Voltage -/+3 in +/-10 out

[ericgibbs]

hi paul,

That makes it clear.

If you are limited to a +/-10V supply rail, a 'rail to rail' opa is a OPA350,
datasheet at www.datasheetarchive.com

Its claimed to swing within +/-10mVolts of the rail voltage.

EDIT: this device is specified to a 5.5V single supply, its NOT suitable for Paul.

Gramo's circuit should get the job done.

Eric

EDIT: Hi Paul, a 741 is an 8 pin DIL

 

[paul200]

I am not limited to a supply of +/- 10 volts, I have orderd a +/- 15 volt power supply.

was gramo's ic an 8 pin?

 

[gramo]

LM741CN can be found at www.futurlec.com for $0.33
**broken link removed**

 

[Nigel Goodwin]

LM741CN can be found at www.futurlec.com for $0.33

But why pay that much for an antique, low spec, low performance, device?, when you can get much higher spec, better performance, opamps for less money?. The TL07x or TL08x series are cheap and just so much better than the crappy old 741- the 741 was a great device back in it's day - but it's day was just so long ago (probably before most members on this forum were even born!).

 

[ericgibbs]

I agree, spend your money on a higher performance opa.
The pin outs will most likely be the same.

Eric

 

[paul200]

I have 741 here thats all, I just need to get this rig tested so I can progress with the project.

Thanks

 

[paul200]

If the output voltage is not dead on zero when my sensor is not moving.
ie input voltage should be zero when not moving.

How can I balance the output to zero volts if is is actually +.2 volts

 

[Nigel Goodwin]

If the output voltage is not dead on zero when my sensor is not moving.
ie input voltage should be zero when not moving.

How can I balance the output to zero volts if is is actually +.2 volts

Add a zero offset control, the 741 even has pins for it, or is easily added seperately.

 

[paul200]

how do I add a zero offset?

 

[Nigel Goodwin]

how do I add a zero offset?

Look at pins 1 and 5 on the diagram Gramo posted above, and consult the datasheet to see how to use them.

 

[Roff]

Look at pins 1 and 5 on the diagram Gramo posted above, and consult the datasheet to see how to use them.

I don't think you want to use pins 1 and 5 to correct offset in another device. The offset circuitry in the 741 is there to correct op amp input offset voltage. Adding input offset to the op amp to correct for the offset of an external device will make the op amp's input offset temperature coefficient worse. Also, I doubt you could even get 0.2V of offset range.

You can add an offset adjustment to the single op amp noninverting circuit that Gramo posted, but the offset adjustment affects the gain unless you add another op amp to buffer the offset adjustment pot. I would probably just use two cascaded inverting stages, with the offset adjustment added to one of them.
Paul2000, is your offset 0.2V before the gain stage, or after it? How stable does your zero point have to be?

 

[paul200]

.2 volts before amp

 

[audioguru]

A 741 opamp has an input offset voltage of only 7.5mV max and its offset adjustment might adjust for 15mV. It certainly won't adjust for a 200mV error somewhere else.

Maybe the resistance of the sensor is very high and the input current of the opamp is creating a voltage drop? Then an opamp with FET inputs like a TL071 will fix it.

 

[Roff]

.2 volts before amp

And how stable does the zero point need to be?
What is the output resistance of your sensor?

 

[paul200]

I just can't get this amp to work. I am using a +/-15 volt supply which pin on the 741 connects to the common from the dual supply

 

[Nigel Goodwin]

I just can't get this amp to work. I am using a +/-15 volt supply which pin on the 741 connects to the common from the dual supply

None!, only to the +ve and -ve.

 

[paul200]

Thats what I done first, but did't work.
Will give it another go thanks

 

[audioguru]

One resistor and one wire of the sensor connect to the common of the dual supply.
The output is also referenced to the common of the dual supply.

 

[paul200]

on the op amp 741 i put 3 volts in and get 2.9 out. whats wrong?

 

[Roff]

on the op amp 741 i put 3 volts in and get 2.9 out. whats wrong?

Post a schematic. or better yet, answer the questions I asked, and I or someone else will draw one for you with offset adjustment included.