Issues when use AD8001 as Adder

Hello Guys,

We are using AD8001 as Adder. We can use it in our previous product, now, we are using it in our new prototype and test board. We didn't change anything in the schematic, only replace and route a test board. As attached imag file shows, there are something like ripples coupling into the Adder's output. What happens on the AD8001?

We have isolated the next stage, and nothing change. The input signal is exponent from function generator, and across AD8264(VGA), then input to AD8001. We also input sine, square and sawtooth signals. It seems the low speed signal can get better result. The output imag as below(violet channel is VGA's output, and blue channel is AD8001's output):

Thanks in advance.

Jerry

  • 0
    •  Analog Employees 
    on Jan 11, 2017 8:09 PM

    Hello again, Jerry.

    Could you attach your schematic please?

    Thanks.

    Jino

  • Hi Jino,

    Here is the AD8001 part schematic, do you need other parts?

    PS: The same circuit can work well in our previous board. This test board is just used to test VGA part - AD8264, unfortunatly, we all don't think the issue happens on AD8001. Below is the test rusut in our previous board, AD8001's output is wondful(violet channel is the AD8001's output, yellow channel is the input signal).

  • 0
    •  Analog Employees 
    on Jan 12, 2017 6:08 PM

    Hi Jerry,

    I don't normally support the AD8001 but I can make a suggestion that may help. What you're seeing is called 'ringing and occurs when an amplifier is operated beyond it's stable bandwidth limit. Ringing occurs during a transition, as you see here. You said you this issue showed up with a new test board with new routing. Both these factors could easily occur with a new board, different passives (especially capacitors), grounding, and rise/fall times of the applied signal. A combination of all these factors can conspire to create ringing where none has been observed before.

    As a first step, and if you have the hardware, look at the same waveform on an old board and new board using the same part and driving signal. If this isn't feasible, you can try adjusting the rise and fall times of the signal source. if you determine that the same part has ringing in the new board but not in the old, then you could examine the board and circuit more closely.

    I hope this helps.

    js

  • Hi Harry,

    1. The DC_offsetNEG1 and DC_offsetPOS1 are from DAC, they are DC voltage.

    2. The test board and old board have same number(8) layers, and their PCB stackups are also same:

    3. No, we don't remove the coppers.

    4. The SUMs are connected to same chip, which is a comparator - Max9601. Now, we remove Max9601 for debugging only AD8001. The problem still there after removing 9601.

    5. Our old board use AD603 as the VGA, the test board changed to AD8264. The VGA's outputs feed to AD8001. We changed to AD8264, just in order to increase the bank-wide from 90Mhz to 150Mhz. According to AD8264's datasheet, our signal's speed no more than 150 MHz. The AD8001's BW is 800MHz, our circut is congigure the gain to be -1. So we only get 370MHz BW, but still enough for us.

    What's the "0.1 dB Flatness" means? 

    BTW, why you don't want to use AD8001? And can you recommend me some parts for our application?

    Thank you very much.

    Jerry

  • Hello Js,

    Thanks for your informations.

    AD8001 works well on our old board. Actually, we designed this test board to test new VGA part - AD8264. And encounter this issue for AD8001 on this new test board. Almost schemaic design is same, just change VGA part from AD603 to AD8264.