Xmega128a1 PWM D->A questions.

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Freaks,

I am working on a design that uses the Maxim 5250 10 bit quad D to A converter. I am looking to reduce cost and board space. The analog voltages do not need to be highly accurate and a small amount of noise will not be a problem. ( I know, specific numbers would be helpful ) The voltages control DC motors that are part of a feedback loop. The standard d/a on the micro are already tied up with other functions.

They do not need to be quick and I will buffer the o/ps.
I am trying to get some idea of what to expect from a PWM output from the counter timers using a simple RC filter.

What can I expect the minimum o/p voltage to be given that the o/p ports never reach a true zero volts?

If I am using an analog regulator and the standard layout for decoupling caps on the micro, can anyone comment on how stable the o/p will be?

Any input is appreciated. (No pun intended)

Thank you,

A.

AVR Studio 4 Ver. 4.18 684
avr-gcc Ver. 4.3.0
ISIS 7
ELECTRA

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With a DC motor, you are far better off applying pure PWM and let the inductance and inertial of the motor "filter" (smooth or average) it.

You can still control it just fine if your feedback is a tacho or quadrature encoder or position sensor.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Jim,
I had not considered that. The DC motors that I am using have built in controllers and only require a 0 to 5vdc control voltage. This input controls the speed.

Thanks,

A.

AVR Studio 4 Ver. 4.18 684
avr-gcc Ver. 4.3.0
ISIS 7
ELECTRA

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OK

My tests show that for modest filter output ripple, the pole frequency of the low pass filter needs to be no higher than 1/10 the PWM repetition frequency. But, on the other side, that filter frequency controls the rise and fall time of the analog (filtered) output.

For more than you ever wanted to know about PWM filters, see:

https://sites.google.com/site/ka...

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Jim,

Very well written and comprehensive.

The intention is to do as you suggest and buffer the
o/p from the filter with an opamp. If the PWM signal is supplied directly from an xmega port, what can I expect the limitations to be with respect to voltage range?

Does it make sense to use an FET driver of some sort between the filter and the port pin?

Thank you for your assistance.

A.

AVR Studio 4 Ver. 4.18 684
avr-gcc Ver. 4.3.0
ISIS 7
ELECTRA