[sdiy] ADc & DAC voltage dividers.. fixed resistors values finally..

Jean-Pierre Desrochers jpdesroc at oricom.ca
Wed Feb 17 16:28:40 CET 2021


Thank you for your kind help Roman..

Because this project won't be a 'multi unit' to come
The actual cost for the final opamp choice is not an issue (for a reasonable cost though).

After reading the 3 suggested opamps datasheets,

my choice will probably be between MCP606-I/P or OPA340PA
Hmmm.

 

 

-----Message d'origine-----
De : Synth-diy [mailto:synth-diy-bounces at synth-diy.org] De la part de Roman Sowa
Envoyé : 17 février 2021 03:12
À : Brian Willoughby; SYNTH DIY
Objet : Re: [sdiy] ADc & DAC voltage dividers.. fixed resistors values finally..

 

All 3 opamps I listed are unity gain stable and cover full input span, and even beyond rails.

I also wrote about RRO and RRIO opamps in earlier post and how to overcome this limitation.

 

Roman

 

W dniu 2021-02-17 o 08:53, Brian Willoughby pisze:

> Yeah, one thing to look out for is that not all rail-to-rail op-amps support the full voltage on both input and output. Quite a lot of them are only rail-to-rail outputs, and assume that you'll be using some gain. Thus, a voltage follower is going to work very poorly with those select chips.

> 

> Always check the data sheets carefully and thoroughly.

> 

> Brian

> 

> 

> On Feb 16, 2021, at 23:30, Roman Sowa wrote:

>> None of them would work in this scenario.

>> My humble suggestion is to try MCP606 as "good enough". It's a tad slow but if you plan to use it only for CV sampling at 1kHz, it's still fast as lighning.

>> Or other interesting opamps: LT1637, OPA340 to mention just 2

>> 

>> Roman

>> 

>> W dniu 2021-02-16 o 21:50, Jean-Pierre Desrochers pisze:

>>> So far the precision opamps I have on hands are:

>>> OPA2277PA

>>> OP177G

>>> OP275G

>>> LF412CP

>>> I don't think they fit the requirements..

>>> Any ideas for a good 16 bits ADC front end opamp ?

>>> 

>>> 

>>> -----Message d'origine-----

>>> De : Mike Bryant

>>> Envoyé : 16 février 2021 14:52

>>> 

>>>> But to me 5V rail to rail opamps will never reach 0 neither +5.0v at their output when these values show at the input..

>>>> Close values but not the original ones.

>>>> Or maybe I am wrong on that.

>>> The ones for ADCs have FET output stages so they'll happily go all 

>>> the way to zero accurately and pretty near to the positive rail.  

>>> And many ADCs don't go all the way up to 5V anyway as they need some 

>>> headroom for the internal op-amp on the voltage reference

> 

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