Nigel Goodwin

Yep! - but I was at work and didn't have time for drawing pictures!

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PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

Roff

Work! I'm retired, vegging out in front of my computer, slowly going broke.

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Ron

Nigel Goodwin

With the price of petrol now £103.9 per litre, you can go bust working over here!.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

jnnewton

Actually, no it wasn't. If vout- is -5 and vout+ is +5, the net is +10 and vice versa. It was right.

which I do which i don't know how to do.

The transistion is a problem, it causes the motor to jump, b/c it sees the 5V prior to the pwm. so what i can try is the 5V from an output pin.

Roff

That might work. If you are lowpass filtering the PWM prior to going to the op amp, you should put the same filter on +5V. If you are not filtering the PWM, you will need to pay attention to op amp bandwidth and slew rate.

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Ron

jnnewton

Yes, i am lp filtering the pwm, i do this in order to get the "analog" signal. For the sake of my education, why should i put the same filter on the +5?

Roff

Filters have delay. Unless you put the same delay in the +5 path, your motor will jump on power up.
Actually, a better solution might be as below. Hold both inputs at zero until you want the motor to start.
You can alternately use 5V without PWM on the other input, as you suggested. Filter it identically to the PWM.
If the motor still twitches, we should be able to come up with an enable. You would need to post a fairly complete schematic.

EDIT: Oops, this still has +/-10V outputs .

Attached Images

single-ended to diff sch1.PNG (17.1 KB, 7 views)

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Ron

jnnewton

Hi all,

I built the attached circuit, but can only get the outputs to swing from 4.9 V to -2.47 V (approximately) and need them to go from -5 to 5. This will give me the 0-10V i need. What do you think could be the issue. Also, by using the complement of the pwm output, the motor doesn't jump at startup. Also at the point where the differential signal is 0 V, the two outputs show 1.14V with respect to ground. Shouldn't this be zero with the circuit I have? Finally the -12 V bus generated with the switched capacitor is only giving 10.08 V when on, can anyone suggest a different setup to get a little more margin for the differential op amp?

Attached Images

PWM Circuit.gif (19.4 KB, 3 views)

Roff

The input voltage of the MCP602 can only handle (Vcc-1.2) on the high side. The output swings (almost) rail-to-rail, with no load. You have a very heavy load on both amplifiers, so the output swing will be reduced.
You don't need that high-speed diff amp. Why don't you do it like this?
TL074 is 4 op amps in one package. I show power supply pins on each amplifier, but this is just for simulation purposes.

Attached Images

PWM amp.PNG (31.7 KB, 1 views)

Attached Files

PWM amp.asc (3.9 KB, 0 views)

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Ron