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Ad8099 cascade .

stenzer

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Hi,

the transfer function of an opamp acts like those of a first order lowpass filter. At the cut-off frequency the magnitude is already attenuated by 3 dB and a phase shift of 45° occures. As the AD8099 has a GBP of 3.9 GHz and your second stage has a gain of 10, your cut-off frequency is 390 MHz. The cut-off frequency of the first stage is 1.95 GHz. Note, the gain-bandwidth-product (GBP) is only applicable for voltage-feedback amplifiers (like the AD8099). As you can see, it depends on the intended/targed frequency. The blue trace (second stage output) results in a phase shift of about ~40° @ 420 MHz.

Please use lables to name your nodes/traces as those lables also appear in the plot and is much easier to identify the related curve. Therefore, make a RMB click on the intended trace, choose "Label Net", choose a name and place the labe on the trace/node. Re-run the simulation, now the vurves jave individual names instead of e.g. n010 within the brackets.

BR
 

Paul98

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Did it! I also understood the GBP (I think). Now I do some tests with the successful stages to have a sufficient output voltage to drive an MMIC.

1.JPG2.JPG
 

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As mentioned by stenzer in post #15, AD8099 has limited slew rate, allowing about 0.3 Vrms output at 500 MHz for a G=10 stage. If you want higher output voltage, you should choose a different OP for the output stage.
 

Paul98

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As mentioned by stenzer in post #15, AD8099 has limited slew rate, allowing about 0.3 Vrms output at 500 MHz for a G=10 stage. If you want higher output voltage, you should choose a different OP for the output stage.
I was looking at it this morning to find another OpAmp that could do well as an intermediate stage and currently I haven't found anything yet. There are actually OpAmp with higher slew rates but they have a limited BW for this purpose. So I was looking for an MMIC that was sensitive enough to be driven by an OpAmp like this.
 

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The MMIC amplifiers are generally not DC-coupled and in so far not compying with your initial requirements - if still valid.

Suitable high slew rate OPs can be e.g. found in the Texas Instruments current feedback category, G=10 with 500 MHz bandwidth isn''t offered unfortunately.
 

stenzer

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Hi,

I used the AD8099 several times for frequencies up to 100 MHz. As it provides limited SR and is sensitive to parasitic capacitances (see page 23 in the DS) I used the AD8000 as a replacement. The AD8000 has the same pin out as the AD8099 and does not use the compensation pin C_c, so it is a suitable replacement if the PCB has been realized. Although the AD8000 has only a GBP of 1.5 GHz, this value is not really meaningful as for the AD8099, as the AD8000 is a current-feedback amplifier. Have a look at page 6 figure 7, there you can see the large signal tranfer function for a gain of 10, which results in a cut-off frequency of about 400 MHz. The AD8000 provides a SR of 4100 V/µs.

BR
 

Paul98

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@stenzer

Hi, I had already seen the AD8000 but I stopped because I had problems with the import on LTSpice I had already tried to integrate the AD8000 on one of the circuits where I already had the AD0899 but I had to stop because I had some problems with pins. I wanted to look at it to understand why I didn't have the time. I was hoping to remove an 8099 and put an 8000 on it but as you can see from the screen there is some problem with the pins ,then I looked at the rest!. I don't want to sound presumptuous but these models seem a little badly made to me. Am I wrong? I can also edit the CIR file via a text editor but then does it work? I copy-paste the pin assignments from 8099 file of the 8000 and then I try.
 

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stenzer

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Hi,

I don't want to sound presumptuous but these models seem a little badly made to me. Am I wrong?
these models are well made by means of modelling the actual behaviour pretty close. I made several measurements with corresponding simulations, and the reaults are in good agreement. How to import these models is an other topic, but LTspice is used by a lot of people and there are several instructions how to import a third order model, you just have to get used to it and do propper web search, heads up!

I copy-paste the pin assignments from 8099 file of the 8000 and then I try.
that wont work, the pins shown in the "header" (.SUBCKT) line have a dedicated name and order. If you have a closer look in the spice file, you will see that these pins (labels/names) are referenced at multiple points. You have to perform the same procedure as for the AD8099 and create a new component. Have a look at the video I linked in post #10 and you linked in #11. For your current simulation you can not replace the AD8099 by your created AD8000 model as it does not has a dedicated feedback pin (although the physical IC has one). As you can see on page one in the datasheet, the feedback pin is directly connected to V_out, so use the output pin to provide the intended feedback.

BR
 

Paul98

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Hi everyone, today I designed the non-inverting circuit for the AD8000. As a test I chose to try with the resistive values to have a G = 2 so I chose RG and RF at 432ohm and inserting a 50ohm resistor in series for RS. I seem to have done everything correctly but the output signal is flat. Maybe it's tired but I don't see any errors. Thank you.


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Presumedly an error in the model pin order.
--- Updated ---

We can check your design much easier if you post a .zip including the .asc, .plt and imported .lib or .sub files.
 

stenzer

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Hi,

connect the power down (PD) pin to +5 V (V_DD). If I remember correctly, for the AD8000 this pin has to be HIGH (fot the spice model).

BR
 

Paul98

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@klauss @stenzer

Solved connecting PD to VCC!. I see a little phase shift. It's normal?
I attach the zip again because I deleted the previous post.

1.JPG2.JPG
 

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Paul98

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Thanks everyone for the help. I abandon the project because what I want to achieve is not possible. If nothing else, I have a good basis for using LTSpice.
 

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