Low temperature characteristics of OPA561

Hi Thomas

Thank you for your quick reply.

Our answer is below.

• Is the OPA561 mounted to a PC board with the PowerPad soldered to a PC board pad connected to V- (ground in this case)?

Yes, the OPA561 is mounted to a PC board with the PowerPad soldered to a PC board pad connected to V-(ground) using recomended PowerPad footprint.

• Does the board include the recommended 100 nF power supply decoupling capacitor from V+ to ground?

No, the board doesn't include 100nF power supply decoupling capacitor.

But we tried various decoupling capacitors from V+ to ground.
e.g,
  1uF(Ceramic Capacitor) + 100uF or 220uF(Aluminum Capacitor)
  1uF(Ceramic Capacitor) + 47uF(Ceramic Capacitor)
And we tried various gain settings from 0dB to 20dB.
However, the oscillation did not stop.

• What does the 3.3 uF output capacitor couple to?  What is the load after the "Out" point?

The load is 50ohm resister.

Thank you

Jen

Hi Jen,

Simulating the circuit you provided in your schematic shows roughly 45 degrees of phase margin.  While 45 degrees is generally considered a stable design, the temperature swing and output loading of this application may push the phase margin below 45 degrees in some cases.

Let's try increasing the phase margin to roughly 60 degrees by placing a 47pF capacitor across the R16 feedback resistor.

Here's a link to learn more about op amp stability:

http://e2e.ti.com/support/amplifiers/precision_amplifiers/w/design_notes/2645.solving-op-amp-stability-issues.aspx

I left all original components in this simulation, but I have a few questions about some of them. 

What are R18 and C35 for? 

What is R19 for?  I'm not sure that R19 is required, it doesn't appear toaffect the transfer function or dc operating point.

Here's the results from the simulations:

Original Circuit:

Modified Circuit

8713.Original_Open_Loop.TSC

3581.Modified_Open_Loop.TSC

Hi Collin

Thank you for your reply.

We have tried your suggestion increasing the phase margin by placing a 47pF capacitor across the R16 feedback resistor.
However, the oscillation did not stop at -40 degree.
And we also tried the 22dB gain setting (R16:6.8k, R17:510) by placing a 7pF capacitor across the R16 feedback resistor.
Though this setting have the phase margin of over 100 degree, the oscillation did not stop at -40 degree.

Could you show the setting that do not oscillate by OPA561 evaluation results of  at low temperature to us?
Could you lend your evaluation board of OPA561 to us if possible?

Our answer is below for your question.

• What are R18 and C35 for?

R18&C35: it's 200Ω AC termination for the former stage amplifire.

• What is R19 for?  I'm not sure that R19 is required, it doesn't appear to affect the transfer function or dc operating point.

R19: it's for a bias of negative input port. It could be redundant.

Best regards,

Jen Jolly

Hi Jen,

Because the most recent customer results did not change very much with the loop compensation, we are starting to believe this is a local output-stage oscillation that seems to get worse at low temperatures.  That said, we recommend implementing the output snubber circuitry described on page 8 in the "Output Stage Compensation" section of the product datasheet.  Let's try 10 Ohms and 0.01uF directly from the output to GND.  If we see improvement we can optimize the snubber from there.

Also, is it possible to provide us with oscilloscope captures of the output signal instead of the spectrum analysis?  We are curious about the shape of the transient oscillation waveform.

Hi Collin

We have tried the snubber of 10 Ohms and 0.01uF directly from the output to GND.  However the oscillation did not stop. And then we are sorry but we could not get the oscilloscope captures of the output signal well.

We are doubting that OPA561 is oscillating inside at low temperature. So we hope you show your OPA561 evaluation result at low temperature to us.

Best regards,
Jen

Hello Jen,

Our colleague Collin is busy with other pressing issues right now so the rest of us here in applications are going to try and assist you with the OPA561 oscillation issue.

Collin's stability analysis appears correct so the oscillation doesn't appear to be due to circuit implementation. The fact that the oscillation is at a frequency much lower than the amplifier's GBW supports his analysis. This suggests that oscillation may be related to a slower, internal loop. That is why Collin suggested adding a snubber at the output, but as you found it does not remedy the problem. If it is an internal loop stability issue the circuit may not be connected directly to the outside world.

You have provided the spectrum analyzer images which show the fundamental frequency and very strong harmonics. That indicates that the oscillation's waveform is not sinusoidal. Would it be possible to connect an oscilloscope probe to the output when the device is oscillating and capture the image? Knowing what it looks like might provide some clues about the oscillation's internal mechanism. The oscillation level looks to be tens of millivolts so a 1:1 scope probe may be required to best view it.

Here are some questions and ideas we have discussed:

• Would it be possible to drive the input with an ac signal, while the output is connected to the 50 Ohm load, and see if the oscillation persists?
• Does adding an additional load to the output such that the output sinks and sources output current, 10 or more milliamps, stop the oscillation?
• We are going to work on constructing your OPA561 circuit here and see if we can reproduce the oscillation.
• Is this circuit application early in its design, or well along in the development? It may take us some time to get to the bottom of this issue. Would it be possible for you to consider the OPA564 for the application? The OPA564 is a newer design on a different process and probably won't behave the same in your application circuit.

http://www.ti.com/lit/ds/symlink/opa564.pdf

Regards, Thomas

PA - Linear Applications Engineering

Hi Thomas

Thank you for your reply.

We have confirmed in order to answer your questions and suggestions.

Please check the attached files. And we have already evaluated OPA564 but its bandwidth was slightly insufficient by our application. So we would like you to consider the countermeasures of the oscillation.

Best regards, Jen

Hello Jen,

Thank you for making the requested measurements. They are very thorough and complete. It appears from them that the oscillation persists being altered somewhat by external influences. If I may impose upon you one more time, I would like to you try a couple of more things:

• Would it be possible to add an I_LIM resistor between the I_LIM pin and GND? The resistor value would selected based on the maximum anticipated current the load will require. Then, connect a 10 nF from the pin to GND.
• Add a 10 nF capacitor from the CLS pin, pin 9, to GND.
• Take the OPA561 down to cold where the oscillation is usually seen and see if it still persists.

We have located some OPA561 boards and will assemble you setup here at our facility. We are going to attempt to reproduce your results in our lab.

I think it is time that we move this item off the E2E forum so that I may ask you further details about you circuit, and that I may discuss anything that we find. Moving forward please contact me at my email address:

kuehl_tom@ti.com

Regards, Thomas

PA - Linear Applications Engineering