PSN-L Email List Message

Subject: RE: line driver question
From: "Keith Payea" kpayea@...........
Date: Wed, 15 Mar 2006 21:12:12 -0800


Barry: 
 
The circuit you describe is the classic way to deal with a large capacitive
load on an op-amp.  The series resistor decouples the capacitance from the
op-amp output, and the local feedback capacitor keeps the whole thing
stable.  There are also op-amps which are designed to drive highly
capacitive loads like this.  It's not so critical if you don't need high
gain in addition to the line driving capability.
 
I use a part called the LM837 in my VBB output to drive the 70 or 80 feet of
cable back to the SDR system.  See the schematic at:
http://www.bryantlabs.net/Images/SenseAmp.pdf  You can see I have a small
resistor in series with the output also.
 
You are right to add a buffer between the internal "Analog Computer" and the
cable.  I agree the issue is not so much the added capacitance as the
potential to feed noise into the process back up the cable.  Opamps are
cheap...
 
Cheers,
 
    Keith

  _____  

From: psn-l-request@.............. [mailto:psn-l-request@............... On
Behalf Of Barry Lotz
Sent: Wednesday, March 15, 2006 8:46 PM
To: psn-l@..............
Subject: Re: line driver question


John,Brett,Richard&Geoffrey
  Thanks for the "feedback" :). All I am trying to do is monitor a voltage
in a feedback circuit from 40' from the circuit without affecting the
performance of the feedback circuit. The monitored voltage goes to a LCD
voltage display in my house.I think I will just try a follower with a
resistor at the output , to prevent transient voltages from entering the
feedback circuit from the shielded line.
regards
Barry

 





Barry: 
 
The circuit you describe is the classic way to = deal with a=20 large capacitive load on an op-amp.  The series resistor decouples = the=20 capacitance from the op-amp output, and the local feedback capacitor = keeps the=20 whole thing stable.  There are also op-amps which are designed to = drive=20 highly capacitive loads like this.  It's not so critical if you = don't need=20 high gain in addition to the line driving = capability.
 
I use a part called the LM837 in my VBB output = to drive the=20 70 or 80 feet of cable back to the SDR system.  See the schematic = at: http://www.bryantl= abs.net/Images/SenseAmp.pdf =20 You can see I have a small resistor in series with the output=20 also.
 
You are right to add a buffer between the = internal "Analog=20 Computer" and the cable.  I agree the issue is not so much the = added=20 capacitance as the potential to feed noise into the process back up the=20 cable.  Opamps are cheap...
 
Cheers,
 
    Keith


From: psn-l-request@................. [mailto:psn-l-request@............... On Behalf Of Barry=20 Lotz
Sent: Wednesday, March 15, 2006 8:46 PM
To:=20 psn-l@..............
Subject: Re: line driver=20 question

John,Brett,Richard&Geoffrey
  Thanks for the "feedback" :). All I am trying to do is = monitor a=20 voltage in a feedback circuit from 40' from the circuit = without=20 affecting the performance of the feedback circuit. The monitored voltage = goes to=20 a LCD voltage display in my house.I think I will just try a follower = with a=20 resistor at the output , to prevent transient voltages from = entering the=20 feedback circuit from the shielded line.
regards
Barry

 

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