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Recommend OP AMP

Hi All,

I am trying to buffer voltage from a resistive voltage divider via a TLV2211 OP Amp. My problem is that when I use any resistors above 100K/220K in the divider I get lower readings than expected. e.g. when I use 470K/1M with a 4V input on my divider I measure 2.55V instead of 2.75V. I assume this is because of the input bias current? but this is specified to be 150pA max....

I would really like to use bigger resistors to cut power down, can anyone recommend a better OP Amp?

Best regards,

AJ

Reply to
AJ
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"AJ" <

** And 1 pA typical at room temp.

You have damaged the thing.

Or your post is a crock of shit.

........ Phil

Reply to
Phil Allison

Sorry I should have mentioned the typ 1pA.... I have tried a few of them, all from the same batch and all have the same problem... I might order some fresh ones..

Regards

AJ

Reply to
AJ

"AJ"

** Hope your DC supply is at least +5 volts.

Do read the data sheet.

The stuff it says matters.

....... Phil

Reply to
Phil Allison

There are a few things to ponder over here.

For an ideal circuit, I get 2.71 as the expected voltage how ever, we know things are not ideal.

Have you tested the values of your resistors with an accurate meter?

Also, have you scoped the divider to make sure there is no noise or some introduced signal near by?

Have you tried a small by pass cap on that divider?

Like I said, things to think about.

I can tell you, I have seen many times OP-AMPs perform differently of the same number but in different packages. I have found if you don't specify mill spec, it may become a turkey shoot.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Reply to
Jamie

How good is the DVM you are using to measure the voltage at the divider? If you have a second meter available, continuously monitor the output while checking the input to see if the output voltage changes when you touch the primary DVM probe to the divider junction. IOW, your DVM may be thing loading down the divider. What voltage are you running the op-amp at?

Reply to
Anthony Fremont

WHERE are you measuring this voltage ?

Graham

Reply to
Eeyore

Thanks for the tip Phil, I am operating at 3.3V, which I guess must be a bit out of range. I reversed the resistors (1M/470K instead of 470K/1M) and I am now reading the voltage I expect.

Thanks to everyone else for their replies as well.

Best regards

AJ

Reply to
AJ

"AJ"

** You are * not * kidding.

THAT info should have been in your first damn post !!!!!!!!!!!

The term " rail to rail" is basically marketing bollocks - it just might apply to the output but never assume it applies to the input.

Like the old saying goes:

" What the LARGE print giveth - the small print taketh away ..... "

........ Phil

Reply to
Phil Allison

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