opamp input cap? - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 26th February 2006, 06:56 PM   #1
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Question opamp input cap?

Hi,
I have just read Soufiane Bendaoud & Giampaolo Marino - Analog Dialogue 38-6 june 2004
They say that inverting opamps can become unstable when one hangs an RF filter on the input pin, due to the capacitor to ground.

He goes on to explain how to modify the normal circuit to avoid the instability.

He says that not all opamps are susceptable.

Now the questions.
Q1. Are noninverting opamps immune?
Q2. Is the OPA134 and 2134 susceptable to this form of instability?
Q3. How/what does one read in the data sheet to find if there is a problem.

Q4. Is the ne5534 immune?
Again I have never seen a specific warning of the problem in the datasheet.
__________________
regards Andrew T.
  Reply With Quote
Old 26th February 2006, 08:51 PM   #2
Banned
 
Join Date: Mar 2003
Location: Lisbon, Portugal
Default Re: opamp input cap?

Quote:
Originally posted by AndrewT
...
They say that inverting opamps can become unstable when one hangs an RF filter on the input pin, due to the capacitor to ground.
You should have another resistor before the inverting pin, no?
Are you saying a cap directly from the inverting input to ground?
Why would you do that?
That input is a virtual ground.
You must use the RF filter before the gain setting resistor (on the inverting input).
The R on the RF filter should be small, or it affects the gain of the opamp.
  Reply With Quote
Old 27th February 2006, 04:02 PM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
following Carlos' comments, I went back to check what the paper said.

Figs 14 to 17 http://www.analog.com/library/analog...e_loading.html show the layout, symptoms and cure.

But, not all PCBs allow for the extra resistor and/or cap to give the fig16 solution.

Help, with confirmation that only inverting are afflicted with this problem?

Could the exaggerated sss in female voice be in part attributable to something like this?
__________________
regards Andrew T.
  Reply With Quote
Old 27th February 2006, 04:27 PM   #4
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi Carlos,
have a look at Mauro's opamp configuration.
http://www.diyaudio.com/forums/attac...amp=1112280850

But, his is inverting.
I want to check out suitability of RF filter in non-inverting mode.
__________________
regards Andrew T.
  Reply With Quote
Old 27th February 2006, 07:30 PM   #5
Banned
 
Join Date: Mar 2003
Location: Lisbon, Portugal
Quote:
Originally posted by AndrewT
Hi Carlos,
have a look at Mauro's opamp configuration.
http://www.diyaudio.com/forums/attac...amp=1112280850

But, his is inverting.
Notice that on the final power op-amp stage he uses the cap across both inputs, not to ground
On the input op-amps he uses a cap to ground on the inverting input, right, but he is using positive feedback
That implementation is different.

Quote:
Originally posted by AndrewT
I want to check out suitability of RF filter in non-inverting mode.
There is no problem with NI.
Attached Images
File Type: jpg ni 1st order lp filter.jpg (87.0 KB, 228 views)
  Reply With Quote
Old 28th February 2006, 07:19 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi Carlos,
are you saying that the non-inverting opamp mode with RF filter on the input pin is immune to the ringing that 38-6 is referring to?

Quote:
There is no problem with NI.
That non-inverting does not need the Fig16 solution implemented?

That 5534 and OPA2134 are all OK for this form of RF suppression (cap to ground on the input pin)?
__________________
regards Andrew T.
  Reply With Quote
Old 28th February 2006, 10:05 AM   #7
diyAudio Member
 
traderbam's Avatar
 
Join Date: Jan 2002
Location: Earth
Quote:
Now the questions.
Q1. Are noninverting opamps immune?
Q2. Is the OPA134 and 2134 susceptable to this form of instability?
Q3. How/what does one read in the data sheet to find if there is a problem.

Q4. Is the ne5534 immune?
Q1. Yes, provided no capacitor is placed from inv input to ac ground.
Q2. Yes.
Q3. See below.
Q4. No, it is not immune.

Hi Andrew. This is a subtle effect and a little tricky to analyze but the cause is quite simple. Let me try to explain.
You are aware that a negative feedback loop will be unstable (oscillate) if the phase shift exceeds 180 deg when the magnitude of the loop gain is greater than 1. I should add that a circuit starts to become unstable, ringing, when the loop phase exceeds 135 deg. So it is best to keep the loop phase less than 135 deg when the loop gain is greater than 1. So your design challenge is to keep the phase shift from inv input to output and then back to inv input less than 135 deg.

Take a look at the open-loop frequency response of the OPA134 at 25C and 2K output load and +/-15V power rails (attached). You'll see that the PHASE shift from input to the output starts at zero at 0.1Hz and quickly increases to 90 deg above 100Hz. It then levels off and starts dropping again above 100kHz to reach 135 deg by 10MHz. The gain is less than 1 above 10MHz. This phase shift is something you cannot do anything about.


In a simple inverting configuration the output voltage is fed back to the inverting input via a simple resistor chain which does not impose any phase shift (assuming an ideal op-amp input). So over the frequency range 0.1Hz to 10MHz the loop phase shift - inv input to output and back to inv input - will never exceed 135 deg while the gain is above 1 and so it will be stable (when the output is resistively loaded).


When a capacitor is placed on the inv input to ground a low-pass filter is created in conjunction with the feedback resistor and this adds phase shift. In the worst case (large feedback resistor and no input resistor) it could add nearly 90 degrees to the loop phase. In this case the system will start to lose stability above 10Hz where the op-amp has 45 deg shift and the feedback filter adds 90 deg, a total of 135 deg, and will oscillate above 100KHz where the total exceeds 180 deg.

You need to arrange the component values so that the total loop phase shift is less than 135 deg at all frequencies where the loop gain is greater than 1. Note that the output load on the op-amp has a significant effect on the forward path phase shift - the datasheet assumes an easy 2k resistive load. If you put a capacitive load on the op-amp output the forward phase shift may have up to 90 deg added to it at some frequencies.

Example:
Suppose you want an inverting amp with a closed loop gain of 10, an input resistance of 10k and as large an input C as possible whilst keeping the system stable. Assume an OP134 and a 2k resistive load on the output.

The easiest way to choose the value of C is to use a simulator. Instead, here's a rough method to choose it...First, to achieve a closed loop gain of 10 the feedback resistor must be 100k. You have an input resistor of 10k and a feedback resistor of 100k. Now, look on the frequency response graph for the frequency at which the voltage gain is 10 (or 20dB). This is about 900kHz. Now see what the open loop phase is...about 100 deg. So, as a first approximation, you want the value of C to impose less than 35 deg of phase shift at 900kHz in the feedback network. You can calculate the phase shift in the feedback loop using the equations in the article. Assuming the 10k input resistor is ac grounded, the value of C for 35 deg shift is about 13pF. Now, subtract the input capacitance of the op-amp itself, about 5pF, and the actual capacitor you need is 8pF. There is some margin included here because I have not taken account of the reduction of loop gain that the capacitor causes...but a little margin is a good thing. To get the exact value you'll need to draw the loop gain and phase graphs.

Note that this assumes the input resistor is driven from an ac ground source. If the source is disconnected for any reason the 10k resistor will no longer load the feedback network and this will greatly increase the phase shift. Indeed, disconecting the input may cause a system to burst into oscillation! In this example the phase shift is about 160 deg when the loop gain is 1...it probably won't oscillate.

Summary:
Adding a capacitor from inv input to ac ground adds loop phase shift and reduces the stability margin. The input capacitance of the op-amp needs to be taken into account as well as the forward path phase shift change with load and temperature and power supply voltage. The system may become unstable if the input is disconnected or the circuit feeding this one has too high an output impedance.
Attached Images
File Type: jpg opa134 fres.jpg (74.9 KB, 164 views)
  Reply With Quote
Old 28th February 2006, 03:12 PM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi Traderbam,
Thanks for taking the time to explain that so comprehensively.

1. This instability afflicts all/most inverting mode opamps.

2. It is compensated for by adopting the Fig16 solution.

3. It is not a problem for an RF fitered input to non-inverting mode opamp with the C to ground from the +input pin.

Are these statements correct?
__________________
regards Andrew T.
  Reply With Quote
Old 28th February 2006, 03:23 PM   #9
Did it Himself
diyAudio Member
 
richie00boy's Avatar
 
Join Date: Nov 2003
Location: Gloucestershire, England, UK
I've only ever used Figure 16 (right) version, seemed the logical way to do it to me.
__________________
www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more.
  Reply With Quote
Old 28th February 2006, 03:39 PM   #10
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Thankyou
__________________
regards Andrew T.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
opamp inverting input sounds better? dorkus Solid State 89 24th November 2011 11:53 AM
Dual OpAmp in input of amp? miligor Solid State 16 17th March 2008 06:08 PM
high voltage input opamp sunrise Chip Amps 7 9th April 2007 06:54 PM
Opamp for A/D input stage mcs Digital Source 0 21st September 2003 12:57 PM
OpAmp used as Input Stage (IS)? sam9 Solid State 14 2nd June 2003 06:33 AM


New To Site? Need Help?

All times are GMT. The time now is 11:21 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2