Author Topic: HPM7177 ADC from CERN  (Read 5059 times)

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Offline maat

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Re: HPM7177 ADC from CERN
« Reply #75 on: February 14, 2020, 12:57:21 pm »
These are usually very cold - Josephson junctions, SQUIDs, superconducting detectors.

Unfortunately superconducting stuff has a rather large output impedance. We usually use a MESFET buffer, but at higher frequencies though, because their LF noise is horrible.
 

Online MegaVolt

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Re: HPM7177 ADC from CERN
« Reply #76 on: February 14, 2020, 01:01:45 pm »
These are usually very cold - Josephson junctions, SQUIDs, superconducting detectors. At room temperature - possibly some batteries. Some time ago I spotted some papers in which they used AD7177-2 for LF noise measurement of Lithium batteries. I can dig up the reference if someone's interested.
Thanks for the clarifications!!!

Link is very interesting. If not difficult, share it.
 

Offline Castorp

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Re: HPM7177 ADC from CERN
« Reply #77 on: February 14, 2020, 01:21:11 pm »
These are usually very cold - Josephson junctions, SQUIDs, superconducting detectors.

Unfortunately superconducting stuff has a rather large output impedance. We usually use a MESFET buffer, but at higher frequencies though, because their LF noise is horrible.

What kind of device are you talking about?

It's true that "superconducting" doesn't mean it's a zero-ohm source. Some of them are close though. Single SQUIDs for instance - the only amplifier that's noise-matched to them is another SQUID. To clarify that - the input of a SQUID amplifier is a superconducting coil. I've worked on such a setup, with one SQUID at <100 mK and a large SQUID array at 4K. Eventually they demonstrated nearly quantum-limited energy resolution, i.e. a few times Planck's constant.

Another example is transition-edge sensors (TES). They can be either calorimeters or bolometers - depending on whether they measure the energy of single photons, or average radiation power. They are very low-resistance and very low-noise. It's basically a small resistance biased on the superconducting transition. It's a steep transition from 0 to a few Ohms, so they have lots of gain on that setpoint. Once again - you need an amplifier suited for very low impedances, e.g. a SQUID.

Sorry about drifting the topic far from ADCs. It just brings back some fond memories  :)

Here's one of those battery LF noise papers (seems the same author has many):
https://www.researchgate.net/publication/338209069_Electrochemical_noise_measurement_methodologies_of_chemical_power_sources
 
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Offline Gerhard_dk4xp

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Re: HPM7177 ADC from CERN
« Reply #78 on: February 14, 2020, 01:58:40 pm »
I did also some measurements on batteries.
<   http://www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf     >

inspired from this NIST report:

https://www.researchgate.net/publication/3622215_Measurement_of_voltage_noise_in_chemical_batteries/link/555b4cc508ae6aea0816a420/download   >

It used to be on the NIST server of the timefreq group, but it seems it is moved around on a regular base.
The NIST people don't dare to disclose / endorse commercial products, but I wanted data on things I can
actually buy.  "I found a NiCd cell in my drawer" is not precise enough.

My preamp was better, but they had cross correlation, so they are better by a few dB over all.

The pretty amplifier from some posts above released its black magical smoke btw.
when measuring 4 Lithium cells.  :-//

You see that wire from the op amp inputs over the DK4XP text to the "wrong" side
of the wet slug tantalum location. That was OK for verifying the skin effect, but was
forgotten to be removed before flight.   :palm: 

"Opfer müssen halt gebracht werden"  -- Otto Lilienthal, early aviation pioneer
("Sometimes sacrifices are necessary")

@Megavolt: A LT3042 features 2 nV/rtHz noise density. I you want to verify that,
it's better to have >10 dB margin in your setup.
« Last Edit: February 14, 2020, 03:08:46 pm by Gerhard_dk4xp »
 

Offline splin

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Re: HPM7177 ADC from CERN
« Reply #79 on: February 14, 2020, 04:04:44 pm »
I did also some measurements on batteries.
<   http://www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf     >

It would be good if you could either rerun the tests or at least add a disclaimer at the start of the article that the results are all wrong below 100Hz due to the 1/f noise of the amplifier used.

https://www.eevblog.com/forum/projects/low-frequency-very-low-level-dc-biased-noise-measurements/msg655965/#msg655965

The results are interesting data for > 100Hz; below 100Hz they are still very useful showing the relative noise performance of the various batteries. Subtracting the estimated 1/f noise of the amplifier used gives a reasonable estimate of the actual low frequency noise of the batteries so all useful data.

I saw that you spotted the 1/f noise problem of the amp long ago but it's a shame you didn't get round to correcting your web pages detailing the amplifier and its performance.
 

Offline maat

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Re: HPM7177 ADC from CERN
« Reply #80 on: February 14, 2020, 07:04:15 pm »
Unfortunately superconducting stuff has a rather large output impedance. We usually use a MESFET buffer, but at higher frequencies though, because their LF noise is horrible.

What kind of device are you talking about?

It's true that "superconducting" doesn't mean it's a zero-ohm source. Some of them are close though. Single SQUIDs for instance - the only amplifier that's noise-matched to them is another SQUID. To clarify that - the input of a SQUID amplifier is a superconducting coil. I've worked on such a setup, with one SQUID at <100 mK and a large SQUID array at 4K. Eventually they demonstrated nearly quantum-limited energy resolution, i.e. a few times Planck's constant.

Another example is transition-edge sensors (TES). They can be either calorimeters or bolometers - depending on whether they measure the energy of single photons, or average radiation power. They are very low-resistance and very low-noise. It's basically a small resistance biased on the superconducting transition. It's a steep transition from 0 to a few Ohms, so they have lots of gain on that setpoint. Once again - you need an amplifier suited for very low impedances, e.g. a SQUID.

Sorry about drifting the topic far from ADCs. It just brings back some fond memories  :)

Last bit of off-topic from me ;). You don't want to draw any current from the superconductor. You are only interested in a voltage fluctuation due an external exitation/disturbance. The superconductor itself has of course 0 resitance (exlcuding type II + magnetic field here), but any electron, that you remove from the sensor will disturb the measurement, hence add noise. Therefore your amplifier should ideally have infinite input impedance.
 

Offline Castorp

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Re: HPM7177 ADC from CERN
« Reply #81 on: February 16, 2020, 12:47:32 pm »
I'm still wondering what kind of device you're talking about  :) Because the term "superconductor" is about as general as "semiconductor".

Sometimes you draw current, e.g. in a voltage-biased SQUID. Another example that should be familiar to the audience here is a microwave-irradiated Josephson junction, biased at a Shapiro step. It should be obvious why you don't want to buffer this voltage with any transistor.
 

Offline Castorp

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Re: HPM7177 ADC from CERN
« Reply #82 on: February 21, 2020, 12:34:23 pm »
Kleinstein's post finally provoked my curiosity to look more carefully at the extra 1/f noise. Here's what I've got.

First - there's a multiplexer inside the HPM7177 that allows you to couple various internal voltages to the ADC. Those can be the +5V (which also goes to Vref), the bare unscaled LTZ1000 voltage (ca. 7.1 V), and a 10 V which is Vref*2. Here's how their noise spectra look like:



Now here's the integrated noise within one decade. I chose 1 to 10 mHz, as it seems to be fully in the 1/f region (also for 0V input). One plot line shows the noise on the internal voltages, the other one is with external 5V and 10V. Those come from 10 sources in parallel, feeding either into a 100 Ohm or 50 Ohm load. The external contribution is small (60 nVRMS/decade at 10 V, half of that at 5V), but I rms-subtracted it anyway. The result:



Now, one step further. I RMS-subtracted the 0V component. So what should be shown is just the voltage-dependent excess 1/f noise:



At the moment I don't have the time and patience to trace it all down, but a rough estimate tells me that the Noise Index of the Vishay bulk metal foil resistors should be somewhere between -40 and -50 dB, i.e. less than 10 nVRMS/dec/V, but more than 3 nVRMS/dec/V. The datasheet spec is for -40 dB. If they are actually much better, i.e. better than -50 dB, then this extra noise must come from the AD7177-2.

It's actually quite a jigsaw puzzle to separate the components. There's one part in the input attenuator and another in the Vref scaling. When you measure the +5V internal voltage, the component from Vref scaling won't show. That could possibly explain why the excess noise doesn't scale linearly with voltage.

Anyway - it looks like it's not as negligible as I thought before. I think I had a wrong number stuck in my head - perhaps I confused the 7.1 V measurement for 10 V. Or maybe I hadn't subtracted the contribution of the 10 V external source that I used before. In any case - the results are still pretty good.

Edit:

For completeness, here are the spectra with external 5V and 10V. Some plots look thinner than others because they were averaged over much longer time (e.g. one week instead of overnight). Also the EMI pickup varies due to the cabling, etc. It doesn't matter for this particular study of 1/f noise. Also, the 50 Hz spike may look big, but actually it isn't. It's the FFT bin width which is very small (0.001 Hz), which results in a large processing gain for narrowband signals. Anyway, those details are not relevant.



Once I tried to measure the excess noise in bulk metal foil resistors. Back then I concluded it must be better than -40 dB NI, which was my measurement limit, and it was sufficient at that time to point my attention elsewhere. It would be nice to solve this mystery eventually.
« Last Edit: February 21, 2020, 02:30:47 pm by Castorp »
 
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Offline Kleinstein

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Re: HPM7177 ADC from CERN
« Reply #83 on: February 21, 2020, 02:47:14 pm »
AFAK some of the AZ OPs also have a little bit of 1/f noise. So it is not only the AD7177 and the resistors as possible sources. I would expect the foil resistors to be much better than -40dB noise index. I would be more surprised if the resistor contribution is significant at all.

Having more noise from the ADC and scaling means the noise contribution from the LTZ1000 reference is a little smaller. The overall noise has not changed, only a slight change in where the noise comes from. The reference still is the largest contribution.

For the really critical parts it would be more useful to have 2 completely separate units in parallel and not use one ADC with 2 x LTZ1000 for the reference. Looks like this is the plan anyway.
 

Offline Castorp

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Re: HPM7177 ADC from CERN
« Reply #84 on: February 21, 2020, 03:01:28 pm »
AFAK some of the AZ OPs also have a little bit of 1/f noise. So it is not only the AD7177 and the resistors as possible sources. I would expect the foil resistors to be much better than -40dB noise index. I would be more surprised if the resistor contribution is significant at all.

Having more noise from the ADC and scaling means the noise contribution from the LTZ1000 reference is a little smaller. The overall noise has not changed, only a slight change in where the noise comes from. The reference still is the largest contribution.

For the really critical parts it would be more useful to have 2 completely separate units in parallel and not use one ADC with 2 x LTZ1000 for the reference. Looks like this is the plan anyway.

If it's 1/f noise (voltage/current) from the AZ op amps, I wouldn't expect it to scale like this with voltage. It would be the same at zero and elsewhere, right?

I also thought the NI should be much better. If LTC5400 has -55 dB and that's thin film, then Vishay are almost certainly under-specifying. However, now I have some doubts again.

You're right, it's still mostly the LTZ1000. That extra bit from the amps/resistors/ADC is less than 30% at 10 V.

And yes - the plan is to have 2 completely separate units. It's always like that. In normal operation the controller uses the average of the two, so we're always sqrt(2) better in terms of noise. This fact is not reflected in the specs, we keep it as a safety margin.
 

Offline Kleinstein

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Re: HPM7177 ADC from CERN
« Reply #85 on: February 21, 2020, 04:17:59 pm »
The OP at the input would add noise independent of the voltage. The OP at the reference would add noise to the reference, and depending on where the 5 V is actually taken to the test input this may give voltage dependent noise.

For the resistor noise, i can imagine the measurement gets tricky much below -50 dB. At some point that can be just temperature variations that cause variation in the resistor ratios. Here the LT5400 can have a slight advantage because of very good thermal coupling and matching. in the extremes, combining resistors with positive and negative TC is not as good as one material with a low TC everywhere.

It is not such a surprise that the ADC itself has some extra 1/f noise also for the reference path. The internal switches also have some resistance and these can have excess noise (I expect the MOSFET ON resistance to have a rather poor noise index). I don't know the details of the switched capacitor SD converters, but in the continuous time version and multi-slope ADC with a high input voltage there is more uneven weight to the resistors if the voltage is large and thus more excess noise from the resistors gets visible. This is what I see in my version. A can imagine a similar effect with the switched capacitor version too.
 

Offline Castorp

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Re: HPM7177 ADC from CERN
« Reply #86 on: February 21, 2020, 07:29:56 pm »
Well, at least I'm sure that these in situ measurements are not affected by temperature fluctuations, turbulent air flow or thermal EMFs. At the moment I really can't pinpoint the origin. I'll dig deeper if I get more time, even if it's just for academic reasons.
 
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