply >> 10kHz max to be measured.

> At 400 mW you can probably do great things with
> LC step-up networks or these Macom wideband transformers.

That principle looks interesting, I did not know of it before. But still seems to use square drive instead of sine?

> We have used audio-type transformers, driven by sine waves...

The "lower end" would be a mains transformer, followed by a Delon circuit. Yes, very simple.
All these are followed by at least one rectifier which produces noise.
And if the frequency rises, trr becomes visible.
https://ibtk.de/project/hzdr/Fameio-presentation/20210522_FAMEIO_SuperSIMS-MV40-Ersatz_S38-39.pdf
The STTH112UFY shows it in Abb. 7.21 (without installed parallel caps).
The smallest SiC devices seem to be for 2A, big and with high Cj.
Is there another small (Schottky?) diode without or very low reverse recovery for >500V?

Chris Jones
13:25 (vor 1 Stunde) 
> It is strange that as far as I know there isn't any off-the-shelf DC-DC
> converter for instrumentation, with a well screened transformer and
> quiet drive circuit. 

I fear you're right...

Thank you!
Cheers, Timo

On Tuesday, May 25, 2021 at 10:54:45 PM UTC+10, timo.k...@ibtk.de wrote:
> Bill Sloman 
> 24.05.2021, 17:11:16 (gestern)
> > Toroidal transformers have no stray field, if they are non-progressively wound.
> Didn't here about that, so I have to learn a bit. 
> 
> > https://www.amazon.com/Coaxial-AC-Bridges-B-Kibble/dp/0852743890 
> 
> $900 and up? Hmmm... 
> Beginning from p.101 it's of practical interest. Thank you! 
> All that stuff is important, if the "DCDC" powers the bias, which is inserted before the TIA. 
> If I can place it on the bottom of the PCB and let drive bias the preamp reference similar to the recent situation, the radiated field is of much less concern, at least I hope that. 
> 
> jla...@highlandsniptechnology.com 
> 24.05.2021, 17:48:05 (gestern)
> > If you ever need a really low noise floating voltage or current 
> > source, consider a PV photodiode. Good for maybe 9 volts and 10s of uA.
> Gerhard Hoffmann 
> 24.05.2021, 18:10:54 (gestern) 
> > Yes, I wanted to propose a small flock of...VO1263... 
> 
> Thanks, looks a lot better than the Toshiba TLP3924.
> > The next best is maybe the Murata dc/dc converters, but still pretty 
> > noisy.
> I'm a little bit done with DCDC modules. Most of them do not specify no load in currents. So I was surprised about the ~8W power consumption of the Fameio module, containing 5 DCDCs. The TDKs consume ~30mA @24V each! 
> I tested a LT8302 flyback with EFD15, it's Iq was ~2mA, the efficiency >80%, which is okay. Additionally it produces all needed voltages from _one_ flyback, except the bias. Because there are linear regs afterwards, this seems to be the better solution. 
> 
> If I could drive the Baxandall sine converter to efficiencies significantly above 50%, this one could produce all voltages.

Jim Williams - in his Linear Technology application notes N49, AN51, AN55, AN61, and AN65 -  was getting around 92% efficiency.The Baxandall Class-D inverter is pretty good.  If you want a a better sine wave my variation on it is only about 50% efficient.

> > Alternatively rising the frequency of the DC/DC above 
> > the frequency band of interest, where it can easily be filtered. That's already the case: 100kHz power supply >> 10kHz max to be measured.
> > At 400 mW you can probably do great things with 
> > LC step-up networks or these Macom wideband transformers.
> That principle looks interesting, I did not know of it before. But still seems to use square drive instead of sine? 
> 
> > We have used audio-type transformers, driven by sine waves... 
> 
> The "lower end" would be a mains transformer, followed by a Delon circuit. Yes, very simple. 
> All these are followed by at least one rectifier which produces noise. 

Synchronous rectifiers, with properly driven MOSFet switches, are much quieter.

<snip>

> > It is strange that as far as I know there isn't any off-the-shelf DC-DC  converter for instrumentation, with a well screened transformer and  quiet drive circuit.

It's not a big market. It's been around since Ralph Morrison wrote the first edition of "Grounding and Shielding Techniques in Instrumentation" back in 1967 - I read it back then, but the copy on my bookshelf is the 4th edition from 1998 - but it isn't a mass market.

-- 
Bill Sloman, Sydney

On Tuesday, May 25, 2021 at 4:25:56 AM UTC-7, Chris Jones wrote:

> It is strange that as far as I know there isn't any off-the-shelf DC-DC 
> converter for instrumentation, with a well screened transformer and 
> quiet drive circuit.

The drive (for a DC low-ripple output) would best be a triangle wave, so
the rectifiers after the transformer only make a short blip twice per cycle.
Some square-triangle generation schemes make a small step at the
turnover, which can be tuned to be the same as the forward voltage step
required to turn on the rectifiers.

That makes for a relatively quiet conversion, but it's not energy-efficient,
which is why it's not similar to the  non-instrumentation DC/DC conversion
we see in mass-produced items.

Bill Sloman
> Coaxial-AC-Bridges-B-Kibble
Found it in the web, thank you!

> Jim Williams - in his Linear Technology application notes N49, AN51, AN55, AN61, and AN65 - was getting around 92% efficiency.
Ahh, I see, it looks good. It seems to be very dependent on the core and it's winding numbers (not only).
Unfortunately I'm restricted concerning the transformer's height to remain below 10mm, so I am fiddling with the small EFD15. Gives up to 80% over all in LTSpice now...

> The Baxandall Class-D inverter is pretty good. If you want a a better sine wave my variation on it is only about 50% efficient.
Maybe I did not understand what means "Baxandall Class-D inverter". Is it the Baxandall oscillator driven/regulated by a classD- or simply a PWM-switcher?
Tight regulation is not necessary in my case, so the constant input voltage and the relatively constant conversion ratio should be sufficient.

> Synchronous rectifiers, with properly driven MOSFet switches, are much quieter.
Is that trua in the case of 500Vpp and a few milliamps also? Properly driven - that could become complex in comparison to the rest of the converter. At least I do not know an appropriate scheme for this situation, working at 100kHz.

whit3rd schrieb am Dienstag, 25. Mai 2021 um 21:08:34 UTC+2:
> The drive (for a DC low-ripple output) would best be a triangle wave, so 
> the rectifiers after the transformer only make a short blip twice per cycle.
The "blip" stimulates trr-ringing, at least in my case.
Does triangle differ from sine significantly in this regard?

I put a short videoclip onto the website, Fameio measures 20pApp (last link):
https://ibtk.de/project/hzdr/Fameio-presentation/20210521_Fameio.html
20pApp rectangle, 0.1Hz, 10 samples moving average.

Thanks!
Cheers, Timo

timo.k...@ibtk.de wrote:
> Bill Sloman
>> Coaxial-AC-Bridges-B-Kibble
> Found it in the web, thank you!

Would you post the link?

Thanks

Phil Hobbs


-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On Friday, May 28, 2021 at 12:04:01 AM UTC+10, timo.k...@ibtk.de wrote:
> Bill Sloman 
> > Coaxial-AC-Bridges-B-Kibble 
> Found it in the web, thank you!
> > Jim Williams - in his Linear Technology application notes N49, AN51, AN55, AN61, and AN65 - was getting around 92% efficiency.
>
> Ahh, I see, it looks good. It seems to be very dependent on the core and it's winding numbers (not only). 

You get losses in the core and in the resistance of the windings. The absence of fast edges and the fairly rapid roll-off of the high frequency harmonics helps - ferrites are more lossy at high frequencies. 

> Unfortunately I'm restricted concerning the transformer's height to remain below 10mm, so I am fiddling with the small EFD15. Gives up to 80% over all in LTSpice now...

It should be just as good on a small core, unless you manage to saturate the core.
You could e-mail me the L-Spice simulation (to bill.sloman at ieee.org)- or post it here as the text version of the .asc file.

> > The Baxandall Class-D inverter is pretty good. If you want a a better sine wave my variation on it is only about 50% efficient.
>
> Maybe I did not understand what means "Baxandall Class-D inverter". Is it the Baxandall oscillator driven/regulated by a classD- or simply a PWM-switcher? 

Back in 1959 Baxandall distinguished his two transistor resonant oscillator from the one-transistor Class-C oscillators by calling it a Class-D oscillator.

Class -D amplifiers came quite a bit later. Somebody like John Woodgate might know the history.

> Tight regulation is not necessary in my case, so the constant input voltage and the relatively constant conversion ratio should be sufficient.
>
> > Synchronous rectifiers, with properly driven MOSFet switches, are much quieter.
>
> Is that true in the case of 500Vpp and a few milliamps also? Properly driven - that could become complex in comparison to the rest of the converter. At least I do not know an appropriate scheme for this situation, working at 100kHz.

Probably. Too many rectifiers act as step recovery diodes when you don't want them to.

> > whit3rd schrieb am Dienstag, 25. Mai 2021 um 21:08:34 UTC+2: 
> > The drive (for a DC low-ripple output) would best be a triangle wave, so 
> > the rectifiers after the transformer only make a short blip twice per cycle.
> The "blip" stimulates trr-ringing, at least in my case. 
> Does triangle differ from sine significantly in this regard? 

Probably not.
 
<snip>

-- 
Bill Sloman, Sydney