Is this a good start to make a 60db probe preamp for my scope? - Page 1
Forum section: electronics / projects

author: robrenz
author_rank: Super Contributor
timestamp: July 19, 2012, 05:15:58 pm
content: I want to buy/build a probe preamp for my scope to measure PS ripple and triboelectric noise in the microvolt range. I dont want to pay $2k for a lab version. I just want something resonably accurate and the 100kHz of this module seems like plenty for what I want to do. Is this thing a good start or does someone have a better circuit they would like to share.http://www.freqdev.com/products/amps/d86.htmlThanks for any help.

author: blackdog
author_rank: Frequent Contributor
timestamp: July 19, 2012, 07:23:19 pm
content: Hi robrenz :-)Bild one yourself..Sample one of these nice instrumentation opamps from Analog DevicesAD8229 High temp + Low NoiseAD8428 Low noise + Fast!AD8429 Low Noise Super Gain 10.000 maxAD8421 Good input protection, medium speed 60dB 100Khz -3dBKind regarts,Bram

author: Unknown
author_rank: Unknown
timestamp: July 19, 2012, 07:27:01 pm
content: An alternative DIY design:http://tangentsoft.net/elec/lnmp/The input is single-ended, although it is battery powered to prevent ground loops. Bandwidth can be more than 100 kHz (the usual trade-off between bandwidth and noise floor).

author: SeanB
author_rank: Super Contributor
timestamp: July 19, 2012, 08:21:51 pm
content: If you want to make a high impedance amp i have some nice LH0033 buffers. 8pF input capacitance, 100MHz bandwidth and 10V input range on +-15V supplies. Some assembly required though, just the nice steel can.

author: Unknown
author_rank: Unknown
timestamp: July 19, 2012, 08:38:39 pm
content: High impedance, 100 MHz bandwidth and uV levels? Good luck! Some voltage gain might also be nice in this application.

author: hans
author_rank: Super Contributor
timestamp: July 19, 2012, 09:46:43 pm
content: Quote from: blackdog on July 19, 2012, 07:23:19 pmThanks a little list, I've been looking for low noise instrumentation amps too, and I haven't seen some of those before.What I found before was more TI/BB parts, like INA103 (very low noise). I've used it in a x1000 setup before and it works fine.Only thing to watch out for with these amplifiers is that every bit of noise or interference will cascade to the output. uV's is very tricky.

author: robrenz
author_rank: Super Contributor
timestamp: July 20, 2012, 12:31:54 pm
content: Thanks alm & blackdog, that input gives me a good start on my climb of a huge learning curve.

author: toli
author_rank: Frequent Contributor
timestamp: July 20, 2012, 06:28:28 pm
content: I have an LNMP I'm using to measure the output noise of linear regulators. One thing I think you should check is the opamp in the first stage. The AD797 used isn't stable in some cases. I've heard about it from other users of the LNMP, and had similar issue with my unit. Replacing the opamp to the OPA228 solved the issue (and gives similar performance).

author: robrenz
author_rank: Super Contributor
timestamp: July 20, 2012, 07:25:14 pm
content: Thanks for the heads up on that.

author: jahonen
author_rank: Super Contributor
timestamp: July 20, 2012, 07:37:57 pm
content: I made one some years ago for measuring regulator output noise, but with 80 dB gain. Worked just fine:http://koti.mbnet.fi/jahonen/Electronics/ScopePreamp/NoisePreamp%20SCH.pdfFor some practical measurements, seehttps://www.eevblog.com/forum/product-reviews-photos-and-discussion/fft-high-precision-mode-with-hameg-hmo-vs-rigol-ds2000-scopes/msg122099/#msg122099.Regards,Janne

author: robrenz
author_rank: Super Contributor
timestamp: July 23, 2012, 03:22:30 pm
content: So I made a rediculously low offer on a Preamble Instruments 1855 Differential Amplifier on ebay and Won it! It appears they are the manufacturer of the Lecroy 1855 diferential amp. So for $300.00 I get 100MHz differential preamp with 1x and 10x gain. I appreciate all the help but this is a cheaper solution in the long run. I might still want to do a 100x gain stage for after this unit to get down to microvolt levels on the scope but I think think that will be easier.Edit: Lecroy bought Preamble Instruments in 1997

author: robrenz
author_rank: Super Contributor
timestamp: July 24, 2012, 01:44:21 am
content: Very nice job! I may use that LNA section to boost the output of the differential unit into my scope to get an additional 40db.Thanks.

author: robrenz
author_rank: Super Contributor
timestamp: July 25, 2012, 01:16:47 pm
content: Just picked up the DCA100X probes for the 1855 differential preamp on eBay for $98.00. Gives 10x and 100x attenuation and matching to keep the CMRR high. At 10x it boosts the common mode voltage to 1550V but is actually limited by the probes max voltage rating. Anxious to try this setup out when it all shows up.I read in the manual that you can get a free 2x additional gain when plugged into 1M scope inputs without using a 50 Ohm feedthru terminator. This only works well on full bandwidth or 100kHz filter settings. With 10x gain and 1x probes and no 50 ohm terminator I will get 100uV/div on my Rigol 1052E and 25uV/div on my Tek 2225.