Preamp for very small signal - Page 1
Forum section: electronics / projects

author: Gavin Melville
author_rank: Contributor
timestamp: June 21, 2023, 04:28:38 am
content: I'm just about to start building a preamp to get a 'scope to see a 30uV pulse at around 48Hz. From looking at all the commercial amplifier/filter combinations that you can't buy right now, I keep coming back to the two amplifier/filters in AN159 from AD. The pulses - I think - look like half sine pulses of 5kHz.My source impedance is low, sub 10 ohms, there is wideband noise <10MHz, so I figured I'd reset the two Butterworth filters to 20kHz and 10KHz.I'm seeing many people talking about them, but nobody I can find has built one, is there any reason _not_ to build either of these ?.

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2023, 05:17:50 am
content: That seems like overkill for your application. Any low noise operational amplifier will be good enough instead of all of those matched differential pairs in parallel.

author: gnuarm
author_rank: Super Contributor
timestamp: June 21, 2023, 05:36:12 am
content: Quote from: Gavin Melville on June 21, 2023, 04:28:38 amMy brother inherited a design of a similar preamp board used to measure the electrical signals from fish in a tank. The instrumentation ADCs are at the mV level and the fish emissions are picked up at voltages similar to your levels. They use an instrumentation amplifier to provide 100, 1,000 or 10,000 gain.An instrumentation amp is designed with a pair of voltage followers to provide a very high input impedance on both inputs. These feed a more conventional, low noise differential op amp to provide gain as set by the resistors used. Because there is a strong focus on common mode rejection, resistors are provided in the same package as the instrumentation amp so they don't have temperature drift.It is smart to use at least a single pole low pass filter on each of the inputs, to reduce any strong noise. No reason to set the corner frequency so high if you are looking at a 48 Hz signal. I'd set the cut off at maybe 1 kHz, but I expect you know your signal better than I do. Additional filtering can be applied after the instrumentation amp, if needed.If you don't have common mode noise concerns, then this is not so helpful. In a single ended signal, there will be tons of noise way larger than 30 uV. I would expect it to be hard to see a 30 uV signal. But maybe the low pass filter will manage that adequately. If this is a general front end tool for the scope, a differential input would not be a bad idea.A lot of the instrumentation amps use chips with part numbers starting with INAxxx. If you want, I will find out the part number they are using.

author: Circlotron
author_rank: Super Contributor
timestamp: June 21, 2023, 06:36:11 am
content: ETI magazine October 1979 page 40 (Australian edition) published details for a moving coil phono cartridge preamp that had a noise figure of 0.3nV root Hz. That or some of the principles involved might be useful.

author: moffy
author_rank: Super Contributor
timestamp: June 21, 2023, 07:55:28 am
content: I did a quick simulation in LTSpice using an OPA1612 low noise opamp with a gain of 1000 and a bandwidth of 1.6kHz (1k to inverting input other side to gnd and 1Meg//100pf feedback). The input signal was 30uV p-p 50Hz square wave, fed into the non inverting input, became unsurprisingly 30mV p-p output. A noise analysis gave a total RMS noise from 0.1Hz to 1MHz of 230uV referred to the output, which I assume would be low enough.P.S. If I change the components to 100, 100k/1nF, the RMS noise becomes 90uv over 0.1Hz to 1MHz referred to the output.

author: magic
author_rank: Super Contributor
timestamp: June 21, 2023, 09:05:06 am
content: Even OPA1612 may be overkill, get the jellybean NE5534. It has some 75% of the former's open loop gain at audio frequencies, still good enough noise and much better price and availability. Closed loop bandwidth at 60dB gain will be 60~80kHz.If using 1k/1M feedback divider, place 10μF in series with 1kΩ (can be electrolytic, polarity doesn't matter). This reduces closed loop gain below ~20Hz towards unity at DC and prevents 1000x amplification of the opamp's offset voltage, which could end up several volts at the output.editSomewhat lower noise will be had with 100/100k feedback divider. Then the offset killing cap has to be 100μF.Beyond that, not much difference with those ICs.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 21, 2023, 09:05:18 am
content: At such low levels ground noise will ruin your day, so I would use a low noise microphone amp. The $5THAT158360db of gain, low noise, no worries.

author: magic
author_rank: Super Contributor
timestamp: June 21, 2023, 09:09:36 am
content: In which universe is 30μV a low level? Audio circuits routinely deal with such signals and jellybean opamps achieve a few μV RMS noise over full audio bandwidth.Ground is not a problem either, unless it's screwed up.

author: moffy
author_rank: Super Contributor
timestamp: June 21, 2023, 09:21:59 am
content: Quote from: magic on June 21, 2023, 09:05:06 amTried it with 100uF, the square wave looks like it has been differentiated so distorted for the purposes, 1000uF looked better but the flat portions of the square wave still sloped.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 21, 2023, 10:01:01 am
content: Looking for a pulse with some 100 µs (halve wave of 5 kHz) suggests that one would need a bandwidth of slightly more than the audio range. Chances are one would want the filters a little higher and one may consider a Bessel type filter to get less destortion of the pulse shape (though more noise BW and thus more noise).This would indeed need low noise to resolve a 30 µV pulse well and the NE5534 may not be sufficient. Things get easier if the signal is repeating and one can average over multiple pulses (scope that offers averaging mode and a good trigger signal).

author: AnalogTodd
author_rank: Regular Contributor
timestamp: June 21, 2023, 12:23:34 pm
content: Quote from: Kleinstein on June 21, 2023, 10:01:01 amLow noise on your amplifiers is needed, but you don't need to get down to the levels that were done in AN159. That was designed for signals that are basically sub microvolt in amplitude. If anything, go look at AN83; that used a single amplifier chain to get the gain needed for signals down in the single digit uV range. You've got plenty of options for filtering, you don't need the 10Hz to 100kHz range those circuits suggest, you could likely get away with simpler filters.

author: jwet
author_rank: Frequent Contributor
timestamp: June 21, 2023, 01:45:11 pm
content: One other trap in this application is the 1/f noise corner of the amp. Noise rises at 1/f and at these low frequencies, parts like the NE5532 are not very suitable. The NE5532 noise takes off at about 1k and is about 10x the wideband noise at your 48 Hz. A really nice amp might be the the LT1028 or modern incarnations- it has a 1/f corner below 5 Hz. I also went back and looked at the DS, the apps section reads like a tutorial on low noise design- worth a look.

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2023, 04:34:59 pm
content: The LT1028 is a great choice if the source impedance is low enough. It has both low wideband noise and a low 1/f corner. The AD797 is a similar part.For slightly higher source impedances, the LT1007/LT1037 are good choices.

author: gnuarm
author_rank: Super Contributor
timestamp: June 21, 2023, 04:44:42 pm
content: Quote from: magic on June 21, 2023, 09:09:36 amGround doesn't have to be screwed up for a low level signal to have useful advantage. This is a preamp for a scope, not an audio amp. You have no idea where the signal is coming from. There can be significant noise in the ground or reference.Don't assume the world is like the problems you have solved.

author: gnuarm
author_rank: Super Contributor
timestamp: June 21, 2023, 04:45:58 pm
content: Quote from: moffy on June 21, 2023, 09:21:59 amWhat is your load impedance??? The corner frequency is set by the RC, not just the C.

author: moffy
author_rank: Super Contributor
timestamp: June 21, 2023, 11:19:10 pm
content: Quote from: gnuarm on June 21, 2023, 04:45:58 pmI know the corner frequency is set by the RC which is 100u*100 or 1000u*100, the cap makes the gain unity at DC. The capacitor is in the feedback loop connected between the 100 ohm resistor and ground. It looks like the apparent differentiation is due to the phase not being flat at the square wave frequency.

author: Gavin Melville
author_rank: Contributor
timestamp: June 22, 2023, 04:04:47 am
content: I probably forgot to mention I need a lot of attenuation at 1.3MHz, like 90dB. Switching power supply....... I'll use an external LC filter for most of that.

author: gnuarm
author_rank: Super Contributor
timestamp: June 22, 2023, 04:31:16 am
content: Quote from: moffy on June 21, 2023, 11:19:10 pmWork with me here. I don't see a schematic anywhere. What is the 100 ohm resistor doing? Oh, it's the input impedance of the amp, i.e. 100/100k. You can raise the resistor a lot more easily and cheaply than raising the cap. You can set the resistor to nearly anything you want if you use multiple stages, say two, with one 10x and another at 100x gain. Or, better yet, use an instrumentation amp which has very high input impedance (non-inverting follower inputs). Then the filter can use whatever value resistor doesn't muck too much with the DC offset from input bias currents.Make life easy. Don't optimize things that don't have to be optimized. Even 1,000 uF is pretty absurd. Someone working for me who was proposing that, would be told to do it over.

author: gnuarm
author_rank: Super Contributor
timestamp: June 22, 2023, 04:35:52 am
content: Quote from: Gavin Melville on June 22, 2023, 04:04:47 amYou might consider a ferrite bead rather than an inductor. Where do you plan to insert this filter? Do you expect 1.3 MHz on the input? Why would a standard filter not catch this? I guess you are saying filter, and I'm reading notch filter. You probably just mean a standard low pass filter? Still, 90 dB is a lot. Inductors have inter winding capacitance that can conduct signal through at higher frequency. So spec your inductor carefully.

author: moffy
author_rank: Super Contributor
timestamp: June 22, 2023, 05:07:53 am
content: Quote from: gnuarm on June 22, 2023, 04:31:16 amSorry for the confusion but I was responding to a comment by 'Magic' we both understood the context, and it wasn't an actual proposal just a simulation showing the effects of the corner frequency on square wave distortion. For you and any others that might have been confused I have included a capture of the sim and its output of a 50Hz square wave, the corner frequency is 15.9Hz as per 'Magic's initial suggestion. The phase really messes up the waveform's shape. Hope that clears up any misunderstanding.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 22, 2023, 05:52:35 am
content: Twice the price of the ready optimised out of the box THAT1583 mic pre-amp?Its got two outputs , handy.

author: moffy
author_rank: Super Contributor
timestamp: June 22, 2023, 06:20:20 am
content: Quote from: Terry Bites on June 22, 2023, 05:52:35 amBut you get 2 opamps in the OPA1612 package which makes it slightly cheaper than the THAT1583, and it has slightly better noise, but the THAT1583 is differential input which can be very handy. Thanks for the recommendation.

author: gnuarm
author_rank: Super Contributor
timestamp: June 22, 2023, 06:25:29 am
content: Quote from: moffy on June 22, 2023, 05:07:53 amActually, I'm much more confused. I have no idea what V1 is generating, but I assume it is a fixed DC voltage, likely 0 volts?It appears you are using the op amp to form an oscillator to produce a square wave by over driving an input. In other words, the op amp is used as a comparator. If IN+ is steady and IN- is rising and falling, I can't think why the output would be anything other than a square wave. I would expect the output to be a 20 Vpp square wave (assuming the op amp output is rail to rail) resulting in 20 mV swings at IN-. I see that size swing at IN-. But something is very wrong with this circuit if you are seeing this waveform on the output.I'm suspecting the model for OPA1612 is not connected to the symbol correctly. But something is wrong. What's the waveform on IN-?Why do you have such a large ratio between R1 and R2? That attenuates the 20 Vpp swing on the output to just 20 mV swing. Make R1 1 kohm, R2 10 kohm, and you get 2 Vpp swings with some noise margin.I would also remove V1 and C2, grounding R1 and IN+. But then, I don't really know what you are trying to do with this.

author: moffy
author_rank: Super Contributor
timestamp: June 22, 2023, 06:36:33 am
content: Apologies, V1 is a 30uV peak to peak, 0v being the low value, 50Hz, 50% duty cycle square wave.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 22, 2023, 07:02:19 am
content: If possible one should go for an DC coupled first amplifier stage. This avoids the rather large coupling capacitor that would be needed to get AC coupling of suppressing DC gain with the low resistance needed to keep the noise down.The slight slope visible in the simulation would be less of an issue for the low on ratio pulses that are described (100 µs pulse with some 20 ms period). So the 100 ohm and 100 µF should be OK in this respect. More like that one would want less than 100 ohm to keep the noise low. 100 ohm have already more noise than an OPA1612.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 22, 2023, 07:48:56 am
content: I get the impression that you need a floating input.Ground is generally pretty filthy in todays switchmode environments.Assume its crappy unless proven otherwise. Just design that problem out.EMI can easily burry a small signal. Low noise opamp amps won't cure that.You'll need some RFI filtering at the input.And you'd be wanting some decent CMRR @50/ 60Hz for 30uV signals unless you're living in a Faraday cage.Inst amps tend to be pretty low bandwidth at high gain. GBPs of 1MHz are typical. So 10kHz @ 60dB.My suggested mic amp has a 192 KHz BW@ 60dB, 10x the max frequency of interest, that's not bad. And no, I dont work for or flog THAT productsThen there are fully differential amps with spectacular specs. Sub nV noise, insane bandwidths. They make for great front ends, ideal for low Z sources.See attched sketch of a possible configuration.

author: AndyC_772
author_rank: Super Contributor
timestamp: June 22, 2023, 08:05:13 am
content: My concern would be common mode noise pickup; as others have said, the signal level itself isn't particularly low, but you'll need excellent CMRR otherwise all you'll do is pick up mains hum. It doesn't help that your signal is 48 Hz and so, more or less, is the mains.I'd be using a transformer to give the necessary CMRR. As a bonus, you can pick one with, say, a 10:1 turns ratio to increase the signal level at the same time.

author: gnuarm
author_rank: Super Contributor
timestamp: June 22, 2023, 08:39:42 am
content: Quote from: moffy on June 22, 2023, 06:36:33 amStill confused. If V1 is a square wave, what is this circuit intended to do?BTW, the RC is not 100 ohms * 100 uF. It's 100,100 ohms * 100 uF, or around 10 seconds. The waveform at VA should still be a square wave of 20 Vpp. That's an output. It is not impacted by any of the circuit connected to it. It's the voltage on IN- that will be an odd waveform that could likely look what you are showing as VA.Until you fix the issues with the simulation you've provided, I certainly can't help you. If this is LTspice, post your schematic file as text and I can run the simulation myself.

author: magic
author_rank: Super Contributor
timestamp: June 22, 2023, 08:41:41 am
content: Quote from: Kleinstein on June 22, 2023, 07:02:19 amI suggested the large capacitor for 50Hz signals, but it's 50Hz repetition rate of a 5kHz signal apparently, so I was overly pessimistic and the capacitance could be greatly reduced.Definitely feedback resistance should be decreased if one is nuts about noise. For 60dB gain, 10/10k is typically good enough and only slightly affects even the lowest noise IC opamps.THAT1510/SSM2019/INA103 for ultimate low noise with balanced differential input. These chips beat DIY instrumentation amps made of opamp ICs by using only one transistor per input pin.

author: moffy
author_rank: Super Contributor
timestamp: June 22, 2023, 11:23:55 am
content: Quote from: Kleinstein on June 22, 2023, 07:02:19 amThanks for the clarification about the low duty cycle, Magic and you are right 100uF in such a case would be more than sufficient.