OPA2890: Max frequency for square wave

ttd

Mastermind 8110 points

Part Number: OPA2890
Other Parts Discussed in Thread: OPA890, THS3491, LMH6703

Tool/software:

Dear Technical Support Team,

What is the maximum frequency of the OPA890 for square wave?

10MHz seems to operate correctly, however 250MHz (500Mhz) is required for next version of my application.

output square wave signal is from 0V to 4.5V.

I think general method of calculating the sine wave slew rate is as follows.

SR= 2πfV=2x3.14x250MHz*4.5V=7,065V/us

Q1.

Is the slew rate calculation method for a square wave the same as for a sine wave?

Q2.

Could you suggest appropriate high speed amp for 500MHz(250MHz) square wave?

www.ti.com/.../

Best Regards,

ttd

4 months ago

0 Clemens Ladisch 4 months ago

Guru 285350 points

1. The theoretical slew rate of an ideal square wave is infinitely large. In practice, you have to choose a slew rate.

2. A square wave sounds like a digital signal, so you should use a comparator instead. And most fast comparators have LVDS outputs. Why do you want to use an opamp? What are you going to do with the output signal?

0 Michael Steffes 4 months ago

Guru 48395 points

FOr a 2nd order small signal frequency response the approx. peak dV/dT is 2.85*Vstep*F-3dB. So, for a 250Mhz type square wave you need 1st 4X that BW, so say 1GHz. Then the peak dV/dT will be 2.85*4.5V*1GHz = 12.8V/nsec slew rate, about your only choice is the THS3491

0 Alec Saebeler 4 months ago

TI__Genius 17376 points

Hello ttd,

We have some good responses here from the community.

Our team will help support this question next week.

Best,

Alec

0 ttd 4 months ago in reply to Clemens Ladisch

Mastermind 8110 points

Hi Clemens Ladisch,

Thank you for your reply.

I'd like to use non-inverting amp(G=2), so it doesn't work with comparator.

Best Regards,

ttd

0 ttd 4 months ago in reply to Michael Steffes

Mastermind 8110 points

Hi Michael Steffes,

Thank you for your advice.

"2.85*Vstep*F-3dB" is a common method of estimation?

Could you attached the URL about it?

I couldn't find any website with explanations.

Best Regards,

ttd

0 Michael Steffes 4 months ago in reply to ttd

Guru 48395 points

No it is not well known, when you are doing high speed amp part defintions, useful to know this. Came out of some work I was doing in this article, essentially an estimate suitable across the 0.7<Q<1.5 range on that curve in this article,

https://www.edn.com/what-is-op-amp-slew-rate-in-a-slew-enhanced-world-part-1/

0 Michael Steffes 4 months ago in reply to Michael Steffes

Guru 48395 points

+1 Alec Saebeler 4 months ago in reply to ttd

TI__Genius 17376 points

Hi ttd,

I saw another thread about THS3491 & LMH6703. Do you still require support on this thread?

Hi Michael, thank you for the good resource here .

Best,

Alec

+1 Michael Steffes 4 months ago in reply to Alec Saebeler

Guru 48395 points

Yea Alec, that article has some good stuff in it - I was doing that while I was working on a number of the higher speed parts that came out in the 2018 to 2020 time frame. I had intended to shows point slope plot on a large signal edge in the THS3491 datasheet but that never happened - but it makes sense (to me at least). That old 20% to 80% fit to get a slew rate harks back to the early scope days, but today with these incredible sampling scopes you have the point by point data, where that slope goes goes constant on an edge is the real slew rate - why not use it.

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OPA2890: Max frequency for square wave