0 TO 33V D/A CONVERTER.

[rudy]

Hi everybody.

Can anyone help me to develop a "simple" d/a converter to simple change the tunning voltage from a analogic tuner?

In my application I just want to change the voltage in the varicap and it draws a very little current sink.

Some idea based on LM358 or something like that?

Regards.

Manoel

[Rocket]

Hi Rudy.
You could use an A to D converter, or use the 10bit PWM output which needs to be low-pass filtered to generate an analogue voltage. That would typically be 0 -> 5V. This voltage would then need to be amplified somehow. A high voltage op-amp like the OPA445AP can be used, or a normal low voltage op-amp configured as a voltage to current converter could be used, where the output current is drawn thru a pull-up resistor tied to the +33V rail. The resultant voltage would be a function of the resistor value and the current drawn thru it. If an A to D converter is used, care must be taken to ensure that the resolution is adequate.
Kind regards
Rudie
PS. J.I.L.
_________________
J.I.L.

[rudy]

Thanks Rocket,

With this explanation, it seems to me that will be more easy than I thought.

I will try something on PROTEUS first, because I am not very expert in OP amps, and I don’t like to deal with analog very much, but sometimes you don’t have any other choice.

Regards.

Manoel.

[asmboy]

How R U going to GENERATE the 33v max ( I'd suggest closer to 36 max - but thats just me).
You probably want a capacitor charge pump or very low current boost inductive converter. If you have a ready source of say 36v or so - the driver can be pretty simple- ( MAYBE - but even that depends ).

Do U have ANY previous experience building varicap tuned oscillators ?

you DO know:

1) that the capacitance /to/ tuning voltage curve is pretty nonlinear, right ?
( and when applied to a resonant circuit oscillator - more yet)

2- you are gonna have a VERY tough time predicting the exact voltages
you want to generate if you have precision and stability in mind.

I've done PIC controlled Varicap VHF VCO'S before - but always where the pic was SPI programming a PLL loop controller IC - NOT trying to direct map say a 10 pit PWM value to an exact frequency tuned by varactor. I would not even dare to go there.

You R 1 brave hombre!!!

[rudy]

asmboy

Ok, I understood, but this is not the case, of course if the tuner was controlled by I2C this task was much more simple. In this case I just want to change the original circuit made with a LM358 that’s generates the tuning voltage according to the channel selected. +33 Volts is applied to one pin, some kind of PWM is applied to another pin and all work fine. Yes, maybe the better way to me that doesn’t understand well OP is to "COPY" the original circuit, its relatively simple one, and attach it to one pin from my new processor.

Based on the previous approaching from rocket, I was seeing the simplest RC low pass filter to work in PWM, until this point no problem at all, but is clearly to me that I have to work out some OP AMP to do the voltage shift properly, let’s say, for 0% PWM I have 33 volts and for 100% PWM I have 0 Volts. This is the critical stuff to me.
In fact, I need a tip to build this OP amp circuit.

Regards;

Manoel.