Offset voltage on voltage follower

T

Tuurbo46

Guest
Hi
I need a bit of help on a circuit im trying to build. I will try to explain
my problem below.

Circuit requirements:

*input to op-amp = 0 to 5v
*output from op-amp = 0.5v to 3.5v


Im am currently using a CA3140E opamp with a 0.5v offset in it, which is
quoted in the data sheet.

Pin wirring on chip:

Pin 1: to left leg of 10k pot
pin 2: connected to pin 6 with no series resistor
pin 3: varible input voltage, 0 to 5v
pin 4: to middle leg of above 10k pot and connected to GND
pin 5: to right leg of above 10k pot
pin 6: connected to pin 2 with no series resistor, and then this voltage is
dropped across 2 resistors in series to produce the output voltage ratio


Im at the stage in the circuit, that when you increase the input voltage
from o to 5v, the output is alway 0.5v higher than the input which is
correct. At this point i drop the output voltage over a potential divider
and i get the correct output voltage ratio over this. The problem starts
when a 1k load resistor is taken from the potential divider. At this point
the voltage drops. When the load resistor is removed the circuit produces
the correct output voltage again.

I think the problem is the circuit cant provide enough corrent, but i cant
see how to change this.

Any ideas?

Or does anyone have any better circuit ideas?
 
What is your power supply?

And why the choice of the CA3140E?
 
On Fri, 20 Aug 2004 09:36:33 -0400, "Michael A. Covington"
<look@ai.uga.edu.for.address> wrote:

And why the choice of the CA3140E?
Click to expand...
Harris/Intersil military temperature range version of the 741, perhaps. But it
may also just be that it's a part in hand. Also, note he's using the "offset
null" to set the offset to 0.5V with that 10k pot!

Jon
 
On Fri, 20 Aug 2004 13:16:41 +0100, "Tuurbo46" <alex@beale55.fsnet.co.uk> wrote:

Im at the stage in the circuit, that when you increase the input voltage
from o to 5v, the output is alway 0.5v higher than the input which is
correct. At this point i drop the output voltage over a potential divider
and i get the correct output voltage ratio over this. The problem starts
when a 1k load resistor is taken from the potential divider. At this point
the voltage drops. When the load resistor is removed the circuit produces
the correct output voltage again.

I think the problem is the circuit cant provide enough corrent, but i cant
see how to change this.
Click to expand...
Well, you are using a divider on the output, after all. Adding a load resistor
changes the divider, so you get a different output.

You could, if you want, still bother cranking that poor opamp's offset voltage
to 0.5V with that 10k pot you are using and also then add another opamp at that
divider output, I suppose. But there is a more traditional way, I think, and it
doesn't depend on you using a pot nor the use of offset nulling to push your
offset way the heck to one side.


30k 27k 90k
0V to 5V input >-----/\/\/----+----/\/\/----, ,-----/\/\/-----,
| | | |
| | | +5 |
\ | | |\| |
/ 45k | '-----|-\ |
+5V \ | | >------+--> OUT
--- | +----------|+/
| | | |/| LT1783
| --- | |
\ gnd | ---
/ 30k | gnd
\ |
| |
| 27k |
+----/\/\/------------------+
| |
| |
\ \
/ 45k / 45k
\ \
| |
| |
--- ---
gnd gnd


I'm assuming a 5V power supply, here, and you can use a variety of rail-to-rail
in/out opamps if that's the case. But you don't say what's on pin 7, so hard to
know for sure.

Jon
 
On Fri, 20 Aug 2004 16:06:57 GMT, Jonathan Kirwan <jkirwan@easystreet.com>
wrote:

I made an error in copying down the resistor values, forgetting to change some
of them. I also forgot to add a resistor! The schematic *should be*:


30k 27k 90k
0V to 5V input >-----/\/\/----+----/\/\/----, ,-----/\/\/-----,
| | | |
| | | +5 |
\ | | |\| |
/ 45k | +-----|-\ |
+5V \ | | | >------+--> OUT
--- | | | ,--|+/
| | | \ | |/| LT1783
| --- | 45k/ | |
\ gnd | \ | ---
/ 180k | | | gnd
\ | | |
| | --- |
| 27k | gnd |
+----/\/\/------------------+ |
| +-------'
| |
\ \
/ 20k / 45k
\ \
| |
| |
--- ---
gnd gnd


Oh, well.

Jon
 
Thanks for your help guys!


"Tuurbo46" <alex@beale55.fsnet.co.uk> wrote in message
news:cg4q37$bqa$1@newsg4.svr.pol.co.uk...
Hi
I need a bit of help on a circuit im trying to build. I will try to
explain my problem below.

Circuit requirements:

*input to op-amp = 0 to 5v
*output from op-amp = 0.5v to 3.5v


Im am currently using a CA3140E opamp with a 0.5v offset in it, which is
quoted in the data sheet.

Pin wirring on chip:

Pin 1: to left leg of 10k pot
pin 2: connected to pin 6 with no series resistor
pin 3: varible input voltage, 0 to 5v
pin 4: to middle leg of above 10k pot and connected to GND
pin 5: to right leg of above 10k pot
pin 6: connected to pin 2 with no series resistor, and then this voltage
is dropped across 2 resistors in series to produce the output voltage
ratio


Im at the stage in the circuit, that when you increase the input voltage
from o to 5v, the output is alway 0.5v higher than the input which is
correct. At this point i drop the output voltage over a potential divider
and i get the correct output voltage ratio over this. The problem starts
when a 1k load resistor is taken from the potential divider. At this point
the voltage drops. When the load resistor is removed the circuit produces
the correct output voltage again.

I think the problem is the circuit cant provide enough corrent, but i cant
see how to change this.

Any ideas?

Or does anyone have any better circuit ideas?
Click to expand...
 
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