[sdiy] Cheap low input current opamps?

Veronica Merryfield veronica at merryfield.ca
Thu Sep 12 21:17:04 CEST 2013


I thought I did.

The recommendation is to use digikey's filters to find a list of parts. Using the column indexing, you can reorder the list to search for your preferences. I use the filters to get my selection in the ball park, and then use the indexing to home in on a device. Most of the time I have to work with multiple constraints with one being volume cost which takes a lot longer to figure out, but if you only have a few, it will be easier. Anyway, I have turned up all sorts of devise this way.


On 2013-09-12, at 10:34 AM, Dave Manley <dlmanley at sonic.net> wrote:

> 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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