Low noise amplifier - inputs and ESD - Page 1
Forum section: electronics / projects

author: Strand17
author_rank: Contributor
timestamp: July 25, 2023, 01:40:03 pm
content: I am working on a low noise amplifier with differential inputs and have concerns about ESD sensitivity of the input stage.1. Of the two alternative input stages (see attached schematics), will there be a significant difference in their sensitivity to ESD?2. What measures can be taken to reduce the ESD sensitivity without compromising noise and input current?BackgroundThe amplifier is designed for use with piezoresistive pressure transducers, for instance Kulite XCQ-062. These sensors are essentially Wheatstone bridges with an output impedance in the range 300 Ohm to 1500 Ohm. The frequency range of interest is from DC to 20 kHz and the AC part of the signal is often barely detectable above the sensor noise. This requires an input stage with a low input current and low (voltage) noise. An input impedance above 1 MOhm and voltage noise below 2 nV/sqrt(Hz) at 1 kHz seem like reasonable targets. I am considering two different alternatives for the input stages, a discrete one using low-noise JFET's and one based on the AD8429 instrumentation amplifier, see the attached schematics.The amplifier is intended for a university research lab. During setup and dismantling of experiments, amplifiers are handled and may be subjected to ESD. During operation, exposure to ESD is less likely.My experience with design for ESD is very limited and I would appreciate suggestions on how to make a reasonably robust input stage for this amplifier.Thanks in advance!Best regards, Jonas

author: moffy
author_rank: Super Contributor
timestamp: July 25, 2023, 02:24:01 pm
content: The AD8429 looks like a good choice as long as it is run at reasonable gain, and the offset voltage of the bridge doesn't force it into saturation, assuming it is DC coupled. You can provide some static protection with a series resistor, say 100R, and two diodes, one to each supply reverse biased. It won't degrade the noise performance too much, just the noise contribution of the 100R resistor.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 25, 2023, 02:45:05 pm
content: Hi Jonas,imho FETs are no good for low-noise with low-source-impedance.Look at all the Moving-coil Phono-preamps (all bjts).When your concern with esd is only while handling, consider a shortening-plug (DE: Kurzschlußstecker).Maybe you can design this into the pcb somehow.Lemo for example has some receptacles with build-in switches.Good luck

author: Martinn
author_rank: Frequent Contributor
timestamp: July 25, 2023, 05:18:48 pm
content: Look at AM 502 an SR560 amplifiers, both have JFET differential input stages and you'll find schematics for both somewhere in the internet. The SR560 even got an honorable mention in AoE "designed by masters", should you be interested in JFET amplifier design.You might also look at the JFE2140 - this dual JFET contains protection diodes and the data sheet has more information on ESD protection.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 25, 2023, 06:34:23 pm
content: Gruezi Martinn,Quote from: Martinn on July 25, 2023, 05:18:48 pmAre we talking about the Tektronix AM502? It's essentially the 7A22 (scope-plug-in 7000-series) as stand-alone plugin for TM500/5000.Have two 7A22s here - you cant use them in the 10/20/50uV/Div.-ranges without a narrow filter-setting.SR560 looks impressive, has 100MOhms Rin - lots of noise just alone from the thermal-noise.To Jonas:Maybe have a look at some Microphone-preamps (SSM2017/2019, INA163/217, That 1510/12/80/83) and their app-notes (for emv/esd).Differential and for the same R-source-range.Good luck

author: Martinn
author_rank: Frequent Contributor
timestamp: July 25, 2023, 07:02:11 pm
content: Salü Röhrenonkel,Quote from: Roehrenonkel on July 25, 2023, 06:34:23 pmYes, I have one here (along with some AM503). Not sure about noise, but I bought mine used and except for a corroded 1:100 switch (giving intermittent problems at the beginning) it worked fine, so the input protection can't be that bad and it is documented in the service manual.On the SR560 however (which I know only from its manual) a blown input JFET seems to be not uncommon.As the TO explicitly asked about ESD protection, looking at how it is done in commercial products is probably a good idea.I initially (after reading the AoE description of it) started to build my own version of a SR560, but as I got hold of a AM502 put this on hold... I would have used the JFE2140, as it has integrated protection diodes already, simplifying ESD protection.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 25, 2023, 07:32:02 pm
content: Salü Martinn,Quote from: Martinn on July 25, 2023, 07:02:11 pmOn the 7A22 there are fuses (63mA FF/superfast) that i had to replace and the dual-FET on the other one.Do you need multiple AM503? I could use a spare. ;-)Yes, service-manuals and application-notes are a good point to start.Ciao4now

author: Martinn
author_rank: Frequent Contributor
timestamp: July 25, 2023, 08:03:23 pm
content: Quote from: Roehrenonkel on July 25, 2023, 07:32:02 pmI also suspected dead fuses and input FETs on my AM502, but both were OK, pushing the 1:100 switch a couple of times fixed that.I really wondered about the fuses. I would expect any fuse, whatever speed, to be much slower to blow than any semiconductor junction. But as it is designed by Tek and they are no beginners I'd expect those fuses to make sense somehow.Quite some time ago, I saved three AM503 (including $$$ A6302) from being thrown in the junk. I lent one out (permanently) to a friend and as the AM503 consists exclusively (except for some linear regulator in the power supply) of unobtainium Tektronix custom circuits, I'd rather keep one as spare (or simply use both when necessary). However, after all that time many contacts are somewhat reluctant to let current pass through (I think both AM502 and 503 contain those superfancy custom cam actuated many-pole rotary switches) and need some encouragement (slap on the top) to work properly. Still, DC to MHz current probes did not get a single cent less expensive during the past decades (quite the contrary) so I consider myself extremely lucky having those on my desk. But I think we are getting carried away...

author: TimFox
author_rank: Super Contributor
timestamp: July 25, 2023, 09:23:54 pm
content: Quote from: Roehrenonkel on July 25, 2023, 02:45:05 pmMC phono cartridges have a very low source resistance, typically < 100 ohms, and the lower noise voltage, higher noise current of one or more BJTs is a good match.For the 1000 ohm ballpark, either well-chosen BJTs or JFETs can give a good noise match. JFETs have lower noise current and higher noise voltage (typically), so the choice depends on source resistance.The input impedance of the amplifier is usually much higher than the source resistance, so the noise depends on the source.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 26, 2023, 05:10:28 am
content: Could a BUF802 be useful here? Has its own input protection, can be servoe’d to DC operation. Never seen it on differential input though, however OP used individual BJTs, not a pair, no degradation there. Perhaps a modern process gives the needed symmetry?

author: Marco
author_rank: Super Contributor
timestamp: July 26, 2023, 07:20:17 am
content: Quote from: Martinn on July 25, 2023, 05:18:48 pmPretty.Though how much do JFET's really need it? It's PN junctions and it's not like these devices are rf small.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 26, 2023, 07:52:00 am
content: With the low source impedance the BJT based INA is likely the better choice. It would need relatively special JFETs (the JFE2140 may be a bit higher noise) to get a comparable noise level.The main element for ESD protection are diodes towards the supplies or similar voltages - as leakage is not that critical these can be relatively large and in 2 layers of protection (1. at the connector and 2. at the amplifier itself). One may want some inductors as series elements in between the 2 layers of protection. These can also help reducing RF interference.As one has the amplifier schematics it would always be possible to do a repair if needed - so no need to overdo it with protection. With some care on handling chances are the INA chip could be OK even without extra protection.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: July 26, 2023, 08:49:30 am
content: You need to look at the protection circuits of multimeters (eg 34401). They have an excellent degree of protection and an input resistance of more than 10 Gom.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 26, 2023, 09:01:41 am
content: The requirements are a lot easier here than with a DMM - no need to be very high impedance and the voltage range is limited. On the other side noise is critial and the ~100 K series resistor as used in the HP34401 is not an option here.

author: tooki
author_rank: Super Contributor
timestamp: July 26, 2023, 09:14:36 am
content: Quote from: Roehrenonkel on July 25, 2023, 02:45:05 pmFYI, it’s called a “shorting plug”, or “jumper”. (“To short” = kurzschliessen; “to shorten” = kürzen.)That’s a good idea, I think. It certainly can’t hurt.

author: tooki
author_rank: Super Contributor
timestamp: July 26, 2023, 09:16:36 am
content: Quote from: moffy on July 25, 2023, 02:24:01 pmYep, and you can get TVS diode arrays specifically for this.

author: Someone
author_rank: Super Contributor
timestamp: July 26, 2023, 09:50:19 am
content: Quote from: Strand17 on July 25, 2023, 01:40:03 pmCould be a candidate for microphone preamps:https://thatcorp.com/low-noise-preamplifier-ics/

author: Terry Bites
author_rank: Super Contributor
timestamp: July 27, 2023, 01:11:21 pm
content: Jfets make great low leakage low capacitance diodes.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 27, 2023, 02:38:18 pm
content: Salüt tooki,Quote from: tooki on July 26, 2023, 09:14:36 amHaha, "Freudscher Versprecher". :-)No, what i meant was like in the 1/4"- & 1/8"-receptacles (headphones/microphones) the build-in switches/shorting-contact.They might work, but i'm sure the University can affort some fancy Lemos.Not even that expensive if you buy directly and Lemo has an outstanding support.Ciao4now,Michel

author: Strand17
author_rank: Contributor
timestamp: July 28, 2023, 08:31:16 am
content: Thank you for all the interesting and helpful suggestion!Examining the AM502 and SR560 turned out to be interesting and a bit puzzling. The AM502 incorporates many of the suggested protection strategies. Each input has a fast acting fuse, diode clamps to positive and negative rail as well as a 100 Ohm resistor in series. Funny enough, the SR560 does not seem to have any protection of the inputs at all, or am I missing something?With respect to ESD sensitivity, will there be any significant difference between an input stage with discrete JFET's and an INA like the ones in the first post?Best regards, Jonas

author: Kleinstein
author_rank: Super Contributor
timestamp: July 28, 2023, 09:07:08 am
content: For a DMS amplifier ESD is usually one of the smaller problems.For the JFET based amplifiers it depends on the added protection how good it can handle ESD events. INA chips usually already contain some diodes to the supplies as a las line of defense.

author: MathWizard
author_rank: Super Contributor
timestamp: July 29, 2023, 01:23:23 am
content: If this is for "low noise", why is it ok to use a JFET current and resistor to set the reference on the U3 op-amp in the 1st pic? Assuming the rails are good too, how much improvement would you get from using just a cheap TL431 ? Or are they actually noisier than just a small resistor and JFET? I know you would still have external resistors, and the TL431 is another op-amp +zener or some other internal reference.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 29, 2023, 06:45:20 am
content: A JFET and resistor can be rather low noise reference - however the BF862 is not a good choice in this respect as this likea JFET with higher threshold. The way the FET amplifier sets the current is a bit strange ( a little like with the 741) and may not be a good choice. A TL431 based current source (or JFET, maybe with a cascode) could be good enough.

author: magic
author_rank: Super Contributor
timestamp: July 29, 2023, 11:11:33 am
content: The answer is found in the original DIY Audio thread.Quote from: scott wurcerIf U1 has rail to rail output, this scheme provides stable input stage bias, all the way down to the negative supply, even if the JFETs are running close to IDSS and pushing the current sink device into saturation.Using another matched JFET to generate the reference current may have some advantages in operation over a wide temperature range. If nothing else, it is guaranteed that input pair Vgs will never become positive.You probably don't care about any of that.TL431 is likely noisier than a JFET. You would have to test if it matters - theoretically bias current noise is rejected differentially by this circuit, but only as well as the input devices and their drain resistors are matched.