Isolated variable resistor function?

On a sunny day (Sat, 25 Aug 2012 13:22:33 -0700) it happened Joerg wrote in :

to gate.

Depends, mine were driving 40 W 220V lightbulbs. (MOSFET in bridge).

A MSP430 clocked at 256MHz? Shazam! That sounds like a Suzuki Alto with a Hayabusa engine in there.

"Blazingly fast" and "10K" may not get along.

to gate.

Why is linearity an issue (feedback, ya' know)? Drive it with a good (5% isn't "good") multiplying DAC.

Well, of course not while at 10k :-)

to gate.

The signal going into this divider can't be distorted more than 5% and with a FET-optocoupler it would be.

Sure, but then I might as well just use my solution "b", a digital potmeter. Reduces that function to one part costing 30-40c, outside the optocoupler to xfer the data lines which I'd need either way.

Hehe. I was just needing a faster PWM (wanted to use a very cheap RC, 20db/decade rolloff, low-pass, but needed an audio spectrum. Couldn't get there with 1/62500 PWM period of standard MSP430. So I looked and... cripes!! They had something else!

"Timer D also includes a dual capture mode reducing loading of capture operation by half. In addition, the combining compare blocks help to control both rising and falling edges of the PWMoutput signal. The Timer-Event-Control-block offers external triggering options as well as internal synchronization of timer instances."

See the MSP430F51x2 parts.

Jon

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Please define "blazingly fast"? ...Jim Thompson

In this case several MHz. In the world of switch mode power supplies that counts as "blazingly fast" :-)

Quote, page 511, Time_D Operation: "The timer input clock frequency can be within the range of 8 MHz to 16 MHz if 16x is selected (TDHMx = 00) or 8 MHz to 25 MHz if 8x (TDHMx = 01) is selected".

That would be an order of magnitude less than 256MHz. Or am I missing something here?

Sure they do. For example, this one can be had for around 30c:

So changing the value can be slow, but the "resistor" must be a good resistor to several MHz? ...Jim Thompson

Yup. Pretty much like the gain control in an IF amplifier. It won't have to react faster than the modulation of the signals after a narrowband filter, typically a few kHz. But the amp has to work at several MHz.

I might just do the digital potmeter thing. Not pretty but works and is cheap.

Blazingly? Every switching supply I've designed in the last nine months has had a Fsw > 2MHz. ;-)

network to gate.

MDAC-opamp-FET

I had problems with power dissipation of DPots. There should be a way of eliminating the MDAC, too.

with

Yes, probably you are missing something. They include a x16 multiplier. But the PWM clocking system can't handle anything faster than 256MHz. It dies after. But the CPU can run at

I think, anyway.

Jon

But not da big ones.