voltage to freq converter

[john jardine]

"PDRUNEN" <pdrunen@aol.com> wrote in message
news:20041008054627.16217.00000411@mb-m01.aol.com...

Hi All,

I want to take an analog voltage, 0-5 volts and convert over to a PWM
signal
with fixed period. The on time of the PWM would be based on the input
voltage.

This PWM signal will travel over a opto device for isolation and then be
converted back to a analog voltage.

I have been reviewing the net and see Analog Devices has a really good
handle
on this type of thing, but they are out of the budget for my application.

Any one have a good "cost effective" solution to such a requirement?

For "cost effective" or not, I'd be inclined to simply go
Voltage-to-frequency. The V to F is basically a cheap opamp cap and
comparator. Intrinsic linearity and stability can be high.
The signal is direct digital so no need then have to consider linearity or
stability of any other components or circuitry down the line. Resolution and
noise rejection is simply down to how long you measure the frequency for.
Response time can be fast. A digital frequency signal will directly input
into a micro etc.
regards
john

 

[Nicholas O. Lindan]

"when stupidity will serve as an explanation, you need look no further".

Variation:

"Never ascribe to malice what can best be explained by stupidity."

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer: Electronics; Informatics; Photonics.
Remove spaces etc. to reply: n o lindan at net com dot com
psst.. want to buy an f-stop timer? nolindan.com/da/fstop/

 

[Joerg]

Hi Nicholas,

Variation:

"Never ascribe to malice what can best be explained by stupidity."


Reminds me of the guys who stole a brand new truck from a dealers lot.

Only problem: They both hopped in and forgot the car they came in.
Realizing that they want back to the dealership to retrieve their old
car. Bad move....

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hi Ken,

Let the chip always run on the internal oscillator. Use software to work
out the average period, in terms of machine cycles, of the mains.
Calibrate the fractional divider from the machine cycles based on that.
If the temperature doesn't change too much during the outage, this would
hold the clock quite well.


That could work. But in most households the furnace will quit and then

the temps rapidly change. Same for the AC in summer and that's where
most power failures used to occur. Not in our house because we went to a
wood stove and hardly ever use the AC. Still, since we live in the hills
east of Sacramento the temp change from day to night is pretty dramatic,
30 degrees difference are pretty normal.

I'd probably swallow my pride and spring for a 32KHz clock crystal and
an MSP430. These crystals are the cheapest in their class. Now the VCR
industry still needs to figure that out as well so we don't have the old
12:00 blink-blink after every outage.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hi Spehro,

If it's actually PWM (which makes a lot more sense than V->F), then
the *exact* period doesn't matter (provided there isn't much short
term change or jitter)- it can vary a few percent with temperature and
unit-to-unit. It still gets turned back into the same analog voltage
on the other side of the barrier. So the internal RC clock, for
example on the chip I mentioned, will work just fine.


Certainly. Ken was just pointing out that a 60Hz link would save parts

and would also allow for a realtime clock.

For this PWM business I'd probably use discrete parts and cheap logic.
Just a good "handmade" astable where the duty cycle changes with input
voltage. If it has temp issues this could be compensated for on the
receiving end if that side has a uC, by building the circuit twice and
alternating signals. I'd probably start with the CD4000 series. In some
of my designs these have beaten even a plain vanilla uC in cost. Except
maybe a Chinese four-bit version.

Regards, Joerg

http://www.analogconsultants.com