[Re: Re: [[DIY-PCB] Scanner Schematic/BOM (update)]]

[Harry Bissell]

Harry Bissell rites (or wrongs?):

The trim pots are supposed to be DC nulls. They are from the Nat Semi data
sheet Apps. Notes. I suppose that they work somewhat like the "bias current
comp." resistor that is found from the non-inverting input of an op-amp to
ground, when you are using an inverting configuration (most people just ground
the positive input, and screw the bias current...) Does this work? I dunna'
know captain....  The one side of the bias pot forms part of the input
attenuator. The other side, the bias. I have some doubts about the circuit,
because at the point you reach DC null, you may have queered the attenuation
ratio.

Oh yeah (Duh) it WILL work, because we are using the input bias diodes. They
are tied up with a 15K resistor to +15, and the 500 ohms of the pot from each
side will send some of that DC bias to ground. But I'd sure like to see
someone actually breadboard at least two stages to know for sure...
Harry (ya learns some'tin new every day) Bissell

WeAreAs1 at aol.com wrote:
Hello Harry,

Thank you for sending the schematic drawing to me.  I was able to extract the 
text portion of the .mime file that first came my way, so I have both parts 
now.

In your new schematic, you have some trimpots at the inputs of each VCA 
stage.  They appear to be configured as gain trims.  Is this correct?  I'm 
not sure that gain trims will be necessary on each stage, but I'm almost 
certain that DC offset null trims will be necessary, especially for anyone 
who wants to scan through the stages at a high rate of speed (in the audio 
range).  Any DC at the VCA outputs would result in unpleasantness, I think.  
Every OTA I've ever used has had some amount of DC in the output, and 
benefitted from trimming.

Am I mistaken about the trims on your schematic?  Do they actually work as DC 
nulls?  The typical DC nulling trim is configured as a pot with its ends 
connected to the plus and minus supplies, and its wiper connecting, through a 
resistor, to one of the OTA inputs.  Your trimmers don't seem to connect to 
any DC source - please explain.  I apologize if I'm interpreting the drawing 
incorrectly.

Best regards,
Michael Bacich


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