impedance problem? in op amp circuit

Discussion in 'General Electronics Chat' started by minkey01, Oct 10, 2016.

Jul 23, 2014
185
0
Ok, I’m back. Hehe. After studying op amps and learning LTSpice, I came up with this cv input to micro-controller circuit. It uses a rail-to-rail op amp (inverting setup) with a single 3.3V supply and a 1.65V virtual ground.

It has a jack for cv input and two pots. One pot is the offset and the other is for attenuation of the input.

I simulated and studied 4 scenarios and they seem to work :

1) For manual control :
nothing plugged into the input
attenuation pot turned all the way up in the schematic
offset pot=0 to 3.3V
output=0 to 3.3V

2) For -10V to 10V input :
offset pot all the way up in the schematic (3.3V)
attenuation pot turned to .165 ratio. This scales the input down to -1.65V to 1.65V
output=0 to 3.3V

3) For 0 to 10V input :
turn the offset pot halfway (1.65V)
turn the attenuation pot to .33 ratio. This scales the input down to 0 to 3.3V
output=0 to 3.3V

4) For 0 to 5V input :
turn the offset pot halfway (1.65V)
turn the attenuation pot to .66 ratio. This scales the input down to 0 to 3.3V
output=0 to 3.3V

* Here is the issue. I faked the potentiometers in LTSpice with sinusoidal volt sources, thinking that it would represent the full motion of the pots. In real life will these changing impedances at the pots mess with my op amp calculations? I'm thinking they will. Any ideas? Thanks for the help!

2. ronv AAC Fanatic!

Nov 12, 2008
3,657
2,800
You could use 2 resistors or you can download a pot from yahoo groups.
If you can't find it there someone will have it here. If not I can do it tomorrow.

3. Alec_t AAC Fanatic!

Sep 17, 2013
7,026
1,454
Here you go. A choice of linear/log/voltage-controlled pots:

• potentiometers.zip
File size:
5.1 KB
Views:
5
ronv likes this.

Jul 23, 2014
185
0
cool. thanks!

do you know how to sweep the pot range and have it show up in the graphical simulation when you hit the run icon?

5. Alec_t AAC Fanatic!

Sep 17, 2013
7,026
1,454
Use a Pulse voltage source to define a ramp voltage (0v-1V) for controlling the voltage-controlled pot. Plot the control voltage.
For the normal pot you can set the wiper as a parameter and step the parameter value from 0-1.