PWM to 0-10V DAC not working. Am I missing the basics?

Projects General Guidance
1

First of all, this is my first post (though I have been lurking for about a year). Great site..wonderful community!

And my first project... I have an Mega 2650 and I put together the DAC from Rugged Circuits (Circuit #20 10V Analog Output) using the TLV2371. Using a modified sketch of AnalogWriteMega example found in Arduino examples folder, I plugged in an LED to PWM pin 12 (program below) and it works fine.

  1. The LED goes back and forth between the two levels, just as it should.

  2. When I hook up my circuit, I read no voltage across pin 6 and ground)

  3. I measure 12VDC across pin 7 and ground.

  4. Arduino ground was tied to ground of board and ground of power supply.

BUT...

  1. I DID use electrolytic capacitors (pretty sure polarity was correct).

  2. I am still running off from USB power

  3. I have rebuilt the circuit three times, hopefully not repeating the same mistake.

Thanks in advance for any help.

//
/** Program **/
//
Mega analogWrite() test

This sketch was written for the Arduino Mega, and will not work on previous boards.

created 8 Feb 2009
by Tom Igoe

This example code is in the public domain.

*/

void setup() {
pinMode(12, OUTPUT);
}

void loop() {
analogWrite(12, 200);
delay(20);
analogWrite(12, 200);
delay(12000);
analogWrite(12, 50);
delay(12000);
}

2

If your voltmeter has high enough impedance, check the voltage on pin 3. That should be half of the output (pin 6). If the voltage on pin 3 is zero, then the output you are getting is accurate. I use an o-scope (1Mohm input impedance).

3

Thanks for the suggestion. (And sorry for the delay...I was away for the weekend.) I'll test it when I get home. Just so that I have a clue (someday), what condition am I testing? (That the opamp is OK?)

4

You are checking which side of pin 3 the problem is on.
If the voltage at pin 3 is zero, then something is wrong with the PWM/low pass filter circuit.
If it is higher than zero and the op-amp output is zero, then it is something in the op-amp circuit.

5

Why are you doing this:-
analogWrite(12, 200);
delay(20);
analogWrite(12, 200);
delay(12000);

The first two statements are totally redundant.

6

SurferTim,

Thanks for the explanation. I will do those tests this evening. I have a couple of references for opamps that I should reread now that I have a problem in front of me.

GrumpyMike,

I realize the redundancy. I wanted to see some variance in the intensity--if for no other reason, just to know that I was varying the correct parameter. (I should have made that clear in the code.)

I'll go measure, read a bit more a report back. Thanks both.