Op amp for amplifiying 5uV of noise to 200uV? - Page 1
Forum section: electronics / projects

author: c4757p
author_rank: Super Contributor
timestamp: January 09, 2014, 01:22:24 pm
content: I want to build a simple amplifier for measuring the output noise of my PSU kit. I'm looking to measure in units of at least 5uV. I want a gain of 40, so that my lowest noise "LSB" is at least 2 real LSB on my Fluke 45 on either "fast 300mV" or "medium 3V" range.Now, 5uV isn't incredibly low. I just pulled up the TL072 datasheet (because I have a shitload of them), and while I've always thought of it as reasonably noisy, it looks like it should work for a 0.1Hz to 1kHz* range. The noise voltage vs. frequency plot only goes down to 10Hz, but it's rising at about 18nV/rtHz/decade, so if it's 45nV/rtHz at 10Hz, I imagine it would be 63nV/rtHz at 1Hz and 81nV/rtHz at 0.1Hz. Am I correct? That translates to 14nV at 10Hz, 63nV at 1Hz, and 256nV at 0.1Hz. Multiplied by 40, that "huge" 256nV is 10uV, which is big, but well under the 100uV LSB of the meter. So, can I use it for this? Or am I missing something?*I'll switch to the faster, but lower-resolution, HP 3437A for noise at this end of the frequency range, or use true-RMS mode.(Of course, I know about Johnson noise, input noisecurrent, the need to power it from a battery to eliminate external PSU noise, and the need to shield it with a thick, metal box.)

author: robrenz
author_rank: Super Contributor
timestamp: January 09, 2014, 01:43:12 pm
content: Isn't it going to need to be differential to block out all the ambient noise?using a 7A22 diff amp herepicture links are broken but the pics are in sequence at the bottom

author: c4757p
author_rank: Super Contributor
timestamp: January 09, 2014, 01:57:20 pm
content: If it's floating on battery power (including the PSU itself), wouldn't it effectively be differential?

author: robrenz
author_rank: Super Contributor
timestamp: January 09, 2014, 02:09:56 pm
content: Does battery power make something immune to general EMI just because there are no ground loop issues?

author: c4757p
author_rank: Super Contributor
timestamp: January 09, 2014, 02:30:18 pm
content: No, but wouldn't the diff amp be equally prone to that, at least, after the differential input stage? That's why it would all have to be well shielded.I can get a reading on the meter stable all the way to the last LSB with no special precautions, so I don't think I'm really operating in Voodoo Land here.

author: robrenz
author_rank: Super Contributor
timestamp: January 09, 2014, 03:06:00 pm
content: I was thinking you were using this for a scope. I think the low bandwidth of the meters ac measurement is masking all the noise that you would see when measuring on a scope. jahonen has a nice buildherefor a scope preamp.I assume you are doing this to get into the range where your meters ac specs are valid?BTW I am way out of my league here, I am the last person that should be giving you advice

author: c4757p
author_rank: Super Contributor
timestamp: January 09, 2014, 03:19:31 pm
content: Quote from: robrenz on January 09, 2014, 03:06:00 pmWell, at the low-frequency end, I will sample DC rapidly over GPIB to get a measurement and then compute RMS in software. But 5uV is too low even for DC, at least at faster acquisition rates.

author: robrenz
author_rank: Super Contributor
timestamp: January 09, 2014, 03:47:16 pm
content: Aren't you going to get bogus info for your RMS calculation at such low DC sampling rates. Unless you use 100NPLC you are going to have noise that is higher than the levels you are trying to measure. At 100NPLC you are only getting a reading every 1.66 seconds at best. That is a 0.6Hz sample rate.

author: c4757p
author_rank: Super Contributor
timestamp: January 09, 2014, 03:52:44 pm
content: Possibly. I don't know off the top of my head how fast I can pump data out of the Fluke or the HP, and I'm a couple hours away from my bench right now, so really I'm more or less publicly brainstorming. Perhaps I could chain two amplifiers for higher gain and feed that into a DSO? I could also experiment with my P6046 for this, though it's in need of a (simple) repair first, and I don't know what its noise specs are.Clearly I have no experience with low-voltage near-DC measurements

author: Andreas
author_rank: Super Contributor
timestamp: January 09, 2014, 08:47:09 pm
content: Quote from: c4757p on January 09, 2014, 01:22:24 pmThe TL072 is optimized for high source impedances and audio frequencies.The lowest noise OP-Amps for above 10 Hz and source impedances < 300 Ohms are the LT1028 and the AD797.http://cds.linear.com/docs/en/design-note/DN355f.pdfFor other source impedances and frequencies you might have to choose other OP-Amps.Below 10 Hz the optimum is a chopper stabilized OP like the LTC2057.The question is also which minimum frequency can be handled by your RMS-Instrument.Usually you measure 0.1 - 10 Hz as peak-peak on the oscilloscope with one filter amplifier and > 10 Hz - 1, 10 or 100kHz as true RMS value with another band width limited amplifier.With best regardsAndreas

author: c4757p
author_rank: Super Contributor
timestamp: January 09, 2014, 09:12:41 pm
content: Those look good. How aboutAD8638(auto-zero) for the low end andAD8597for the high end? The LT part looks good, but Mouser doesn't carry LT and DigiKey has crap prices on a lot of the things I want to order at the same time. Looks like AD8597 is similar to 797 - higher noise, but still well within my limits.

author: Andreas
author_rank: Super Contributor
timestamp: January 09, 2014, 09:37:15 pm
content: Quote from: c4757p on January 09, 2014, 09:12:41 pmThe AD8597 looks very good. Forget the AD8638 and use the 8597 for the whole range if you can't get the 2057 for the 0.1 - 10 Hz.With best regards.

author: c4757p
author_rank: Super Contributor
timestamp: January 09, 2014, 09:38:27 pm
content: Very well, it does look excellent. Thank you for your help!Now, are there any noise-related gotchas with polypropylene film capacitors? That's what I am planning on using to implement the filters.

author: SeanB
author_rank: Super Contributor
timestamp: January 09, 2014, 09:47:28 pm
content: Black stripe end is outer foil, connect to low impedance side or ground. Grip lead end when forming so you do not stress the epoxy bond to the schoop end electrode. Try not to burn them with a soldering iron tip while mounting. Otherwise nothing really, clean board preferably without getting the capacitor wet with any solvents that might permeate the casing. If they are high value big units have 2 holes on the board so that you can use some lacing cord to tie them down in a high vibration environment.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 09, 2014, 09:55:23 pm
content: This doesn't sound that tough with a decent opamp and filters. There are no noise problems with film caps. I'd be starting with something like OP-27 or OP-37 for their good DC specs, and a 4th order filter. PM or email me if you want a good single opamp 4-pole filter design. Just as a reference point, I find the low pass filter selections on the Tek 1A7A plug-in very well chosen. One thing to watch out for is popcorn noise. It's not that common now, but will drive you nuts in DC work. It's process dependent and now and then you get some parts that have a bit too much.

author: c4757p
author_rank: Super Contributor
timestamp: January 09, 2014, 10:10:39 pm
content: Quote from: SeanB on January 09, 2014, 09:47:28 pmI can't seem to find any with such a mark. What about this method to test? Wrap a wire once or twice around the capacitor, and feed a high-amplitude, high-rise-time square wave through the wire. Probe across the capacitor in both orientations; the orientation which displays more noise has the probe tip on the outer foil.Quote from: Conrad Hoffman on January 09, 2014, 09:55:23 pmIt looks worse than the AD8597 in most ways. Is there something I'm missing?

author: blackdog
author_rank: Frequent Contributor
timestamp: January 09, 2014, 10:24:55 pm
content: Hi c4757p, :-)Take a look @ the AD8429 instumentation amplifier.Ask a sample from Analog Devices.Use it @ gains of about 100x for the lowest noise.Symetrical amplifiers is the best way, i use a Audio Precision for noise measurements of my PSU designs.I never want to measure these low levels asymetrical :-)Please be aware about protection of your measurement inputs.You need big capacitors for low noise measurements @ the input,This wil give large charge currents through the input.But, probably you know this...The best lownoise PSU i have build with a 22Khz BW, was about 3uV.It is a hell to measure this, because of al the EM Fields in my lab.I hope this helps.Kind regarts,Blackodg

author: Andreas
author_rank: Super Contributor
timestamp: January 09, 2014, 10:25:20 pm
content: Quote from: c4757p on January 09, 2014, 10:10:39 pmOp 27, 37 (or LT1007, LT1037) are optimized for higher source impedances in the 1K (-10K) range.The trade off for low voltage noise is high current noise. So with the AD8597 you will have to use low ohmic resistors and (very) large capacitors.The typical circuit you can find here:http://tangentsoft.net/elec/lnmp/misc/schematic3.pdfWith best regardsAndreas

author: c4757p
author_rank: Super Contributor
timestamp: January 09, 2014, 10:29:51 pm
content: I think I will go with the OP-27.Quote from: Andreas on January 09, 2014, 10:25:20 pmIt looks like they are using an electrolytic capacitor on the input. How much noise is there to the relatively high leakage current?

author: Andreas
author_rank: Super Contributor
timestamp: January 09, 2014, 10:40:32 pm
content: Hello,typically you will select the capacitors for low leakage current. (e.g. < 20nA)Use standard 85 degrees capacitors with high nominal voltage for best results.I use a pre-charging cirquit with a 9V battery to keep the capacitor charged always to minimize noise on my 0.1 Hz - 10 Hz amplifier.As I have to keep the source impedance to >= 1k Ohms I use the LT1037 (OP37 would also work). Noise with input shorted is around 120nV pp in the 0.1 - 10 Hz range. (either with precharged capacitor or after 24 hours run in phase).With best regardsAndreas

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 10, 2014, 01:11:28 am
content: Thought this was a DC application. That circuit is AC coupled and inverting, with a very low input impedance. Not something I'd want for a general purpose measurement circuit. The first stage should be non-inverting, with as much gain as possible, since that stage will establish the signal to noise ratio. The input should have some protection in the form of a series resistor and some diodes so you don't lose the opamp if you probe something err... unexpected. Maybe a ferrite bead or some RF supression as well. Follow that stage with a lo-pass filter to knock out the HF noise. For a scope this becomes way more effective visually at 3rd order and above, but for a meter it probably doesn't matter as much, unless the meter has a very wide bandwidth.

author: c4757p
author_rank: Super Contributor
timestamp: January 10, 2014, 02:02:19 am
content: It has to be AC-coupled, it's looking at PSU output noise. The gain applied to the actual DC level will just saturate the amplifier.Desired range is 0.1 Hz to 20 Hz, if I didn't mention.I've got a decent filter designed, but I'm in my car now so I can't post it. Later

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 10, 2014, 04:29:34 am
content: OK, sorry! In that case the low input impedance isn't a problem, other than requiring very large coupling caps. I'd still add some protection diodes and RF suppression due to paranoia if nothing else. If cap leakage is an issue, there's a trick where you use two in series, with a moderately high resistor to ground. That keeps the junction near zero, so leakage in the next cap isn't an issue and the opamp doesn't produce a big offset.

author: c4757p
author_rank: Super Contributor
timestamp: January 10, 2014, 05:46:32 am
content: How's this? Here are the figures:BandsLF55.9dB at the top54.6dB at 20 Hz55.4dB at 100 mHz52.9dB (-3dB) at 23 mHz52.9dB (-3dB) at 33.5 HzHF55.9dB at the top55.7dB at 20 Hz52.9dB (-3dB) at 12 Hz52.9dB (-3dB) at 1 kHzNoiseLF25.4 uV RMS integrated over 100 mHz - 20 Hz:Input at 1 uV RMS: 27.8 dB SNRInput at 100 uV RMS: 67.8 dB SNRHF28.4 uV RMS integrated over 20 Hz - 1 kHz:Input at 1 uV RMS: 26.9 dB SNRInput at 100 uV RMS: 66.9 dB SNRThe SPICE models used do model both voltage and current noise.Quote from: Conrad Hoffman on January 10, 2014, 04:29:34 amGood idea.

author: SeanB
author_rank: Super Contributor
timestamp: January 10, 2014, 06:36:03 am
content: The band may not be very visible on some, even can just be a line on the one side of the value or just in a datasheet that outer foil is to the left when reading the value. You will likely be using 47uF ( about the biggest value in a wire ended package) 63V units, and those are very large. You will find you need a big PCB that will have very low component density on it around the filter. Do not bother using SMT unit resistors there, use through hole and DIP packages to make assembly easier.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 10, 2014, 02:48:42 pm
content: Good point about resistors. Especially don't use any thick films- stick with ordinary metal films. They say wire wound can be better but I doubt the change would be noticeable. I see you're using the nice 4-pole filter from the old Burr-Brown book by Wong, one of my favorites.

author: C
author_rank: Super Contributor
timestamp: January 10, 2014, 04:38:42 pm
content: You state that you want to measure the low level noise on a power supply.Your ltspice circuit is shorting a bunch of different noise signal sources to your common ground.In the real world there three signals you do not show in your ltspice circuit. One is below VI, One is below R10 and one is below R17.Just connecting your circuit only to the power supply will increase the noise at one or both terminals of the power supply. Due to the UN-balanced ground connection of VI that connection will add a different amount of noise then the other connection.The same can be said about the two other connections(scope, DMM).Think of your scope connection. If you just connected one of those old big TEK medal boxes with no power connection. You just added a big medal plate( part of a cap) that also acts like an antenna. Plugging in the power cord will increase/decrease the noise but not remove it. If you touch the scope, you have changed the noise.Here you are shorting what exists at one terminal on the power supply to what exists at the ground connection of the scope.I would think that you would get much better readings by using a fully differential circuit.C

author: c4757p
author_rank: Super Contributor
timestamp: January 10, 2014, 07:47:55 pm
content: C, do you think this would work?Quote from: blackdog on January 09, 2014, 10:24:55 pmCouple in both + and - with identical capacitive highpass filters, subtract and amplify with an AD8429, then use the same filter configuration to separate the low and high bands (without gain this time)?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 10, 2014, 08:45:11 pm
content: Your signal to noise ratio is always established at the first stage. After that you can only make it worse, rather than better. Thus, one puts as much gain as possible right up front. That way you can do whatever summing and processing after without much concern.IMO, for most power supply work you won't see much difference between single ended and differential- unless you do! FWIW, I haven't, but I work at low (audio to 100 kHz) frequencies most of the time. That said, it's so easy to make the system differential when building up the unit, why not? That way you have it should any doubt arise.

author: robrenz
author_rank: Super Contributor
timestamp: January 10, 2014, 09:21:18 pm
content: Looks like we have circled back to reply#1

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 10, 2014, 09:40:03 pm
content: Pretty much, but after a certain age maybe we just like to talk a lot! FWIW, my Tek diff scope plug-in gets more use than any other. I use the lo-pass filters all the time, but very rarely the high pass. I also rarely use it in differential mode. Tek sells a similar add-on gadget (ADA400A) for the newer scopes, and it has much better bandwidth than my old 1A7A, but it's less flexible and a bit of a PITA to use. It also only works with scopes having the right power interface connections around the input BNC.

author: C
author_rank: Super Contributor
timestamp: January 11, 2014, 12:32:27 am
content: If you look at Ti, Analog Devices ... all the big names you will note that when trying to push speed or low noise they use fully differential circuits with next to no connections to ground as you are using it. So in simple terms, for every signal you have going positive, you have a matching signal going negative. You also have a signal center. This matching and inverted signal chain also helps in the power supply paths if you keep it in place in the power paths. Balance, every thing is balanced.My first choice would be a differential in and differential out opamp as the chip maker has a better chance of reducing the errors. If this does not exist with the spec's you need then matching opamps with one building the signal in the positive direction and the second the negative direction. Again with balanced signal paths.You want a circuit that has an input with two connections. You want to amplify and connect that to two different devices each with two connections.Think on it a sec, The reading you want should not change if you were to ground ether terminal of the power supply. It also should also not change if you connected a signal generator to one of the power supply leads and ground. This signal generator would act the same as EMI or any of other external noise sources that you do not want to measure.Working up to the base idea. If you were to connect two resistors in series across the power supply, That resistor connection junction would be ZERO Power supply noise. This connection will still have some of all the other noise riding on it. You want the AC part with no DC so you need matching cap's. Your input signal is the three connections to the resistors. Will come back to this side in a sec.You have a scope connection that is single ended. As I tried to say that ground side of the scope connection will have noise riding on it. What you really want to happen is to add your desired signal on top of that noise such that the ground connection noise does not effect the signal. If you had a fully differential scope input this could be easer. At the scope end you need to convert from fully differential to single ended in a way that will let you use the noise that exists on the scope's ground connection to compensate for it's noise & offset. If your amp's differential output was two 1M resistors in series you would have a differential center zero. If you then paralleled this with the scope and a network matching the scope's input in series. You would then have the + & - signal. An equal to the scope's ground connection with out the noise and the scope's ground connection with the noise.Your circuit should be a gradual change from the three inputs to the network that drives the scope.Looking at what you have, you need the negative version of C5, R5 & U1. As you progress across the circuit you need to decide if the junction of +-R5(the noise center) should be connected to the next stage or skip it and how much resistance this connection should be. Remember the three signal paths, the positive signal, the negative signal & the noise center. Looking from the input side the noise center should see resistance to the scope's ground connection the noise center at the scope end of circuit.With the current circuit, If you were to put resistors inline to U1's power connections, you would see unequal currents flowing. With the resistors inline to the +-U1 setup these currents should match. Keep this idea in mind for the complete circuit. Just after U1 with the balanced signals you would really have five signal paths. The noise center, +-Signal, +-Power. Note NO ground so the noise that exists on ground is not getting in the signal path all over the place.So if you were to use two AD8429's, One for the positive and one for the negative the REF input would be a great assist in getting the signal centered properly on the scope's noisy ground connection.The sad thing is that the BNC Jack of the scope might be the only part of the scope's internal signal path that is single ended.Sorry, Not sure If I stated everything clearly. It actually sounds more complicated to say then the final fully balanced circuit that does this.C

author: robrenz
author_rank: Super Contributor
timestamp: January 11, 2014, 12:42:07 am
content: Quote from: Conrad Hoffman on January 10, 2014, 09:40:03 pmGlad to see you back ConradMy 7A22 and 7A13 diff amps are my most used also.

author: c4757p
author_rank: Super Contributor
timestamp: January 11, 2014, 01:55:59 am
content: I don't really see how that method differs from doing the differential->single-ended conversion at the beginning rather than the end, except for having more parts. The amplifier would be powered by batteries, so it's not like there would be ground loop issues between it and the scope. It's just whether you subtract the signals at the left side or the right side of the circuit board, as far as I can see

author: c4757p
author_rank: Super Contributor
timestamp: January 11, 2014, 03:35:06 am
content: This is what I have now.The part number of the instrumentation amp is a mistake, it's of course AD8429.I decreased R16 to 30k to eliminate the peak in the HF curve.

author: C
author_rank: Super Contributor
timestamp: January 11, 2014, 04:16:26 am
content: You need to look at all the different current loops or signal loops.With single ended your signal path return is changing from the positive rail to the negative rail depending of which direction the signal is changing.All the current differences in single ended design have effects on other parts of the circuit. Current demands are not balanced and opposite so they can not be localized as well as with balanced circuits.The ground connection of R8 is changing based on what all the other connections to ground are doing.With a balanced circuit any change to the positive power rail is matched by an equal but opposite change to the negative power rail.R8 no longer needs a connection to ground as it is connected to it's matching negative half. There fore no noise or errors from the rest of the circuit effects the R7 & R8 divider.Need to Look closer as it not just more parts for the sake of adding parts.There are some down sides to balanced but if single ended is so great then balanced would not exist.Do a simple thing, in LTSpice put in some resistance to equal the copper resistance you would have on a PC board. Add the other real world things in and check the results.Then really go to town with a fully balanced version of the circuit and compare the results.C

author: c4757p
author_rank: Super Contributor
timestamp: January 11, 2014, 04:20:06 am
content: I'll give it a try tomorrow. Going to bed now.I don't doubt that it would work. I just suspect it's unnecessary. Remember, I only want a resolution of 5uV (that's not a 5uVsignal, that's a 5uV "LSB").

author: robrenz
author_rank: Super Contributor
timestamp: January 11, 2014, 04:36:45 am
content: I got a HP 3410 for measuring PS ripplethread here.I have not had time to restore it yet but it looks promising.

author: c4757p
author_rank: Super Contributor
timestamp: January 19, 2014, 02:56:34 am
content: OK, I built this. I know there were a good few changes suggested, but I was relatively confident it would work (perhaps with a couple tweaks) and really wanted to get on with it...It seems to work quite well. More testing will follow (and I've only posted screenshots of the low frequency output, though the high frequency one works just as well).Couple things -- I of course missed the input bias current of the instrumentation amp, which at 250nA on 470k and a gain of 200 would have the thing shooting for 24V... so I removed the 33k differential lowpass resistor and changed both 470k bias resistors to 33k.- Added a switch to DC-null the inputs. Not really necessary, but it does stop the amplifier from saturating after power is applied, which allowed me to actually watch the settling time.- RC time constant of the first input coupling stage is 1034 seconds, and 73 seconds for the second stage. Yes, this takes along timeto settle (plot attached).- No need for batteries. The noise floor is indiscernible even powered externally.- Clearly, from the layout of the panel parts (jacks and battery clips) I didn't plan the housing very wellPictures:- Inside the box. Yes, I built it in an electrical junction box. Works nicely.- Closeup of the circuitry.- LF noise floor with both inputs terminated.- Output noise of my Power Designs TP343A.- Settling time.- Schematic.The only thing I think I'd change if I redesigned it would be to add an offset nulling control to the LF output, so I don't have to use the oscilloscope to do it.