[GPS_Standard] a parts setback

[Dave Platt]

Chris Howard wrote:
> I really appreciate the Digi-Key BOM.  My head starts to swim
> when I get into the Digi-Key website.
> 
> One complaint:
> 
> The LTC1485 on the sample BOM turns out to be a surface mount part.
> I'll either have to make some kind of adapter under my microscope
> or go back to Digi-Key for the 8-DIP part.  I should have looked closer
> at the list.

Yeah, that should be the LT1485CN8 (DIP) rather than CS8 (SOIC).

When I was procuring parts for mine, I decided to buy from Mouser
rather than Digi-Key since there were a few parts I wanted for
other purposes that Digi-Key doesn't carry.  Unfortunately, Mouser
doesn't carry Linear Technologies parts, so the LT1485 wasn't an
option for me.  The OPA2705 wasn't available, and I couldn't get the
suggested LMC6482 through Mouser.

What I ended up buying and using (successfully) was

	SN75LBC176APE4 for the bus transceiver, and
	OPA2340PA for the op amp.

These are both DIP-package parts by TI (or Burr-Brown).

I did run into one snag I hadn't expected, when testing out the
oscillator-to-controller linkup.  At first, I wasn't getting
any pulses into the counter circuitry, even though the oscillator
was running just fine.  This turned out to be due to the fact
that the oscillator I'm using (an Efratom) has a DC-coupled
output - it swings from just above ground, up to just below
the supply voltage.  The bus transceiver IC requires a couple
of hundred millivolts of negative differential voltage to change
state, and since one input is grounded this means that the oscillator
voltage must swing some distance below ground.  The fix for this
was easy - I just installed the oscillator-termination resistor
on the controller board, and coupled the oscillator to the controller
through a 0.1 uF capacitor rather than connecting it directly.

This fix wouldn't be required with oscillators that have AC-
coupled outputs.

As far as I can tell, the originally-specified LT1485 transceiver
IC would have the same "must swing negative" requirement as the
SN75LBC176 that I used.