[time-nuts] Input filter for data logger

[Bruce Griffiths]

Hoi Attila

That's a fairly standard JFET BJT negative feedback amp that's not usually unstable.

The unity gain version has been employed as the input stage of various high impedance oscilloscope preamps.

Bruce

> 
>     On 20 November 2017 at 11:21 Attila Kinali <attila at kinali.ch> wrote:
> 
>     On Sun, 19 Nov 2017 16:10:54 -0500
>     Vlad <time at patoka.org> wrote:
> 
>         > > 
> >         Here is my schematic:
> > 
> >         http://www.patoka.ca/OCXO/LOGGER/IMG_20171119_155907272.jpg
> > 
> >     > 
>     Ok.. I am surprised, this doesn't oscillate.
> 
>     You have a two stage amplifier, where the second stage
>     has a negative feedback path into the first stage.
> 
>     When a pulse comes in, the jfet will turn on and conduct
>     current through its drain and source resistors. When the
>     current reaches something around 6-8mA the pnp will start
>     conducting. But the collector current of the pnp goes into
>     the source resistor of the jfet. This will increase the
>     voltage on the source, thus decreasing the gate-source
>     voltage, thus turn the jfet off, which in turn will turn
>     the pnp off, which in then will stop conducting, thus
>     no current into the source resistor, thus the jfet will
>     start conducting again... I guess you get it.
> 
>         > > 
> >         I did some simple tests for this. In it seems it was OK up to 10Mhz.
> > 
> >             > > > 
> > >             But guessing from what you showed, I would say that your amplifier
> > >             circuit isn't stable and has some gain peaking at around 10MHz.
> > >             There are two ways to proceed: Either optimize your circuit or
> > >             simplify it using modern components to the input signal you expect.
> > > 
> > >         > > 
> >         The main purpose for this circuit is to protect the MCU input and make
> >         some sine to square conversion.
> > 
> >     > 
>     Use a biased 74AC04. That's the easiest. And you will have very
>     little noise degradation.
> 
>     I would think that the MCU can probably take more abuse than the
>     74AC. Modern ASICs have quite a bit of protection circuits on
>     their inputs. I am not sure whether the 74-families have seen
>     upgrades on their protection circuits in the last 30-40 years.
> 
>     Attila Kinali
> 
>     --
>     You know, the very powerful and the very stupid have one thing in common.
>     They don't alters their views to fit the facts, they alter the facts to
>     fit the views, which can be uncomfortable if you happen to be one of the
>     facts that needs altering. -- The Doctor
> 
>     _______________________________________________
>     time-nuts mailing list -- time-nuts at febo.com
>     To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>     and follow the instructions there.
>