# Convert a PWM to analogue with double the voltage range?

I have a PWM output (0-5v) that I need to convert to an analogue voltage. Simple = RC filter. However, the voltage range from that analogue voltage needs to be twice that of the input voltage, and I'm just not sure of the best approach.

Basically: 5v PWM to 10v analogue

A colleague suggested to use a voltage doubler (such as 7660) to power an op-amp, configured with a gain of 2.

Any other suggestions would be greatly appreciated.

Thanks

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What current do you need on the 0 - 10 volt output? And what other voltages do you have to work with? – Peter Bennett Feb 25 '14 at 16:48
I have easy access to 5v from a dedicated digital vreg (mcp1702). 6v is available too, but it's more of a pain to use. The 0-10v needs to drive 4-20mA for a proportional pressure control regulator (catalogue.camozzi.com/CATALOGUES/CCC-GENCAT/00049/PDF/…) – raaymaan Feb 26 '14 at 10:19

Filter the PWM with a suitable LPF. Use a DC-DC converter to get +12V or + 15V from the 5V rail. Use a gain of two single-supply amplifier powered from the 12V rail to double the voltage input. Done.

simulate this circuit – Schematic created using CircuitLab

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A voltage doubler won't be effective especially if the PWM duty cycle 30% or less - the 7660 needs at least 1.5V to power it and that is the first problem. The second problem would be finding an op-amp with perfect rail-to-rail outputs. Again a problem made worse at low voltages.

Apart from anything else the 7660 converts its input to a negative voltage and that is of little use here.

The only solution I can think of is to provide a 10.5V supply to a rail-to-rail op-amp with a gain of +2.

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Thanks Andy. Regarding the 7660, I have a few lying around and was going to give this circuit a try craigsarea.com/images/voltage_doubler.jpg – raaymaan Feb 25 '14 at 16:20
He's going to need at least a 10 V supply for the opamp, preferably a little more so that you don't bump into headroom issues. – Olin Lathrop Feb 25 '14 at 17:23
@olin oops my what an error. Thanks for the reminder Olin – Andy aka Feb 25 '14 at 17:44

Something close to yours friend solution, use small boost SMPS with MC34063 (cheap&good) to convert 5V to 12V and power OP-amp with the gain of 2, same effort way better results. For OP-amp you can use LM358 single supply rail to rail cheap and decent if you do need some higher precision (then OPA2180 is good with lower offset) Also you can put little bit integration cap in the feedback to additionally suppress PWM noise.

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A six-pin or eight-pin microcontroller that reads the PWM input cycle, and outputs to, for example, an I2C potentiometer or DAC, or uses a built-in DAC, would be a lot simpler than shown here.

You can even output a high-frequency PWM to a RC filter through a MOSFET if you want your own "ghetto" DAC.

The suggested circuit can be realized with an AtTiny85, and has an output bandwidth of about 160 Hz. The input precision is in the sub-microsecond range. All the software has to do is read the width of input pulses, and set the duty cycle of the output PWM (inverted.)

simulate this circuit – Schematic created using CircuitLab

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