Preamplifier for noise and small signal measurements - Page 1
Forum section: electronics / testgear

author: srce
author_rank: Regular Contributor
timestamp: May 18, 2020, 10:01:01 am
content: I'm looking for a somewhat general purpose preamplifier for measuring noise (from LDOs etc) and small signals (from DACs) connecting to an oscilloscope with a noise floor of 40uVrms / 190uVpp. I've found the following devices:VendorDeviceNoise (nV/rtHz)1/f corner (Hz)InputInput ZMax Input (Vpp)GainCouplingBandwidthPriceDatasheetStandford ResearchSR5604 @1kHz200Differential or single100M || 25pF31-50kAC/DCDC-1MHz$2,595LinkSierra AmpsSA10.4 @ 100Hz, 0.3 @ 1kHz1kSingle-ended0.0011kDC3Hz - 10MHz$1,500LinkPicotestJ2180A30 @ 100Hz, 4 @ 1kHz, 2.4>10kHz4kSingle-ended1M || 10pF0.310AC0.1Hz - 100MHz$1,995LinkLecroyDA1855A90 @ 10kHz, 4 @ 1MHz400kDifferential1M / 100M || 20pF0.051, 10AC/DCDC - 100MHz$5,000LinkAnfatecAFT4001 > 100Hz100Differential or single1M || 39pF1, 10, 100, 1000AC/DC1MHz (1/10) / 600kHz (100/1000)€999LinkLinearTechAN1590.6 @ 100Hz, 0.5 @ 1kHz200Differential or single0.0003510kAC10Hz - 3MHzDIYLinkTektronixADA400A30?Differential or single1M || 55pF1, 10, 100AC/DCDC - 1MHz£2,100LinkAlphaLabLNA104 > 100Hz100Differential or single500k10, 100, 1000AC/DCDC - 1MHz$270LinkHoffman0.22 > 100 Hz100Single ended10k10, 100, 1000, 10000AC1.3MHzDIYLinkSignal Recovery51840.8 > 1k100Differential5M || 50pF0.011000AC0.5-1MHzLinkNFCorpSA-200F30.5 > 100100Single-ended1k, 10k, 100k || 150pF0.5100DCDC-800kHzLinkNFCorpSA-410F30.8 > 100100Differential1k, 10k, 100k || 100pF1100DCDC-1MHzLinkNFCorpSA-421F53 @ 100, 1 @ 1k, 0.5 > 100k Hz20kDifferential1M || 85pF4200AC30Hz-30MHzLinkCan anyone recommend any others? Any thoughts on these?

author: jemangedeslolos
author_rank: Frequent Contributor
timestamp: May 18, 2020, 12:40:01 pm
content: Hello,I was looking for the same things few weeks ago and I found this :https://www.alphalabinc.com/product/lna10/Maybe it will fit your needs.I haven't bought it yet so I don't know the actual performance.

author: Sighound36
author_rank: Frequent Contributor
timestamp: May 18, 2020, 04:00:05 pm
content: The Picotest is very quiet and I have not had any issues with finding decent results, its all in the set up and patience and good quality short test leads.The AFT400 looks interesting though not seen this one before any experience with this unit anyone?TES have a refurbished SR 560 for sale around the £1750 markhttps://www.testequipmenthq.com/product/Stanford-Research-SR560?gclid=CjwKCAjw5Ij2BRBdEiwA0Frc9dZzRT4ZzgIj41pzh4__wGdfqhame8TsiHNcmO3vsCGdWw6I3AGq4BoCGokQAvD_BwEThey also have a Lecroy for £3400https://www.testequipmenthq.com/product/lecroy-da1855aEbay US has a few but unless you are looking for a tear-down and repair not many reputable sellers plus it Ebay

author: awallin
author_rank: Frequent Contributor
timestamp: May 18, 2020, 04:29:42 pm
content: another DIY option which has been discussed elsewhere on the forum:http://www.hoffmann-hochfrequenz.de/downloads/lono.pdf

author: nctnico
author_rank: Super Contributor
timestamp: May 18, 2020, 05:24:32 pm
content: Quote from: awallin on May 18, 2020, 04:29:42 pmNot sure whether this is the best design where it comes to distortion. I see a whole lot of MLCCs in the signal path which have a voltage dependant capacitance and thus the amplifier has an amplitude dependant frequency response.

author: David Hess
author_rank: Super Contributor
timestamp: May 19, 2020, 12:56:57 am
content: There is also the Tektronix AM502 which is a 5A22/7A22 vertical amplifier with a BNC output for use with any high input impedance instrument. It has a bandwidth of up to 1 MHz with adjustable low and high cutoff frequencies and a broadband input voltage noise density of 6 nV/SqrtHz with the customary increase below 1 kHz due to flicker noise.They are reliable considering their age and fully documented but not as common as the equivalent 5A22/7A22 oscilloscope plug-ins making them more expensive. This is mitigated somewhat since the more common and less expensive 5A22/7A22 plug-ins can be used as a source of parts if neccessary. A couple years ago, the AM502 could be had on Ebay for about $100 within a couple months. The others went for half that or less.Linear Technology application note 124 has a discussion about noise measurements and a list of instruments which overlaps yours:https://www.analog.com/media/en/technical-documentation/application-notes/an124f.pdf

author: JonM
author_rank: Unknown
timestamp: May 19, 2020, 03:52:07 am
content: I have a SR560 that was purchased in the 1990's. I'm amazed that they are still being sold as new instruments, they even appear in full-page Physics Today ads about once a year. The SR560 is loaded with 7400 series TTL. My intent was to break the SR560 into two pieces with the front end being near, or in, the bore of a superconducting magnet. I remember the sales guy telling me it should be possible... In the end I used a custom pre-amp and the SR560 has not done anything exciting so far. A few years ago I bought new batteries for it (it can run off AC or the batteries). So, need to make it be useful!

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: May 20, 2020, 10:49:05 pm
content: Quote from: nctnico on May 18, 2020, 05:24:32 pmThe early versions had foil capacitors, truckloads of 'em, and it bought nothing. And capacitors withno voltage across them don't change. When there is AC across them, they are too small.Really, performance at the low end suffered because even the bank of foil capacitor at the input wastoo small, resulting in a much worse than 1/f rise of noise below 60 Hz. The current op amp versionhas a wet a slug tantalum at the input but this is problematic, too. It needs some sequencing when youattach the preamp to a low impedance DUT with some DC, and using it with non-low-impedance DUTis pearls before swine.When you mis-sequence, the transient through the huge tantal may zener the op amp's input transistors.There is now a version with many small jFETs (forget IF3601/02 !) but it is next to impossible tomake it exhibit no negative real part of the input impedance somewhere. Another wording for instable,and shared by all published designs that I analyzed. When you are aiming for 1 MHz BW, the feedbacksimply comes to late. It seems that i can get 100 KHz with REALLY fast op amps in the feedback loop,but their 1/f corners do hurt, even with the FETs as a first stage. Between a rock and a hard place.( I talk about CS FET, Cascode, op amp for loop gain, feedback to source)Cheers, Gerhard(above web site is mine)

author: nctnico
author_rank: Super Contributor
timestamp: May 20, 2020, 11:01:26 pm
content: Did you consider using PET capacitors? I know these are expensive especially in large capacitances but they are quite good.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: May 20, 2020, 11:07:57 pm
content: I had WIMA 10u/50V, PP IIRC. 20 in parallel.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: May 21, 2020, 06:34:29 pm
content: https://www.mathews-engineering.com/store/p1/High_Impedance_Buffer_Amplifier.htmlTektronix 650-A900-00, 0 - 3.0 Ghz, RF Precision PreAmphttps://www.ameteksi.com/products/signal-recovery/preamplifiers(also the older products, which were under Princeton Applied Research - I have their Model113 preamp)https://www.ee-quipment.com/products/differential-preamplifier?variant=35410631368http://www.nfcorp.co.jp/english/pro/mi/loc/pre/index.html

author: Frex
author_rank: Regular Contributor
timestamp: May 22, 2020, 04:51:30 am
content: Hello,If you want to spend a little bit less, I designed few months ago a small wide-bandlow noise amplifier to do power supply wide-band noise measurements.It include two stages with x10 and x100 gain output with flat bandwidth over 20Hz - 20 MHz.It can accept DC offset of + or - 20V max. The total bom count is about 30€,and the full project is open source and available on GithubHere.You can also lookherewhere i describe the project with some measurements results.I also have design a more complex LNA for audio bandwidth (10Hz-100kHz)that allow noise measurement down to micro-volts.You can look about this projecthere.Some measurements resultshere.Regards.Frex

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: May 23, 2020, 05:15:00 pm
content: Did you discard the EE-quipment amplifier due to insufficient specs or due to it being currently sold out (which I discovered only later)?

author: srce
author_rank: Regular Contributor
timestamp: May 23, 2020, 07:11:48 pm
content: Quote from: Neomys Sapiens on May 23, 2020, 05:15:00 pmThe latter. I'd already seen it on another site where it said it was discontinued. Although seems noisy too.

author: exe
author_rank: Unknown
timestamp: May 24, 2020, 10:06:36 pm
content: I bought ad8428 for the same purpose, but didn't play much with it because it needs a proper environment to be useful (shielding, supply from batteries, etc). Considering its cost, and cost of a typical lna, I'd say it's worth trying.https://www.analog.com/en/analog-dialogue/articles/low-noise-inamp-nanovolt-sensitivity.html

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: May 25, 2020, 03:47:48 am
content: Interesting thread. SR560 has a lot of competition now. Looks like Sierra Amps beats everyone on performance and AlphaLab beats everyone on price. Does anyone know any commercial pre-amps using zero-drift op-amps or other techniques to get good noise level at 1 Hz and below?

author: n3mmr
author_rank: Regular Contributor
timestamp: May 25, 2020, 07:48:05 am
content: Quote from: Frex on May 22, 2020, 04:51:30 amThe low frequency limits of 20 and 10 hz respectively, seem a bit high?

author: Frex
author_rank: Regular Contributor
timestamp: May 25, 2020, 02:45:55 pm
content: The low frequency limits of 20 and 10 hz respectively, seem a bit high?It's a very common low-cutoff frequencies for these kind of measurements.f required it can be lowered with your preferred one by using proper couplingcapacitor value (with a little increase on noise floor level of course).Frex

author: MegaVolt
author_rank: Frequent Contributor
timestamp: May 31, 2020, 03:55:21 pm
content: http://www.emelectronics.co.uk/our-products/