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ADC3563: Protection circuitry for AINP/M input pins

Maik Ritter
Prodigy 30 points

Part Number: ADC3563
Other Parts Discussed in Thread: THS4541, LM7705

Hi there,

We are designing an ADC3563 based IF sampling circuit and we are at a point where we are discussing protection methods for the AINP/M pins.

Since the absolute maximum ratings for those pins in the datasheet specify a range of -0.3V to MIN(2.1, AVDD+0.3)V, and our AVDD is 1.8V, the effective maximum rating for our case is -0.3V to 2.1V.

The analog front end driving the ADC3563 contains a single-ended to differential stage using a THS4541 which is supplied with -0.23V (via LM7705) and 3.3V.

Under normal operation, the ADC3563 should never see voltages exceeding its ratings. However, since the positive rail of the THS4541 is 3.3V, there is a risk that the amplifier output could drive the ADC inputs above 2.1V (e.g. during startup transients or faults), which would destroy the ADC.

What is the best way to protect the AINP/M inputs in this specific design?

We considered inserting a BAT54S Schottky diode pair between the initial amplifier stage and the THS4541 input to limit the signal before the final amplification. However, I have concerns about this approach:

  1. An error in the final amplification stage (e.g. wrong gain resistor placement during production) could still result in the ADC being destroyed.

  2. I would prefer to avoid the non-linear capacitance of the BAT54S in the signal path.

I have read about TVS diodes and using current limiting series resistors, but I would appreciate advice from experienced engineers on how to proceed in this specific case to ensure the ADC is safe without compromising AC performance.

Thanks in advance!

Maikcircuitry.png

 

  • +1
    TI__Mastermind 45670 points

    Hi Maik,

    I can appreciate your concerns. The analog input stage protection problem has been an on-going question for as long as I can remember.

    Unfortunately, I won't be much help as you are already pointed in the right direction.

    Using the back 2 back Schottky diode pair between the initial amplifier stage and the THS4541 is probably the best approach.

    If your application is time domain, this should work fine, there are a number of diode pairs like this out on the market and some work better than others in this situation.

    if your application is frequency domain, the limiting factor on using an diode pair on the analog inputs, is early distortion when trying to achieve near full-scale signals. The diodes start to turn on and hamper your SFDR.

    In either situation, you may want to get a handful of pin2pin diodes that you can try to make sure that adding the diode does affect the ADC performance and you are still meeting your application performance requirements.

    BTW: Marki Microwave has some of the best diode out there on the market that limit the performance lesser than others. However, they are typically more costly and big/bulky. If you need a list of other diodes please let me know.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 30 points

    Hi Rob,

    Many thanks for the reply. Even though you could not give me the answer I was looking for, I now at least understand why I haven't found any design examples for the problem I face: There is no real solution for this problem, only compromises.

    By understanding this, I am now thinking about skipping the protection entirely, since it degrades my signals in any case – only the grade of degradation is dependent on the kind of diodes I am going to use. And in the end, if something is wrong with the THS circuitry, then I destroy the ADC anyway.

    Nevertheless, I would appreciate if you can suggest any of those Marki Microwave diodes. The IF frequencies I work with are in the 20 MHz to 80 MHz range.

    Best regards,

    Maik

  • +1 in reply to Maik Ritter
    TI__Mastermind 45670 points

    Hi Maik,

    Here are a few diodes to try:

    https://markimicrowave.com/assets/54b98937-6622-45c4-9222-730410690c9d/DLM-10SM-Tunable%20Differential%20Limiter.pdf

    SMS7621-005LF

    RB851YT2R

    BAT54CQ-7-F

    SMP1330-040LF

    MBRS410ET3G

    MBR0530T1G

    The top three, are known to have the best linearity performance from pervious experiments and work in the lab.

    Regards,

    Rob

  • 0 in reply to Rob Reeder
    Prodigy 30 points

    Hi Rob,

    thank you for the list.

    The Marki Microwave devices with adjustable clipping level look very interesting but they are probably overkill for our application.

    I experimented a little bit with the spice models of some of your suggestions an my findings and as you said all have partly massive impact on the SFDR. Luckily, your reply pointed me into that direction and I realized that in my application it may be better to skip the protection entirely, so that my signal does not get disturbed in any way. Since I intend to use bandpass sampling and massive decimation in the ADC3563 (16 or even 32) all the diode based harmonics will convolute into my baseband and will introduce trouble I cannot yet imagine....

    Thanks for the discussion, it helped a lot!

    Best regards,

    Maik

  • 0 in reply to Maik Ritter
    TI__Mastermind 45670 points

    You are very welcome Maik!