[sdiy] RE: [AH] Prophet 5 ADC output?

[phillip m gallo]

Tim,

Check the DAC's reference voltage.  Since all the de-mux chips are
producing the same voltage it's would seem unlikely that the chips are
the source of the problem.

I can't readily find a spec sheet on the part, so if the reference is
external check this reference perhaps it has a trim.  If the reference
is internal check for adjustment mechanism. 

regards,
p
   

-----Original Message-----
From: Tim Curtis [mailto:sexsymbol at execpc.com] 
Sent: Tuesday, January 27, 2004 4:31 PM
To: 'analogue'
Subject: [AH] Prophet 5 ADC output?


Hi Everyone -

I've been away for the last week and I just returned to get into this  
Rev. 3.3 Prophet.

I'm having a problem with the ADC adjustment.  The design uses a Burr 
Brown DAC71 with a LF356 on the output that's used to convert the 
current to Voltage.

The voltage at the test point (after the DEMUXes) is supposed to be 
rimmed to +10.00V, but the best that I can get is +7.5V or so. 

When I got this Prophet, the LF356 had been shorted out, and not having 
one handy at the moment, I've swapped in LF351s, AD744s, TL071s, and 
even a few others, but they all give me about the same voltage output.  
I simply can't get it to trim out.

Is there something about the LF356 that would make it the only op-amp 
that would work in this application?  (Of course I plan to replace it 
with one, but that takes time in transit.)  All of the op-amps I've 
tried give me exactly the same reading.

Thanks!

Tim