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26th February 2006, 07:56 PM  #1 
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opamp input cap?
Hi,
I have just read Soufiane Bendaoud & Giampaolo Marino  Analog Dialogue 386 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.
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26th February 2006, 09:51 PM  #2  
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Re: opamp input cap?
Quote:
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. 

27th February 2006, 05:02 PM  #3 
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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?
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27th February 2006, 05:27 PM  #4 
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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 noninverting mode.
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27th February 2006, 08:30 PM  #5  
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Quote:
On the input opamps he uses a cap to ground on the inverting input, right, but he is using positive feedback That implementation is different. Quote:


28th February 2006, 08:19 AM  #6  
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Hi Carlos,
are you saying that the noninverting opamp mode with RF filter on the input pin is immune to the ringing that 386 is referring to? Quote:
That 5534 and OPA2134 are all OK for this form of RF suppression (cap to ground on the input pin)?
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28th February 2006, 11:05 AM  #7  
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Quote:
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 openloop 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 opamp 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 lowpass 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 opamp 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 opamp 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 opamp 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 opamp 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 opamp 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. 

28th February 2006, 04:12 PM  #8 
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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 noninverting mode opamp with the C to ground from the +input pin. Are these statements correct?
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28th February 2006, 04:23 PM  #9 
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I've only ever used Figure 16 (right) version, seemed the logical way to do it to me.
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28th February 2006, 04:39 PM  #10 
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Thankyou
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