PSN-L Email List Message

Subject: Re: line driver question
From: Brett Nordgren Brett3kg@.............
Date: Wed, 15 Mar 2006 10:17:33 -0500

Hi Barry,

At 08:59 PM 3/14/2006 -0800, you wrote:
>Hi All
>  I was planning to put a line driver on the position voltage output of my 
> vbb style sensor. Currently I run it directly about 40' to my house but I 
> think it affects the triple feedback circuit.

That is possible, though not certain.  I think that the position ouput 
comes off from the capacitor of the integrator branch, putting the cable 
capacitance in parallel with the relatively large integrator 
capacitor.  Adding a few (hundred) pf of cable capacitance to that, 
shouldn't affect things much, unless the loop is already close to 
oscillating.  Small amounts of DC introduced back into the cable or 
resistive loading would be a more serious problem.

>The simple way is to put a non-inverting follower at the circuit output. 
>The question is (lets see if I can explain this) the non-inverting opamp 
>on the velocity output of the sensor has a small resistor between the 
>output of the opamp (~10 ohms) and the line going back to the negative 
>opamp input and a small (pf) capacitor between the opamp output and the 
>negative input . I think this is for damping high frequency oscillations.

The RC was possibly a trial and error solution to kill a high frequency 
oscillation.  A good analysis of why the loop was oscillating in the first 
place might suggest some better solution for that issue.

>Should I use this with the unity gain follower? I'm not sure if its 
>necessary or how to do this. The only way I can think of is to put a high 
>resistor (~100k) before the positive opamp input and a 100k in the line 
>going back to the negative input and then put the small cap and resistor 
>as noted above. I'm not sure that this will acomplish anything.

A unity gain follower couldn't hurt and would isolate the signal path from 
AC and DC leakage feeding back into it from the cable and load 
circuits.  Input signal from the integrator capacitor goes to the plus op 
amp input, negative input connects to its output which also drives the 
cable.  You might want to add 1k or so in series between the op amp output 
and cable input end to reduce the danger to the op amp from accidental 
shorts or voltage transients fed back through the cable.  At the low 
frequencies involved, the added resistance shouldn't have much of an effect.

>I didn't want to have a voltage divider before the non inverting opamp to 
>reduce the voltage in half and then have a 2X noninverting opamp line 
>driver. I think the voltage divider will affect the current flow to my 
>feedback coil.

It would help if I could see the circuit.  Is there an image file 
available?  The circuit in my head may be far from what you have.


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