Re: Low absorption SMD capacitors


Winfield Hill wrote:
news@xxxxxxxxxxxxxxxxx wrote...

Steve Goldstein wrote:

Some years ago I found that NPO ceramic microwave capacitors worked very
well.

[snip]

Steve is right. I have taken measurements on three brands of NP0 C0G
0805-size capacitors and found them to be either perfect, or below
0.1% D.A. over a 100Hz to 250kHz range (this is close to my hp 4192's
supposed 0.2% measurement accuracy), which is as good as my "pefect"
polypropylene capacitor measurements. This compares to 5 to 15% D.A.
that I measured for other ceramic-material 0805 capacitors.

OK, great. These seem to be a lot more readily available in SMD.

[more snippage]

We are trying to measure photocurrents of about 200 pA upwards, so I
will have to keep a close eye on leakage paths. Is ordinary FR4 PCB
with conformal coating adequate for this job or will I need to look
at ceramic hybrids etc?

You have to clean the board shortly after soldering, then add a
conformal layer to a dry board after test. FR4 should be OK in the
region you're using, provided you have appropriate guards. In some
instances you'll want to add rows of closely-spaced guarded vias to
intercept the currents that could flow along fibres inside the PCB.

By 'guards' I assume you mean a ring around the sensitive nodes
connected to GND?

All our boards are made by a subcontractor so I don't have much control
over the cleaning - some have been fine, some have come back covered in
white detergent residue and other cr@p which has played havoc with even
fairly low-impedance circuits. If I scrub them myself with a
toothbrush and isopropanol after they've been sitting on a shelf for a
week will this be OK or does the contamination leach into the FR4
itself?

I have used the LT1112 (plastic DIP package) for the prototype, it
seems to work fine but I note from the data *** that the worst-case
input bias current and offset voltage could be a problem. In one sense
it doesn't matter because the photodiode dark current will usually
dominate, and I subtract this out by taking a dark measurement at the
same integration time immediately afterwards, but if there is a large
enough negative bias current the ADC will clip at zero and this method
won't work.
Another 'oddity' is that the SO8 package of this device (which I'm
planning on using for production) has significantly worse specs than
the PDIP - any ideas why? I always assumed the die was identical
between different packages.

.

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