Ok Russel,  I like opamp solution. I have found in LM358 AppNote a VCO
example with few low cost components. The only problem is a bigger current
consumption than 4046 VCO, if I'm not wrong (my logger must operate for 4-6
months with 3 AA cells), so I will try to build and compare the two types of
circuits.
Many thanks for support

Marco



----- Original Message -----
From: "Russell McMahon" <apptech@CLEAR.NET.NZ>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: 21 febbraio, 2002 23.03
Subject: Re: A cheap A/D interface


> Look at Scott Datallo's "Sigma Delta Converter".
> Can't get cheaper than that.
> RA4 would not be the best pin for that though.
>
> 4046 may be marginal on linearity to achieve 8 bits without quite a lot of
> work.
> Still be cheapish though.
>
> You can do a fair dual slope VCO with an opamp (even a cheap LM324) plus
> perhaps 1 transistor
> NatSemi do an app note for this in the LM324 data sheet AFAIR.
>
>
>       Russell McMahon
>
>
> > I have to interface my 16F84 Datalogger ( 4.5V supply) with sensors that
> > have a voltage output, but in my board I have only one free pin (RA4),
so
> > I'm thinking to a voltage-frequency converter. I need 8 bit resolution.
> > For a lot of reasons, I would like a really cheap solution, preferably
not
> > over 2-3$.
> > I have seen that a 4046 CMOS has a voltage-controlled oscillator, but I
> > haven't never used it: can it be an acceptable solution for my specs or
> I'm
> > going in a wrong way? Have anyone any suggestion about 4046 VCO use or
> other
> > low cost alternatives?
>
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