# Op amp gain trouble.

Discussion in 'Electronic Design' started by bunny, Dec 1, 2005.

1. ### bunnyGuest

I am using the below schematic for a OpAmp TLC272CP. According to the
formula of G = (R1/R2) the gain on this circuit should only be a matter
of 11. however as you can see below it's closer to 15/16. I'm
wondering what could cause this anomoly. I have put the circuit
below(sorry for the Ascii) and what i put into the + of the opamp and
what i got as output.

Thanks

base=2.0mv
output=33.4mV

+9V +9V
| (+). |
100k pot<------|\|
| (-)| \_____.__R3____Meter__.
| .-| / | |
| | |/| | |
| | | | |
| .------R2-' |
| | | |
| R1 | |
| | | |
Gnd Gnd Gnd Gnd

R1 = 1k
R2 = 10k
R3 = 10k

2. ### Meindert SprangGuest

No, the gain is (R1+R2)/R1. Which indeed is 11 in your case.
If you check the datasheet, you'll see that the typical offset voltage of
this opamp is 1.1mV. This offset gets added to the input voltage. So in your
case, (2.0mV + 1.1mV) * 11 = 34.1mV. Almost exactly what you measured....

Meindert

3. ### bunnyGuest

R3 is because i'm using one of those radio-hack board that has the stuff
like switches and multimeter and led's and pots and crap all in it
the r3 10k resistor is meant to drop the voltage reading to the mV area
so it doesn't destroy the multimeter on the board. I have a seperate
digital multimeter which is what i used ot test for the values.

4. ### bunnyGuest

I have a radio-shack electronics lab with a breadboard in the middle and
pots and stuff all around. It includes a multimeter that can only read
mV's so i had to include 10k resistor in series with the multimeter to
drop the Voltage down to a level the multimeter can safely read. I have
a Digital Multimeter taht i used to take the values. The one on the
board is analog.

5. ### Pooh BearGuest

You're forgetting about DC offset errors. A TLC272 is not suited to
precision DC amplification.

What's the point of R3 btw ?

Graham

6. ### Allen BongGuest

from Texas Instrumenents.

http://www.futurlec.com/TI/TLC272CP.shtml

On P5 & P6, the VIO for TLC272C are TYP = 1.1mV and MAX = 10mV for both
VDD = 10 & 5V.

What is the parameter Rs stood for and how do I make VO=1.4V?
P28 does memtion RS on a graph but I couldn't get any clue out of it.

Will blowing hot air onto the chip while operating make the offset
voltage raise up to 11mV?

Allen

7. ### Spehro PefhanyGuest

Yuk. It's like doing some kind of arthroscopic surgery. Why not go to
the horse's mouth.. TI's website?

Rs would be source resistance. It's not particularly important for Vio
except to notice that it's very low, so as not to include much in the
way of input bias current effects.

You make Vo = 1.4V by adjusting the input voltage(s). It's unclear how
exactly they do the test. Another version of the data sheet for the
same part has this footnote:

* All characteristics are measured under open-loop conditions with
zero common-mode voltage unless otherwise specified. Unless otherwise
noted, an output load resistor is connected from the output to the
ground pin.
It could cause thermocouple voltages that happen to add to existing
Vio. You should avoid thermal gradients and fast changes if you want
to measure the actual Vio. But the effect will generally be << 1mV
unless you're doing something silly like pointing a heat gun at the
poor thing.

Best regards,
Spehro Pefhany

8. ### Winfield HillGuest

Spehro Pefhany wrote...
Perhaps he wanted the old 1994 version b of the datasheet,