Recommended DC Blocking Cap for Ultra Low Noise Measurements - Page 1
Forum section: electronics / projects

author: Cigarsnob
author_rank: Regular Contributor
timestamp: March 03, 2021, 07:47:40 pm
content: Hey Everyone! Looking for a recommendation on some DC blocking capacitor to be used in a low measurement test box. I'll be used to measure low noise laser drivers, such as Wavelength Electronics WLD3343 or the Koheron CTL200. I will be using an AD797 low noise amp to boost the noise and a DC blocking cap from the laser driver load to the input of the amp.I'm looking for an ultra low leakage cap to not only function as DC block, but to also act as my highpass filter with a cutoff frequency of 1-10Hz. I found a good app note from Jim Williams that talks about such a cap, but damn is it pricy.https://www.analog.com/media/en/technical-documentation/application-notes/an124f.pdfIn my first attempt, I used the following cap, but only 2 in parallel:https://www.digikey.com/en/products/detail/rubycon/16MU226MD35750/4969522?s=N4IgTCBcDaIIwDYC2BXMZkBMDMBWA7LgAwgC6AvkAThis time around, I'm making a PCB with everything on it and into a metal box to hopefully get more consistent measurements as my original test set up consisted of eval boards, batteries, cables, etc etc.At the moment, I may be sticking with the cap given above and just create a very large parallel combination to get up into the mF. Resulting in hundreds of dollars just for the caps alone.Any recommendations/alternatives is much appreciated.

author: macaba
author_rank: Regular Contributor
timestamp: March 03, 2021, 10:51:21 pm
content: I bought 10 decent 1500uF electrolytics and selected the one with the lowest leakage after a certain duration. I’ve also heard good things about the Wima DCP4 400uF.

author: Cigarsnob
author_rank: Regular Contributor
timestamp: March 04, 2021, 12:18:38 am
content: Thank you! This does have a higher insulation resistance than the 22uF one and comes in larger values to help with that filter. I've give it a shot.

author: vindoline
author_rank: Unknown
timestamp: March 04, 2021, 01:12:38 am
content: Take a look here:https://www.eevblog.com/forum/metrology/measuring-capacitor-leakage/25/

author: David Hess
author_rank: Super Contributor
timestamp: March 04, 2021, 11:06:49 pm
content: For a high input bias current low voltage operational amplifier, unless the price of the wet tantalum capacitor Jim Williams mentioned is acceptable, then low leakage aluminum electrolytics are about the only option.However low voltage noise JFET and MOSFET input amplifiers are almost as good and their low input bias current considerably relaxes the size of the input coupling capacitor making film capacitors viable.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 05, 2021, 12:02:31 am
content: Selected low leakage series aluminums should be very good, or buy some military surplus thing with wet slug caps in it.

author: bdunham7
author_rank: Super Contributor
timestamp: March 05, 2021, 12:26:50 am
content: 13 of these would be my choice, but obviously not cheap or small.https://www.parts-express.com/Dayton-Audio-DMPC-100-100uF-250V-Polypropylene-Capacitor-027-447

author: mawyatt
author_rank: Super Contributor
timestamp: March 05, 2021, 02:16:15 pm
content: Second the vote for the wet-slug tantalums. Recall they support small reverse voltages without bad effects.Best,