[sdiy] Cheap low input current opamps?

[David G Dixon]

Sorry, I thought it was copied to the list.  Basically her recommendation
was to use the filter feature in Digikey to find low input current opamps,
which is what I did.  There are a bunch of cheap 1 and 2pA models from NJR
on there.
 

> -----Original Message-----
> From: Dave Manley [mailto:dlmanley at sonic.net] 
> Sent: Thursday, September 12, 2013 10:35 AM
> To: David G Dixon
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: Re: [sdiy] Cheap low input current opamps?
> 
> Was Veronica's email not sent to the list?  What were her 
> recommendations?
> 
> -Dave
> 
> David G Dixon <dixon at mail.ubc.ca> wrote:
> >Wow, the OPA4348 looks good.  However, a quick squizz at the 
> datasheet 
> >says that the maximum power supply voltage range is 7.5V.  I 
> need 30V.
> >
> >I found some good ones on Digikey, following Veronica's suggestion. 
> >All
> >made by NJR, incidentally.
> >
> >
> >> -----Original Message-----
> >> From: rsdio at sounds.wa.com [mailto:rsdio at sounds.wa.com]
> >> Sent: Thursday, September 12, 2013 1:15 AM
> >> To: David G Dixon
> >> Cc: synth-diy at dropmix.xs4all.nl
> >> Subject: Re: [sdiy] Cheap low input current opamps?
> >> 
> >> Since I have the numbers in my notes, I'll put them in 
> your format.  
> >> The list starts with the cheapest and goes up to the more 
> expensive 
> >> parts. Where documented, I'm also adding the input impedance after 
> >> the typical currents.
> >> 
> >> $0.39 LM224D {2000/20000}
> >> $0.52 TL084 {5/30}
> >> $0.54 TL074 {5/30}
> >> $0.62 LF353MX {25/50} 1.0 TΩ
> >> $0.63 LMV324I {11000/11000}
> >> $0.68 LF347D {25/50}
> >> $1.05 OPA4348 {0.5/0.5} 10.0 TΩ
> >> $1.65 LF444 {5/10} 1.0 TΩ
> >> 
> >> Prices are obviously less in quantity.
> >> 
> >> 
> >> On Sep 11, 2013, at 17:11, David G Dixon wrote:
> >> > Thanks for your reply.  Here's my assessment:
> >> >
> >> > Opamp {Typical input offset current/input bias current in pA}:
> >> >
> >> > TL06X {5/30}
> >> > TL07X {5/65}
> >> > TL08X {5/30}
> >> >
> >> > Hence, it would appear as if either the 06 or the 08 beats
> >> the 07.   
> >> > Here's
> >> > LF444:
> >> >
> >> > LF444 {5/10}
> >> >
> >> > I have used these in my updated CGS ASR design, and no one has 
> >> > complained about droop.  I think I'll be happy with 06 or
> >> 08, but I'd
> >> > prefer something a little better.
> >> >
> >> > I'll definitely have a look at the others you mentioned.
> >> >
> >> >
> >> >
> >> >> I have shipped commercial products with S&H based on the
> >> OPA4374 (0.5
> >> >> pA) and TLV2374 (1 pA), but those are more expensive than
> >> the LF444.
> >> >>
> >> >> Looking back at my selection notes, I considered the
> >> LM224D, TL084CD,
> >> >> TL074CD, LF353MX, LMV324I, LF347D, OPA4348, and compared
> >> them all to
> >> >> the venerable S&H LF398. Of those, the
> >> >> TL084 might meet your specifications, and it's been used
> >> in classics
> >> >> like the Prophet 5 (IIRC).
> >> >>
> >> >> Your best choice might be the OPA4348. It's cheaper than the
> >> >> LF444 and has typical offset and bias currents of only 0.5 pA.
> >> >>
> >> >> Brian Willoughby
> >> >> Sound Consulting
> >> >>
> >> >>
> >> >> On Sep 11, 2013, at 11:19, David G Dixon wrote:
> >> >>> I'm looking for a cheap alternative to the LF442/LF444
> >> opamps, which
> >> >>> have very decent input current specs and therefore recommend 
> >> >>> themselves for use as S&H buffers (typical offset 
> current 5 pA, 
> >> >>> typical bias current 10 pA).
> >> >>> I'm coming up short with Google searches.  Does anyone
> >> here have any
> >> >>> suggestions?  The TL06X series is pretty good with 
> typical bias 
> >> >>> current of 30 pA, but I'm not finding anything else that
> >> isn't much
> >> >>> more expensive.
> >> >
> >> 
> >
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