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

If you want a DC-DC converter, think about using a a Baxandall reseonant Class-D inverter rather than Royer.

The problem with square wave inverters are the switching spikes, and Baxanadall resonant inverters switch more smoothly.

You still have to worry pumping capacitative currents across isolation barriers, but there's less high-frequency trash around.

Peter Baxandall invented the circuit back in 1959, but he published it in a British journal and Americans didn't know about it. Jim Williams eventually picked it up and ran with it, but clearly didn't know where it came from. He first mentioned it in the Linear Technology application note AN-45 which is dated as June 1991, and but AN49, AN51, AN55, AN61, AN65 concentrate on using it to generate high voltages to drive cold-cathode back-lights.

Hello, Bill Sloman schrieb am Freitag, 21. Mai 2021 um 15:25:53 UTC+2:

Oh yes, I did not know about Baxandall, nevertheless I used it already (and mixed the names, excuse me, please). A magic circuit, started below 1V(!) and worked nicely to 30V on my bench... Please see the updated link, I added it's first simulations and measures (in German):

If the circuit stays inside the module, these currents are not harmful. The bias generation is GND-related. It will be a problem, if located outside, in series to the TIA input. So the first option still seems to be easier.

I'll have to go through the information you provided, thank you!

Cheers, Timo

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.

Hello Chris, thank you for the hints!

Chris Jones schrieb am Montag, 24. Mai 2021 um 11:59:35 UTC+2:

Here are some interesting ideas to minimise the coupling capacitance:

Yes, at first. But it measured ungly, maybe due to the "non-optimum" layout. So I put the inner shield back to GND with reasonable results - on the bench.

Yes, the bias drives the common and the supplies of the floating DCDC used for all the OPAs related to the TIA, to bee seen at p. 13 of the document: 20210316_SuperSIMS_FAMEIO_MV40-Ersatz_Praesentation_Kirschke.pdf.

The U7-A at p10 is an INA149, a TI HV-Diffamp. I like to simplify the structure a bit, omitting the composite amp. It's power is GND-referenced, of course. But it's input draws already ~0,5mA from bias and ~0.5mA from the TIA's output. Because the bias current possibly available at any in/out of the module must be limited to remain <3mA under all conditions for safety purposes, all power sinks should be kept at minimum.

Yes, the INAout-connection is a mistake, it is really connected to R107 (U6-B: composite, to bring the voltage to +/-10V inside the loop, lowering the noise a bit). As well as the schematic was not actualised after correcting all the failures. Excuse me please. The schematics, layouts and production are done by our technicians, sometimes the information flow is not good enough. So my second job for troubleshootimg remains necessary. ;)

Our "customer" physicists asked for an idea of such a version, so I simulated one, see

Thank you! Cheers, Timo

Sorry I didn't have time to read your documents more thoroughly before replying so I missed some things you had already provided.

I have to do something like this, I would use two cascaded toroidal transformers, on opposite sides of a metal wall with two holes in it, each toroidal core having its axis of symmetry parallel to the plane of the wall. The first transformer would have many turns primary : 2 turns secondary, and the ends of the 2-turn winding would pass through the holes in the metal wall. On the other side, there would be the other transformer with 2 turns primary to many turns secondary. The centre-tap of one 2-turn winding would be grounded to the metal wall so that the wires passing through the wall have equal and opposite voltages on them. Even with this arrangement and with PTFE insulated wire, I would not expect it to be useful below nanoampere signal levels unless I could arrange for the transformer operating frequency to be so far above the frequency of any wanted signals that it could be filtered out very well.

I can't understand why that would happen, but it might be interesting to investigate it further, as I think that the problems are more likely to be ones that can be solved than with the other approach.

But I presume with zero bias....

It might be interesting to float the ADC and just transfer the digital result from the vbias-referenced ADC to the rest of the system with an optocoupler or one of the newer equivalent transformer or capacitive coupler devices. Especially since you are not going to get super-high accuracy with teraohm resistors, the ADC and voltage reference might as well be cheap ones (even the built-in ones of a microcontroller) so there is less incentive to share one expensive reference + ADC amongst multiple channels.

Perhaps you can have a try at home. Sometimes it is faster and if it is promising then you can guide them to replicate it at work.

Thanks for that info. I did not even really know where to get ones that good, I think mine are 30% or worse, even before I take them out of the packet.

Nor do I. I think the main challenges would be bandwidth variation and getting the compensation right for all current levels, as well as getting good accuracy in spite of temperature gradients and poor log conformance at high currents. If the input current signal has a high peak-to-average ratio and higher bandwidth than the circuit, then such a scheme might also suffer from the same problem as older spectrum analysers where one can measure "the average of the log of the signal" which is not the same as "the log of the average of the signal" that one would prefer to measure.

I've used them, and they do work. Bob Pease has published quite a bit on the practical problems.

Setting up a circuit where an op amp makes the voltage at the collector is the same as the at the base can give you good logarithmic behavior over about seven orders of magnitude.

"The Art of Electronics" makes the point in fig 2.41 in the third edition, 2.39 in the second edition.

If you use one half of a monolithic pair of transistors as your logging device, temperature compensation can be pretty accurate. The voltage drop in the base resistance can be a problem at higher currents, but you can compensate it out if you monitor the base current and correct for the drop - I worked with hyperboloc function generator which did that.

That's always a problem with non-linear circuits.

Chris Jones schrieb am Montag, 24. Mai 2021 um 14:23:28 UTC+2: Hello Chris,

You couldn't do that, because I added some afterwards. ;)

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):

I'm lucky to have just this case. The (higher) currents to be measured are well below 10kHz, the rest will be 4th order lowpassed before the ADC, the (sine driven) transformer will be ~100kHz or above.

For a short period with bias too. Very noisy, but seemed to be stable. And when I switched off the bias, it's 1uF buffer cap put the charge into the +In of the ADA4530-1. One shot only. Expensive and sometimes hard to get... The layout is a little bit complex, the Rogers is sensitive to multiple times soldering, there are some more Guard-related mistakes, so went back to design a more robust version, hopefully.

It would throw out the INA at least, worth to think about, thanks.

Maybe I find the chance to build up a log version too...

I added the offer list from SRT to the small website, if you are interested.

Thank you! Cheers, Timo

Hello,

Thank you Bill, for your hints! I will study the AoE chapters. So it seems to be worth an practical try.

Cheers, Timo

Toroidal transformers have no stray field, if they are non-progressively wound.

If you just wind wire around the toroid, you end up with a turn in the plane of the toroid, and you have to organise the winding to avoid this. One scheme is to wind clockwise half-way around the toroid, then wind back anticlockwise all the way around the toroid - eventually going over the top of the first winding - and then wind back anticlockwise over the top of the second layer until you get to your original starting point.

I first found out about this by reading

Toroids are a pig to wind. There are special machines for the job, but RM core parts are a lot easier to put together in production.

Pot cores (and RM cores are particular sort of pot core ) don't have much stray field. Topologically they are the inverse of the toroid - you wrap the core material around the winding, rather than the winding around the core - but the slots to let the wires out means that they aren't as perfect as a toroid.

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.

Am 24.05.21 um 17:47 schrieb snipped-for-privacy@highlandsniptechnology.com:

Yes, I wanted to propose a small flock of <

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.

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

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

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.

snipped-for-privacy@highlandsniptechnology.com

Gerhard Hoffmann

Thanks, looks a lot better than the Toshiba TLP3924.

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.

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

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

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.

Chris Jones

I fear you're right...

Thank you! Cheers, Timo

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.

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

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.

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

Found it in the web, thank you!

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...

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.

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 "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):

Thanks! Cheers, Timo

Would you post the link?

Thanks

Phil Hobbs

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.

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.

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.

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

Probably not. <snip>