More help with OpAmps.

[jgalak]

This time, it's a practical circuit I'm building.  I have a source voltage that varies 0-5V (right now I'm just manually setting the voltage, eventually it'll be fed a sine waves).  I need to resale that to be 0-2.5V.  Since the source isn't very stiff (it's an R2R ladder), and since the IC I have has two OpAmps in it, I decided to first use a buffer, for its hight input impedance, followed by an inverting amplifier with G=1/2.

I am using an LMC6482 single supply rail-to-rail opamp, fed 5V and ground at its V+ and V- pins.

Circuit attached. 

The buffer is working fine.  Voltage at point B is almost exactly the same as at point A (it's.7V when Vin is 0, but I can probably live with that).

However, the second stage doesn't seem to work.  In my first attempt, I grounded the non-inverting input.  That resulted in Vout staying around zero, regardless of Vin.  Having done some reading, I tried binding the non-inverting input with a voltage divider.  Now Vout sits around 2.5V, regardless of Vin.

Am I doing something obviously wrong?  OpAmps are fairly new to me, so I can't tell if it's a design problem or if I just effed up the breadboarding.

[jgalak]

Never mind.  Breadboarding error.   

I can't seem to delete the thread.  Could an admin please do so?

[grouchobyte]

Get yourself a spice simulator like LTSPICE. As you've probably already discovered, or soon to discover if you get more into this analog stuff, it can and many times saves you the
However, don't get too addicted to it. Its alot like porn in that its not like the real thing...

@grouchobyte

[jgalak]

LT Spice I'm having a lot of issues with.  Need to dig deeper into it.  But circuitlab.com was quite nice for this - showed me that the design was correct, and motivated me to start re-tracing the breadboarding for the N-th time. 

Now if only it was as easy to do DC sweeps in real life as it is in the simulator...

[danadak]

You can do a simple DC sweep easily with a PSOC, 8 bits (or PWM with
filter to 16+ bits) and measure any deviation from it with a PSOC,
to +/- .1% accuracy. The sweep ramp amplitude 4V, if more is needed
an external OpAmp could easily enhance.

See attached.

You can control ramp amplitude and frequency solely in code or by sensing
external pot by adding mux to A/D to respond to controls.

UART transmits error to windows or Linux terminal session, or to file. Or
with some coding over USB. Or datalog to a SDCARD.

Or an Arduino and PWM approach. To << accuracy.

Basically a one chip approach.



Regards, Dana.