Searching \ for '[EE]:unwanted hi frequency osc with opamp design ?' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/timers.htm?key=osc
Search entire site for: 'unwanted hi frequency osc with opamp design ?'.

Exact match. Not showing close matches.
PICList Thread
'[EE]:unwanted hi frequency osc with opamp design ?'
2001\07\20@044757 by Mathew

flavicon
face
Hi all,

I have built a volume control circuit using opamps and a digital pot.
The opamps I am using are NE5532N. The input buffer and output driver amps
are configured as inverting amplifiers. This is so I can add gain later if
need. My problem is that as the circuit reaches unity i.e. digital pot is at
0. The input amp has a hi frequency osc. This is not audible nor does it
noticabley affect the audio but should not be there. I have read somewhere
that I should have some capacitance somewhere to stop this but can not find
the reference :-((

Also what value input decoupling capacitor do you recomend


Thanks a bundle

Matt

--
http://www.piclist.com hint: To leave the PICList
spam_OUTpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu


2001\07\20@081843 by Olin Lathrop

face picon face
> The input amp has a hi frequency osc.

I don't know any details of your circuit, so this is only a general answer.
Some amps are unstable without compensation, especially a low gains.  Try
adding a "compenstation capacitor".  Some amps have explicit pins for this,
otherwise put it directly from output to the negative input.  Of course this
assumes your circuit doesn't drive the negative input with a low impedance
at high frequencies.  Start with the smallest capacitor you can find and
work up slowly.  Usually only a few 10s of pF will do it.  Too much
compenstation will cut into the desired bandwidth.

Of course this assumes you have good clean and well bypassed power supplies
at the amp.  All else is irrelevant until this is taken care of.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, .....olinKILLspamspam@spam@embedinc.com, http://www.embedinc.com

--
http://www.piclist.com hint: To leave the PICList
piclist-unsubscribe-requestspamKILLspammitvma.mit.edu


2001\07\20@115900 by Dan Michaels

flavicon
face
Matthew wrote:
>Hi all,
>
>I have built a volume control circuit using opamps and a digital pot.
>The opamps I am using are NE5532N. The input buffer and output driver amps
>are configured as inverting amplifiers. This is so I can add gain later if
>need. My problem is that as the circuit reaches unity i.e. digital pot is at
>0. The input amp has a hi frequency osc. This is not audible nor does it
>noticabley affect the audio but should not be there. I have read somewhere
>that I should have some capacitance somewhere to stop this but can not find
>the reference :-((
>

For a normal inverting opamp situation, you can prevent oscillation
and also filter hi-freq noise by simply adding a small cap between
output and neg-input nodes. You normally have a feedback [gain-setting]
resistor [Rf] between those nodes. You select the cap value based upon
the value of Rf and the BW [F3db] you desire for the ckt, using the
following formula:

Cf = 1/(2*pi * F3db * Rf)

The cap will both filter out noise higher in freq than F3db, and
also prevent hi-freq oscillations - in "most" situations.

However, if you vary Rf to change gain, then this analysis is
more complicated, because the BW of the ckt will vary as Rf
varies. Also, if the gain-setting element is a digital pot in
the feedback pathway, rather than resistor Rf, then its
characteristics will affect the analysis too.

- dan michaels
http://www.oricomtech.com
======================

--
http://www.piclist.com hint: To leave the PICList
.....piclist-unsubscribe-requestKILLspamspam.....mitvma.mit.edu


2001\07\20@145737 by Paul Hutchinson

flavicon
face
In addition to the good suggestions others have made here's something I've
seen often.

If the load you're driving has too much capacitance you can get oscillation
on the output of most op-amps. With a LM324 type it takes only a few tens of
pF to make the output oscillate. Some op-amps can tolerate a lot more load
capacitance. To decouple the op-amp from the capacitive load simply add a
small (10 to 100 ohm) resistor in series with the output. If the series
resistance causes too much drop in the output you can move the feedback
takeoff from the output pin to the other side of the resistor. National Semi
used to have a good description of this problem and cure in the LM324
datasheet.


Paul

{Quote hidden}

--
http://www.piclist.com hint: To leave the PICList
EraseMEpiclist-unsubscribe-requestspam_OUTspamTakeThisOuTmitvma.mit.edu


2001\07\21@170844 by J Nagy

flavicon
face
Paul Hutchinson wrote:

>
>In addition to the good suggestions others have made here's something I've
>seen often.
>
>If the load you're driving has too much capacitance you can get oscillation
>on the output of most op-amps. With a LM324 type it takes only a few tens of
>pF to make the output oscillate. Some op-amps can tolerate a lot more load
>capacitance. To decouple the op-amp from the capacitive load simply add a
>small (10 to 100 ohm) resistor in series with the output. If the series
>resistance causes too much drop in the output you can move the feedback
>takeoff from the output pin to the other side of the resistor. National Semi
>used to have a good description of this problem and cure in the LM324
>datasheet.
>

       This is quite true. Sometimes you won't even see an oscillation, it
just appears as a DC offset from hell (nothing you do will make it go
away). I've had to use the resistor several times, with the DC feedback
resistor moved to the 'load side' of the series resistor (so that my gain
is not affected). Do not move your compensation capacitor though! You're
asking for trouble if you do, as you are dropping one of the pole
frequencies, reducing phase margin and contributing to the instability.
Keep the capacitor connected between the op amps output and the inverting
input.


       Jim Nagy
       Elm Electronics
 ICs for Experimenters
http://www.elmelectronics.com/

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads


2001\07\22@102338 by Dave Dilatush

picon face
Matt wrote...

>I have built a volume control circuit using opamps and a digital pot.
>The opamps I am using are NE5532N. The input buffer and output driver amps
>are configured as inverting amplifiers. This is so I can add gain later if
>need.
>My problem is that as the circuit reaches unity i.e. digital pot is at
>0. The input amp has a hi frequency osc. This is not audible nor does it
>noticabley affect the audio but should not be there.
You're right; it shouldn't be there.  It may not be audible, but it
very likely will cause significant distortion since the oscillation
will generally vary- sometimes a little, sometimes a lot- with the
instantaneous voltage of your audio signal.

>I have read somewhere
>that I should have some capacitance somewhere to stop this but can not find
>the reference :-((

Here are a couple of application notes at the Analog Devices, Inc.
website that might help:

"An I.C. Amplifier User's Guide to Decoupling, Grounding, and Making
Things Go Right for a Change"
http://www.analog.com/techsupt/application_notes/AN-202.pdf

"Careful Design Tames High Speed Opamps"
http://www.analog.com/techsupt/application_notes/AN257.pdf

"Analog Signal Handling for High Speed and Accuracy"
http://www.analog.com/techsupt/application_notes/AN-342.pdf

Based on your description of the problem, my guess (and it's only a
guess) is that you might be having problems caused by capacitive
loading on the opamp outputs; if that's so, putting a small capacitor
between the opamp output and the summing junction of your amplifier
(i.e., the opamp's inverting input) won't help matters.  For that, you
need to isolate the opamp output from the capacitive load with a small
series resistance, maybe ten to a hundred ohms.

Again, that's a guess; there are lots of things that can make an opamp
oscillate.

Perhaps the best thing you can do is get yourself a copy of Bob
Pease's book, "Troubleshooting Analog Circuits" (US$20.26 at
Amazon.com), which is packed with practical information on how to make
analog stuff work right.

>Also what value input decoupling capacitor do you recomend

What's an "input decoupling capacitor"?  Do you mean input coupling
capacitor?  Or power supply DEcoupling capacitor?  :)

Dave

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics


More... (looser matching)
- Last day of these posts
- In 2001 , 2002 only
- Today
- New search...