PSN-L Email List Message

Subject: Re: line driver question
From: John Popelish jpopelish@........
Date: Wed, 15 Mar 2006 00:26:18 -0500

Barry Lotz 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. The simple way is to put a non-inverting follower
 > at the circuit output.

The simple way to accomplish what?

 > 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.

It improves the opamps stability in the presence of a capacitive load, 
like long wiring has.  I seldom see this between opamps on the same board.

 > Should I use this with the unity gain follower?

It probably can't hurt.  But I don't understand the purpose of the 
follower, to begin with.

 > 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.

The input resistor in the + input leg is necessary, only if the bias 
current error of the opamp is significant to you.  Some opamps have 
such low bias current that the drop across the 100k feedback resistor 
is less than the offset voltage of the input.

 > I'm not sure that this will acomplish anything.

You may be solving a non problem.  Will this follower drive a long wire?

 >   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 agree that this is not the way to go, if it can be avoided.

 > I think the voltage divider will affect the current flow
 > to my feedback coil.

It will not improve the accuracy of the follower.  But, if this opamp 
drives a feedback current, how 'bout configuring it as a voltage 
controlled current source, so that wiring resistance will have little 
effect on the accuracy?


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