Recommend OP AMP

[AJ]

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

 

[Phil Allison]

"AJ" <

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....

** And 1 pA typical at room temp.

You have damaged the thing.

Or your post is a crock of shit.




......... Phil

 

[AJ]

"AJ" <


** And 1 pA typical at room temp.

You have damaged the thing.

Or your post is a crock of shit.




........ Phil

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

 

[Phil Allison]

"AJ"

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..

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

Do read the data sheet.

The stuff it says matters.



........ Phil

 

[Jamie]

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

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.

 

[Anthony Fremont]

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?

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?

 

[Eeyore]

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.

WHERE are you measuring this voltage ?

Graham

 

[AJ]

"AJ"


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

Do read the data sheet.

The stuff it says matters.



....... Phil

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

 

[Phil Allison]

"AJ"

"Phil Allison"

Thanks for the tip Phil, I am operating at 3.3V, which I guess must be a
bit out of range.

** 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