A Few Simple Questions

[Fraser, Colin J]

> -----Original Message-----
> From: inman at interpath.com [mailto:inman at interpath.com]
> Sent: 09 March 1999 15:48
> To: synth-diy at mailhost.bpa.nl
> Subject: A Few Simple Questions
> 
> 
> Would someone please define these terms / answer these questions?
> 
> Rail to Rail
> Does that mean possible output voltage goes from ground to V+?

If you have a single rail supply it does. 
Usually, op-amp based circuits have a dual rail supply - ie, there are two
power supply outputs at a positive and a negative voltage, usually +/- 12 or
15 volts. There will be a ground (0 volt) line as well as a reference.
Most op-amps will not manage to drive their output voltage all the way to
the voltage at either the positive or negative supply.
Some don't even like their inputs driven all the way to the supply rails.

> What does it mean to say an AD/DA converter is 2 or 4 or 8 channels?
> Are these inputs summed in the output or sent in parallel?

Multi-channel converters will either have more than one converter on the
chip, or they will have a built in multiplexer or demultiplexer - that's
just a multiway switch that can select different inputs or outputs for the
ADC/DAC to connect to. A multichannel DAC with a demultiplexer may also need
a circuit to store the output voltage when the DAC is not switched to that
output channel. 

> Is there a simple 8-bit DAC with parallel in?  Is there an 8-bit DAC
> for dummies?  Colin said he had had great success making a DAC with a
> chip.  Colin -- which one?

There are several - I have used ZN558 and ZN428.
Given that there are inexpensive 12 bit DACs about, you may be just as well
to use one of these. Burr Brown make a nice 12 bit DAC that has an 8 bit
parallel input and a register select line, so you write the lower 8 bits,
then the upper 4 bits. There are several others that work like this.
With a look up table in ROM, you can use one of these to give a linear
(Hz/v) output.
Many of the 12 and 16 bit converters have serial inputs, but there is plenty
of documentation on the web showing how to use these.
Some DACs are designed more for outputing digital audio than contant
voltages tho'.

> Any comments on frequency to voltage converters?  Not for tracking a
> musical note, but used more as a DAC?  They clearly have some error
> and tracking times are a concern?  Were these the DAC of the past?

Never used/built one...

Colin f