Part Number: OPA699
Greetings TI engineers! I am using an OPA699 for a sweep circuit and am witnessing some unexpected distortion. Here are the facts:
1. Dual supply = +/- 5V. G = +6 with Rf = 750 ohms. No capacitors. Opamp and limit pins bypassed with 0.1uF
2. Noninverting input fed by another opamp generating a -0.3V -> +0.3V ramp at 50V/uS, overall signal peaks lie at at almost +/- 0.6V.
3. With limit pins open, I get a straight looking ramp on the output as expected. However, with limits set to +/- 2V (or less), I get a noticeably distorted ramp. Not "severe" but "noticeable".
I'm puzzled as It does not seem that I am pushing the amp with too fast an input ramp. Any suggestions? Could it be that the amplified signal peak (G=+6X0.6V = 3.6V) lying beyond the input voltage range of 3.2V causes the problem?
Thanks in advance.
Best - Ram
Should work, what is your load and perhaps measure the clamp voltages as you are doing this.
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In reply to Michael Steffes:
Hi Michael. I just edited my post to add the following observation: overall input signal peaks lie at at almost +/- 0.6V. Could it be that the amplified signal peak (G=+6X0.6V = 3.6V) lying beyond the input voltage range of 3.2V causes the problem, possibly the amp is slow to recover?
btw the limits are set using 3.3k/2.2k 1% resistors off a stable +/-5V supply. I've tried loads of nil and a 560R
In reply to Dr. Ramkumar Ramaswamy:
I've attached a couple of photos.
Photo 1: G=+6, Liimit pins open
Photo 2 G = +4.33 (Rf = 500), Limits set to +/- 1.07V using 900R and 3k3.
You can clearly see the second ramp develops a bit of a hump which is quite annoying.
SO you have changed conditions and not probed the limit pins, and what is your load?
at your lower gain, if you have the +/-0.3*4.333 that is trying to hit +/-1.33V exceeding your new limit voltages of 1.07V - should go non-linear,
Michael, I actually changed conditions in a way that I felt more sharply illustrated the problem. With the lower gain the input peak V * Gain does not hit the 3.2V input limit, yet the waveform is distorted. Thanks for goading me to probe the limit pin - I do see some noise on the limit pin despite the 0.1uF cap, so let me try some more aggressive filtering and see if it works.
wrt the last sentence you wrote - yes, I would expect the nonlinearity to set in after hitting the limits. But what I observe is that there is nonlinearity inside the limiting range as well. In the second photo above, the lower limit is more than 3 divisions below the center but you can actually see the hump appearing just one division below the center.
Well if the part is not broken you should see a good ramp right up to the DC clip voltage, I was suspecting there might be some interaction in your set up pulling the limit voltage down as you ramp up.
You might also try intentionally driving hard into the limit like say a +/-1V input triangle wave and see if you produce nice trapezoidal waveform with the peaks at your limit voltages -if not something odd is going on.
This is a decomp part, so be careful not to probe with a cap probe - if you must - probe through a 100ohm isolating R to avoid issues. Load??? if it is accidentally very low, that will also give you Vout induced curvature. (<20ohm for instance).
Thanks for the thoughts Michael. Load as I had mentioned, was tried at 560R and no load...to no avail. Current photos are with no load, just the feedback resistor network. Also tried adding 10nf and 100pf caps across the limit pin 0.1uFs and managed to reduce noise further, but no improvement with that hump. I'm actually using the 1:21 probe described in Johnson's black magic book so that's not causing any issues as the first photo itself shows.
Interestingly, the large signal pulse response graph in the datasheet does show similar nonlinearity around the zero crossing - there is a distinct change in slope. I'm wondering if that and this are related.
Broken part? Hmmm...just bought them directly from the TI eStore so I assumed they'd be good. Maybe I'll just try swapping it out and see.
Thanks for all the assistance!
It seems that a similar type of response appears when simulating this circuit as well.
In reply to Hasan Babiker31:
Oh wow - thank you so much Hasan, for taking the time to simulate and verify this.
Since this drops out from the simulation itself I suppose then it should - in principle at least - be possible to make a statement about the conditions under which this sort of thing could happen, so that one may avoid them by design. It's tempting to say it is large-signal distortion but then one should have seen it when the limit pins were open as well! So in that sense perhaps the data sheet is incomplete.
I tried some more values in TINA for gains of 4, 5 and 6. With the input ramp as specified in the simulation model, no form of limiting seems to work. The "limit" does not distort only if the limit value is greater than the peak input voltage X gain - which is quite meaningless since then you might as well keep the limit pins open. I'm quite disappointed by this chip. The datasheet looked so good!
I tried the simulation with a very gentle amplitude/rise time combination of 0.4V/100nS with a gain of +6. The simulation (which corroborates my tests) shows the IC cannot handle even this. Specifically, the 1.9s recovery time specified in the datasheet seems to be not met, and by a pretty wide margin. Would that be correct?
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