SparkFun Forums

[czarate]

I recently purchased a LM 358 op amp hoping to amplify 0-1V Dc coming from a COM-10613 digital pot to 0-2V Dc. I can't figure out what I am doing wrong however at the maximum end with a 1V in, I am only getting .7V out. Any help anyone could provide would be greatly appreciated. Attached is a wiring schematic of what I hooked up.

http://img850.imageshack.us/img850/6029 ... 2237pm.jpg

To make you all aware of the overall purpose of the circuit. I am attempting to simulate the trigger on a cordless drill. I was able to use the digital pot to ramp the voltage allowing for a smooth actuation from off to running. However, the wiper arm on the pot killed half of my voltage not allowing me to reach maximum speed. So I was hoping to use an op amp to amplify the trigger output voltage back to 2V.

Thank you in advance.

[waltr]

Circuit looks ok as a non-inverting amp with a gain of 2x.
Some simple troubleshooting:
Is the Vdd 5V. Measure pin 8 to pin 4 to ensure 5V is being applied.
Measure the Input Voltage. Is it what you expect from the Digital Pot?
You could try a simple resistive pot instead of the digital pot just to test the op-amp circuit.

Do you have any de-coupling caps across the LM358's power pins? If not add a 100nF cap from pin 8 to pin 4 and as close as you can to the op-amp pins. Without this cap the op-amp could oscillate and give the wrong output voltage.

Check and try these and let us know what you found.

[Blackfin]

A few thoughts:

a) You might try a different op-amp, one that has a rail-to-rail rating.
b) Add some bypass caps to pin 8, close to the device.
b) You might also try changing the connections at pins 5 and 6 so that one is tied to 5V. With both tied to ground the op-amp may be oscillating like crazy.

Can you scope the output? Or are you seeing 0.7V from a DVM (if so it may be lying to you and showing an "average" reading resulting from oscillation...)

[Mee_n_Mac]

What is the op-amp driving ? What is connected to the "high end" of the pot, the Arduino's 5v ?