northwest Adjustable Frontend Gain. Pros an Cons ? Digital Pot or Fixed R's ?
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australian electronic gold prospecting forum.com  |  DIY forum  |  DIY forum  |  Topic: Adjustable Frontend Gain. Pros an Cons ? Digital Pot or Fixed R's ? 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: Adjustable Frontend Gain. Pros an Cons ? Digital Pot or Fixed R's ?  (Read 205 times)
greylourie
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« on: Thursday July 23 2015 18:38:40 AEST PM »

The subject title basically asks the question. Which way could be easier for the experimenter ?
If for example, you had a two stage front-end (similar to a GS5 or SD), and decided to leave the first amp well alone..

You could either (dunno if I'm using the right words/terms), change the value of the input impedance to the next stage opamp input.
Or you could change the value of the feedback R on that opamp.

Would there be any dis/advantage to using a set of switchable fixed R's, as opposed to a digital pot ?

What say you, all ?
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greylourie
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« Reply #1 on: Friday July 24 2015 20:31:25 AEST PM »

Or we could leave our two stages well alone, and do stuff on the output feeding to demod/integrators. (I have been staring at a schematic for days - will try to chop it up in Paint to show. Then maybe members will be more willing to comment.)

Lets forget about digital pots. And look at the idea, with switched R's placed wherever...

Having switchable gain before any sampling stages - suddenly turns a dumb detector, into something more versatile. Because now the ability to adjust for use from one extreme to another is possible.
Or is it more like a case of a dog chasing its tail ? Let me go do try make a picture.
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greylourie
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« Reply #2 on: Friday July 24 2015 21:15:30 AEST PM »

I have seen the option in early QED schematics posted here, for adjustable preamp gain.
I also vaguely recall some mention made in posts, maybe on Geotech/GPOZ. I cannot remember all the details in the thread, but I do remember Aziz saying something which stuck in my head, for some odd reason.

Anyway, have made a simple image. Hopefully to encourage comment.

I chopped up a schematic I saw here :   
It is not part of a metal detector, but for the purposes of discussion think its a pretty good example of what could be done with a metal detector - at the front end.
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greylourie
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« Reply #3 on: Friday July 24 2015 21:52:01 AEST PM »

I think this is the concept Aziz mentioned somewhere. Hopefully I got him right.
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