Differential Pair Signal Limiter: An Impossible Ask? - Page 1
Forum section: electronics / general

author: marks2c
author_rank: Contributor
timestamp: March 23, 2022, 05:20:39 pm
content: I recently released the magic smoke on my audio analyzer when it was hooked up to a 100V-Line amplifier without an attenuator. A quick and expensive mistake.Cutting a long story short: to test an amplifier you typically need an attenuator to match the output's peak-peak voltage to the analyzer's full-scale input voltage. Many analyzers don't list their 'damage level' - but typically it is up to about twice the full-scale voltage.So... I'm looking for a differential signal limiter:* that is self-resetting (no fusible links).* that can have 230VAC mains connected to its input continuously without damage to the connected ADC.* 24+ bit ENOB performance (ie transparent to the measurement when not limiting).* Has an excellent CMRR.* Is fast enough to survive connection to a valve amp's HT supply.* Has a switchable attenuator position to match the analyzers full-scale input voltage range and input impedance (change resistors to suit the analyzer's need).* Protected in both attenuator positions (typ 0dB and 15-25dB).Any thoughts?

author: fluxgate
author_rank: Contributor
timestamp: March 23, 2022, 07:38:13 pm
content: For high Voltage:Resistive divider, use Low Output impedance ~1k Ohm.For 0db Range: use low capacitance TVS Diode + series Fuse+ little series resistor (200ohm) at the Input for User Error Protection.

author: MasterT
author_rank: Frequent Contributor
timestamp: March 23, 2022, 09:09:24 pm
content: Have a look:In both cases buffer amp's using inverting topology, LME49721 10k-10k, so I'm sure it would survive 220V if I put 10k - 5W. 22 mA probably slightly above 10mA OPA input current limit, than 20k would be fine

author: marks2c
author_rank: Contributor
timestamp: March 23, 2022, 11:14:29 pm
content: Thank you x3, greatly appreciated.I should have added the requirement that the ideal solution would be passive(ish) - ie unpowered and not so not have any nice opamps.Something like the outboard limiters that are often used on the input of spectrum analyzers and VNAs (they typically have a performance hit).Easy/commercial solutions are not apparent (or existent...).Edit: grammar

author: jonpaul
author_rank: Super Contributor
timestamp: March 24, 2022, 09:38:20 am
content: Divider x10 or x100, eg 900 ohm power resistance low inductance in series with 100 ohm precision resistanceWhat was the Analyzer?Jon

author: Brianf
author_rank: Regular Contributor
timestamp: March 24, 2022, 04:01:07 pm
content: Quote from: marks2c on March 23, 2022, 05:20:39 pmA straightforward balanced attenuator, with a couple of different ranges, but with the attenuation setting selected by relays and set to default to maximum attenuation. And arranged such that any range selection falls back to default when power cycled. So that YOU have to make a conscious decision to reduce the attenuation. Or even have the control side drop back to maximum attenuation every few minutes, and needing a button push to go more sensitive.

author: marks2c
author_rank: Contributor
timestamp: March 24, 2022, 05:16:09 pm
content: Thank you. NTI XL2: This is a lovely handheld device that is now fixed and even calibrated after being abused. The voltage may have been very high as 24V powered 100V-Line amplifiers that make rated power at 19V DC can (with enough input drive) do a whole lot more with 28V-30V DC input.... aka PBKAC, 'OSI layer 8 problem', 1D10T etcAnatoly: please advise what the oscillator/analyzer you were using in the tests are?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 24, 2022, 05:27:14 pm
content: Limting the singnal without introduing nonlinearity is a rather difficult task. One would usually need some series resistance of some sort and than some clamping device. The series resistor will intrduce some attenuation and may need to be a high power one (e.g. 100 W range).The actual clamping part would need to be low leakage and low capacitance - there may no suitabale ones. Low capacitance TVS would be a chance, but not clear of good enough ones exist. The problem is that the TVS capacitance / leakage is nonlinearOne step less leakage / capacitance could be obtained with a 2 stage circuit, that is bootstrapped for the first stage. So something liek 2 back to back diodes in sereis with a TVS, so that the TVS gets about the right voltage from an auxiliary amplifier.

author: MasterT
author_rank: Frequent Contributor
timestamp: March 24, 2022, 06:33:33 pm
content: Additional info to video linked in reply #3.Ultra low THD obtained out of very high Q (1000 - 2000) selective filter based on tween-T. Circuits doesn't have magnitude equalisation capability, input driven by external SG (Rigol x1000 series), 100-200 mVp-p.THD below <-70 dBc is o'k for SG, filter shows -130dB at the output, may be better - no means to verify.ADC buffer amp shown on second drawings.I prefer Inverting topology whenever it's possible, no input capacitance non-linearity issues. My tests show, that even Industry grade SD ADC quite noisy, so 10k at the input doesn't degrade overall SNR compare to results with ADC inputs shorted.Audio ADC likely much worse on SNR, I have really disappointing results with PCM4202.Noise was a problem in 1970-x 1980-x when THD+Noise was measured nulling out prime tone, but since DSP was brought into perspective things changed .There is a software I uploaded a while ago in public domain, though not the latest version.https://drive.google.com/file/d/1DzUVrgU4mdnkKblT_ltabVeJS97TnC5g/view?usp=sharing

author: marks2c
author_rank: Contributor
timestamp: March 24, 2022, 08:42:35 pm
content: Quote from: Kleinstein on March 24, 2022, 05:27:14 pmThank you, this is the path I'm following. It isn't trivial...

author: David Hess
author_rank: Super Contributor
timestamp: March 26, 2022, 03:01:37 am
content: Quote from: marks2c on March 23, 2022, 05:20:39 pmMy usual recommendation of a current driven diode bridge will not meet that linearity requirement.I would place a pair of back-to-back high voltage depletion mode MOSFETs in series with each line to limit the current. A series resistor between the gate and source sets the current limit of both MOSFETs for AC operation. Only one resistor is needed for both MOSFETs.

author: marks2c
author_rank: Contributor
timestamp: March 26, 2022, 09:34:19 am
content: Yes, the depletion mode MOSFETs are a winner.3mA and 1k total series resistance & 400V rating is achievable. Vebo transistor clippers for the win too.Make it all very survivable.

author: MasterT
author_rank: Frequent Contributor
timestamp: March 26, 2022, 10:31:28 pm
content: Can;t remember where I get this circuits, some audio analysers, anyway, check it out

author: marks2c
author_rank: Contributor
timestamp: March 27, 2022, 01:51:36 am
content: Quote from: MasterT on March 26, 2022, 10:31:28 pmHat-tilt: you have to respect the performance of precision resistors, audio switching with expensive sealed relays, and active overload detection.That said, modern audio ADCs/DACs, and software analyzers are becoming so good and so cheap.Just needs good passive input signal chain protection and a single attenuator step to be good enough for 95% of audio analyzer work.