[sdiy] Cheap low input current opamps?

[Dave Manley]

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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