Hi,
We have a potential customer who wants us to modify one of our ADC modules to increase its input impedance to > 10M. It uses three series
0805 resistors in the front-end divider, so we need to find a thinfilm, low-TC, 0805 resistor above 3.3M. Something like 3.6 or 3.9M would work. We can find Vishays at 3.6M, 50 PPM/degree C, but lower TC would be good.Susumu doesn't go that high. IMS stops at 3.3M, but still 50 PPM.
Anybody know of sources? High value thinfilms are hard to find.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Teepee some lower values? Should double the R.
George H.
Barbarian! I'd spin the PCB rev first!
(Do barbarians live in teepees?)
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
I've successfuly used Ohmcraft and Riedon high-value precision resistors up to 10M with good TCR.
kevin
Ohmcraft, not that I use them, But just because I know they listen to their customers.
Cheers
At 4M as long as the board is washed properly he should be ok. No clean flux would cause problems before the FR4 resistivity comes into play.
Cheers
I wonder what type of PCB he is using? With a dielectric constant of
5 or so with FR4, kind of makes you wonder what happens with high Z circuits using high value R's, when not taking precautions in board design?Jamie
Somewhere on this NG i think i say a "trick" to find low TC resistors out of a batch. Or was that trick a way to make precision resistance - which might be adapted for a "find low TC" method.
Caddock has 25 PPM.
They start at 10M though...
Farnell/Newark have pages of them. The list started with Vishay/ Beschlag 0204 cylindrical parts, at 2.4 Eurocents each (minimum order
25).A tighter specification could shrink the list - I limited it to 1M to
10M, 50ppm/C or better, +/-1% or better, and metal film.-- Bill Sloman, Nijmegen
Uggh, Barbarian live in woods, sleep under star, get wet in rain.
George (Conan only in my dreams) Herold
.highlandtechnology.com=A0 jlarkin at highlandtechnology dot com
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