Hi all. Let me start off by saying that analog design is not my forte.
As some of you may recall, a few months ago I asked about an audio
signal switch. I've been drifting in and out of this project, but I
finally decided on using the MAX4570
(http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1934) dual 4x2
crosspoint switch. I'll run it off a charge pump inverter so I can use
it in its dual supply mode.

So I think I have an idea of how I'll control the unit (it's my first
time using SPI). I do have a few questions though:

1. Is it usual to use a pull up or pull down on the serial lines (CS,
SCLK and DIN)?
2. I don't fully understand the internal bias resistors that this unit
has. If I run off dual supplies, and use a 1uF cap on each input and
output line, do I need the bias set?
3a. Unfortunately Maxim hasn't included any "typical application"
circuits in the datasheet. They do have an application note than
includes this chip (well, the MAX4550 I2C version) as part of an audio
subsystem. I've put up a copy of the schematic here:
http://sites.google.com/a/designpartners.ca/dumpzone/Home/HFRD34A_Sch.pdf
- the relevant section appears on page 6 of the PDF. They're feeding
audio directly into the chip through a few discretes. There is a 1uF
cap in between the jack and the chip. There's also a parallel resistor
and capacitor leading to ground off each audio line. The cap has a
stated value of 10pF but the resistor has a value of DNI. Any idea of
what value the resistor should be or if the whole arrangement is even
needed?
3b. On the output side there is a slightly different arrangement. The
audio again passes through a 1uF cap (DC block, right?) then through
two 7.5K resistors in series. In the middle of the two resistors is a
1200pF cap to ground. It then goes to a line driver IC. I haven't been
able to figure out if that arrangement is for the switch or for the
driver. Any ideas?

Thanks for the help everyone! I know some of these questions are a bit
basic but I'm a little outside of my knowledge base here.

Josh
-- 
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
 -Douglas Adams
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist