1. UG-755 is a "universal" evaluation board for Single SO-8 amplifiers. Meaning, it caters not only AD8000 or AD8001 but all other amplifiers that are pin compatible with the board. Some amplifiers, not too fast, are not too critical for the ground plane under the pins/pads. However, AD8000/AD8001 are GHz amplifiers and parasitics are very critical for the devices performance and response, that's why it is recommended that no ground planes should be below/under the pins/pads of the amplifier.
2. Looking on figure 6 of UG-755, you would see that R5 and R6 ensure the isolation or connection of the trace to the feedback loop.
If you remove either R5 or R6, you'll make the red trace connected to either FB or OUT pins. It's okay for AD8001 but not with AD8000. The extra trace would degrade the performance of the AD8000 because of its parasitics. For amplifiers with dedicated feedback pin, R5 and R6 both in DNI ensures that the trace is disconnected from the signal path.
Thanks for you reply quickly.
I have a question about your answer: Is that "universal" demo baord suitable for testing AD8000&AD8001? For, as you said, that evaluation board didn't remove ground plane under/below pin/pad.
Now, I want to confirm one thing again. Is it means that i can design one type compatible circuit by using AD8001 or AD8000, and keep R5 and R6? If this is the case, I'll follow figure 6 of UG-755's pcb placement and routing.
As above figure shows, we used two different pcb placement and routing for AD8001 and AD8000 respectively. It means that we have to use two different parts (both AD8000 and AD8001). If we can make above pcb design same(just like ug-755), then we can use same part or different part (ignore performance, for both are ok after tested on our test board). It will be more convenient.
BTW, I didn't find pin description for pin number 8 from AD8000's datasheet, just named as /PowerDown. As above picture shows, we connected this pin to +5V, however, ADI demo board seems leave this pin floating according ug-755. If i want to design compatible with AD8000&AD8001, what's your recommendation about this pin (pin number 8)? Floating or connecting to VCC?
I failed to ask what your application is, would you mind sharing it?
Anyway, yes the board can be used to evaluate AD8000/AD8001 but for very fast (GHz) applications the ground layer below the pins of the part, especially below IN- pin would have a parasitic capacitance which in effect introduce a pole. And we recommend that for "optimum" device performance, certain layout considerations should be observed as discussed in the datasheets.
Yes, R5 and R6 will give you the option to use amplifiers with either have a dedicated feedback pin or none without major variations in the layout. Pin 8, power down pin, is an active low pin so it's okay to leave it floating if the part will be ENABLE at all times. However, it is always safe to connect the power down pin to a certain voltage level (> +VS – 1.9) instead of floating, to make sure that it is always on it's desired operating condition. A DNI resistor between pin 7 and pin 8 would be a great option for you as well.
I'm glad to get your reply.
1. Our application is PET, so there are many ADI parts used in our system (AD8264/AD603,AD8065,AD8001/8000,AD5391&AD5383,AD9218&AD8138). The AD8001 and AD8000 are used to implement adder, which will provide source for CFD or Leading-edge circuit.
2. Now, we have almost finished the pcb design. I attached our board stack-up and pcb placement&routing picture, please help us to figure out how to remove the ground plane under AD8000/8001 chip(pins/pads)?
Note: The numbers on the left of the picture are those layers thickness, the board total thicknees will be about 2mm. All PP's DK value are 3.94, and core's DK value is 4.12.
3. The engineer gives pcb placement&routing picture with two types (left AD8000/8001 and right AD8000/8001) solutions, do you think which will be better? I like the right one, for shorter routing length between those two optional resistors.