[sdiy] Cheap low input current opamps?

[David G Dixon]

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.
> >
>