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OPA145: Slew Rate induced distortion

Part Number: OPA145
Other Parts Discussed in Thread: OPA140, OPA189, OPA209

OPA145 is marketed as 20 V/µs slew rate. Figure 6-37 of the datasheet shows slew-rate induced distortion already at ~25 kHz for a 30 V supply.

I am wondering if this is a mistake in the datasheet or how it is determined..

For comparison the OPA140 datasheet (nominally the same slew rate of 20 V/µs) shows the correct frequency (way above 100 kHz).

  • Hello Tobias,

    Actually, OPA145 datasheet Figure 6-37 is correct. The graph information indicates that the maximum slew rate to avoid distortion is limited to about 2.2 V/us at 20 kHz [SR = 2 pi x f x Vp]. This is certainly a significantly lower number than the typical 20 V/us listed in the Electrical Characteristics table.

    The reason for the difference is the OPA145 is a slew boosted op amp where over a wide portion of its output range its slew rate is boosted or much faster than it is at the ends of its range. The OPA145 has two slew rates and ranges it operates with - the faster boosted range, and the slower natural range over the wider swing range. The latter limit helps keep the output from slamming to a stop and ringing for a long time which is undesirable in most applications. What you see in Figure 6-37 Maximum Output Voltage Amplitude vs Frequency is based on the slower, natural slew rate of the OPA145.

    For a more complete explanation of slew boost have a look at the TI Precision Labs - Op amps session about Slew Rate.

    https://training.ti.com/ti-precision-labs-op-amps-slew-rate-introduction

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Thank you, Thomas,

    this is a very informative and swift response!

    I wish TI would *prominently* indicate the reality of slew boost circuits in the datasheets. The OPA145 datasheet provides no hint of it. Even the large signal step responses look pretty usual. When browsing for parts in parametric searches, the high slew rate figure would falsely make some parts relevant for some high frequency linear tasks.

    Moreover, the OPA145 spice model does not reveal the slew boost behavior either. It behaves pretty much like a UniversalOpamp set to 20 V/µs slew rate and produces no slew-induced distortion when configured as a voltage follower for a 20Vpp 100 kHz sine wave.

    For example, the OPA189 also has slew boosting. While its datasheet hides this fact for the most part and while the OPA189 spice model, too, doesn't reveal the slew boost, at least the block diagram under (8.2.) provides a clue.

    Is there an overview that shows which opamps suffer from the slew boost, or rather from this much lower "natural" slew-rate than given in the parametric searches?

  • Hello Tobias,

    There certainly have been a number of discussions about what information to provide about slew boost when an op amp has that property. I agree providing full disclosure about what it is and what role it plays in the op amp's slew performance would be more helpful to customers. When we view the OPA145 Figure 6-34, Large-Signal Step Response the signal level is such that there is no evidence of the slower, natural slew and the full 20 V/us is all that really is evident. I will say that at least for the OPA145 the Figure 6-37, Maximum Output Voltage Amplitude vs Frequency is based on the natural slew so if the curves are followed slew induced distortion can be avoided.

    I checked the OPA145 simulation model with TINA and verified that the slew is about 20 V/us, the boosted slew rate value. I do not see any evidence of the slower natural slew at the swing extremes in the waveform. Then I checked the OPA209 model associated with the Precision Amps discussion I referenced and it does produce the slew boost characteristics. I'll bring the subject up with our modeling engineer next time I see him and request that all models moving forward do produce slew boosting when applicable. 

    Thanks for your inputs. We do need to do a better job including and describing the slew boost characteristics.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Thanks for replying, Thomas,

    the general issue is that in case of the OPA145, all the evidence provided to customers (datasheet, spice model, parametric search) actually suggests that it is *not* a slew-boosted op-amp and that Figure 6-37 is wrong (hence my question).

    I would welcome any appropriate customer relation policy change as you outlined in your latest reply. I will mark this as resolved, but I would be glad if you let me know in case there is an update on this, e.g. the "Overview of hidden op-amp specs" that I mentioned before.

    Besides the presence of slew-boosting (and natural slew rates) such a table could also contain info on presence of back-to-back input diode clamps as well as on output saturation behavior (increased current draw?). The latter two are especially meaningful to judge start-up/shutdown behavior at a glance and for low-cost op-amps that might end up being used as comparators.

    I understand that those things are possibly not tested by default, but at least the design teams should have data on it and if TI makes this available, I can only anticipate a strengthening of customer trust.