OPA820: High current output (46mA) rising edge slew rate

There is really hardly enough to go on here, a picture (schematic) would help Tina file preferred, 

Essentially you need to arrive first at the small signal response shape to see what it to produce assuming a fast input edge (and what is that). 

Some of the required layers are considered here, 

https://www.planetanalog.com/separating-linear-from-slew-limited-performance-in-high-speed-amplifiers-part-2-of-2-the-signal-sped-up-insight-15/

Hello,

   Also, would you be able to post the figures again; we are unable to view them.

Thank you,
Sima

Hallo,

the additional informations:

The schematic is that of the TI demo-board.

C1, C2, C3, C4, J1, U1=OPA820, R2, J2 assembled

R1=100k, R6=0R assembled

all other components are not assembled

J1 is connected to the waveform generator, Output Impedance 50Ohm

J2 is connected to the oscilloscope, Input Impedance 50Ohm

Figure1, (C1) OPA820-Output at J2 Demo-board, (C2) wafeform generator at J1 Demo-Board

Figure2: expected pulse response (OPA820 Data Sheet)

regards K-H.

Hello K-H,

     I am still unable to view the image/file; you can use the insert file or insert/edit media icon within the reply window. Follow this post to correctly insert a file or image: https://e2e.ti.com/support/site-support/f/1024/t/761613.

Thank you,

Sima

those larger step plots certainly look slew limited, be careful not to be clipping. Here is a more detailed pair of slew rate articles discussing the edge issues, 

https://www.planetanalog.com/separating-linear-from-slew-limited-performance-in-amplifiers-part-1-of-2-the-signal-sped-up-insight-14/

https://www.planetanalog.com/separating-linear-from-slew-limited-performance-in-high-speed-amplifiers-part-2-of-2-the-signal-sped-up-insight-15/

Hello K-H,

   Thank you, it worked. As Michael mentioned, the large signal response looks slew limited in your application. Output saturation could be a concern for a rise in supply current. Would you be able to share the schematic using the same method? That seems to be the only figure missing compared to your original reply. Also, what is the range of your input signal requirements for your application?

Thank you,

Sima

Hello sima,

the schematic is that of the TI demo-board.

The Input Signal Range is 1V ... 4.6V.

Regards K-H.

Hi,

just ran a simulation: The TINA-TI reference design does not show an output voltage toggling any more with this low load impedance this high output voltage.

Kai

The sim of the modified EVM works fine, 

It does show about 460Mhz SSBW - that is pretty fast for such a slow slew rate device. 

Your measured plots look like they are labelled backwards, and are clearly slew limited. That single overshoot and decay is indicative of the loop opening as it slew limits then closing again to a final value. 

The details of that edge shape also depend on the input edge rate which you have not reported. 

What is the question again? 

Hi Michael,

I get these results:

kh_opa820.TSC

Kai

Yes Kai, that is what the sim shows. 

the output swing does decrease as you get into an internal IR drop at higher load currents. In that time frame, we had been showing that in the plots as this curve which is a nice 4 quadrant thing. Later, this became what is now known as a "claw" curve. In any case, the physical device is right on the edge of being able to support that 4.6V output into 100ohm with 6.2V supply - this transistor based model is trying to tell you that. 

If I increase the positive supply to 7V, it pops back in showing the slew limited transitions. slower negative slew rate vs. positive in this model? Been too long for me to remember why - but, yes this device is very slew limited vs. the SSBW.