Anyone ever attempt a DIY LT1088? - Page 1
Forum section: electronics / projects

author: cellularmitosis
author_rank: Unknown
timestamp: March 16, 2015, 06:23:13 pm
content: I've noticed a lot of Jim Williams app notes use a thermal RMS-to-DC technique to perform precision noise measurements, and these all used the LT1088 chip.Now that this chip has been out of production for a long time, it seems almost legendary.Has anyone ever attempted to create a DIY replacement for this chip out of discrete components? The equivalent circuit certainly looks simple enough.It seems the only drawbacks would be much slower response times (much larger thermal mass because of discrete components), and you'd have to go buy an old HP meter which had an LT1088 in order to calibrate it anyway...Nevertheless, it certainly seems like a fun DIY project for us metrology geeks.Thoughts?

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 16, 2015, 06:34:26 pm
content: Interesting idea! It resembles the idea behind theIL300, that uses 2 diodes to get a linear transfer function within a optocoupler.QuoteThere you have it .. in 2015 there are better approaches to get RMS value.However, I would be interested do to some noise measurements. I saw a video from Jim Williams but that was very complicated to build. On the other hand, on my 34401A I cannot see any noise on my references.

author: Vgkid
author_rank: Super Contributor
timestamp: March 16, 2015, 07:09:25 pm
content: I remember reading an appnote about using dual matched thermocouples to give similar performance to the lt1088, at up to a few MHZ. I actually found the thermocouples on ebay, but for a good amount(around$60.00 each) of money.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 16, 2015, 07:53:49 pm
content: Hi,I Have 3 LT1008 IC's and made a long time ago a RMS converther with them.Now i have a HP3400A and an Fluke 8920A <= a beautiful machine...The prblem then was the wide band amplifier, now its relatif easy, use A LT1206 :-)You cant buy the LT1088 any more @ Linear Technology.JohnnyBerg,The 34401A is not very useful at measuring noise levels from a reference.The RMS converter in the 34401A is not designed for signals below 1mV.Read the app notes of Jim, about measuring noise from references, take a look at the LTC6655 reference.Kind regartsBlackdog

author: Vgkid
author_rank: Super Contributor
timestamp: March 16, 2015, 08:04:41 pm
content: Found it ysi-44018, now sold by Omega, $28.00.http://cds.linear.com/docs/en/datasheet/10134fd.pdfstill around $60 worth of thermocouples.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: March 16, 2015, 08:17:23 pm
content: I missed that one. Linear Tech's App note 5 also has a discrete RMS converter but not as elegant. It uses a few more 0.1% resistors.www.linear.com/docs/4101

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 16, 2015, 08:21:46 pm
content: Quote from: blackdog on March 16, 2015, 07:53:49 pmI had a look at this one:but it seemed immense complicated to me?

author: blackdog
author_rank: Frequent Contributor
timestamp: March 16, 2015, 08:40:46 pm
content: Hi JohnnyBerg,Jim explane's it, it NOT simple, you can cut a part of the schematic, only use the preamp.Take time to select a bunch of Capacitors for low leakage (1000 to 1500uF).Every capacitor cost u at least 2 a 3 days to measure at say 15V DC and after the 15V format voltage another day for testing at 10v DC.This testing makes you humbleIt is not Easy to measure from 0.1 to 10 Hz with low noise levels, use de doubble shielding!Kind regarts,Blackdog

author: juani_c
author_rank: Contributor
timestamp: March 16, 2015, 09:23:40 pm
content: A while back I wanted to do the same, never finished it. BAV99 diodes as sensors with some 50 ohm 0805 resistors as heaters, something similar to this:http://www.vk2zay.net/article/210It worked for Alan, so it seems plausible

author: dom0
author_rank: Super Contributor
timestamp: March 16, 2015, 09:46:45 pm
content: Today it seems much more practical to me to either buy a good RMS voltmeter (e.g. HP 3400A/B) or to build a low-noise wideband preamplifier for a DSO, which then can easily calculate FFT and RMS of the amplified signal.If you're concerned with 0.1 - 10 Hz noise... well... measuring that ain't simple. I personally didn't bother with it, but there's a loooong thread (in German) on mikrocontroller.net ; user ArnoR posted a very interesting schematic there with a special JFET achieving quite some less than 300 nV residual noise, if I remember correctly.

author: cellularmitosis
author_rank: Unknown
timestamp: March 16, 2015, 11:15:39 pm
content: Quote from: dom0 on March 16, 2015, 09:46:45 pmWonderful article!

author: cellularmitosis
author_rank: Unknown
timestamp: March 16, 2015, 11:17:18 pm
content: Inspriation:https://www.eevblog.com/forum/projects/worlds-smallest-cheapest-diy-temperature-controlled-oven/

author: paulie
author_rank: Frequent Contributor
timestamp: March 17, 2015, 12:37:30 am
content: But I don't understand.... where is the PCB? How is this possible?

author: cellularmitosis
author_rank: Unknown
timestamp: March 17, 2015, 05:56:13 am
content: Background:http://readingjimwilliams.blogspot.com/2012/04/app-note-106.html

author: cellularmitosis
author_rank: Unknown
timestamp: March 17, 2015, 06:14:44 am
content: Hmm, perhaps two transistors in one package could be useful? Use one BJT as the heater, use the other as the temperature sensor...http://www.seekic.com/circuit_diagram/Sensor_Circuit/TRANSISTOR_SENSOR_1.html(interestingly, it looks like the above was also a Jim Williams circuit, though I'm having trouble finding the original source online, which appears to be "Designer's Guide to: Temperature Sensing, EDN Magazine, May 5, 1977, p 77-84").

author: dannyf
author_rank: Super Contributor
timestamp: March 17, 2015, 11:19:21 am
content: Doesn't seem to be that difficult, other than the pair of diodes that need to be matched in terms of their thermal charactersitics.If the desire is RF->DC conversion, there are other chips, like log-amps that can do the trick. Check AD for that.

author: LaserSteve
author_rank: Super Contributor
timestamp: March 17, 2015, 02:39:24 pm
content: HFA3096 is in limited production and replaced the CA3096. Its rated to UHF... ~~ 4.00 a chip.That might be a start.Matched pair chips are rare like unicorns, one should always buy some stock when available.A further check shows Digikey has some pairs and quads that are cheap, but not exactly matched.Steve

author: dannyf
author_rank: Super Contributor
timestamp: March 17, 2015, 02:55:44 pm
content: QuoteNot likely. The diodes should have identical thermal characteristics but otherwise thermally ISOLATED -> the opposite of what you expect from a matched pair.I think the charge transfer approach is the most likely alternative.