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ADS6225 impedance restriction

user4293780
Prodigy 40 points
Other Parts Discussed in Thread: ADS6225

Dear Sir/Madam,

ADS6225 is applicated in my design as 80Msps ADC. I want to the impedance requirements for both input and output signal.  (As the Smith circle showed, 50 ohm differential for input? that may require big change on my PCB design)

Also, i add a AD8138 amplifier in its input side.

Please kindly help answer my question.

thanks!

  • 0
    TI__Genius 17290 points
    Hi,

    The ADS6225 input impedance is considered high impedance at low frequency. See figure 79 for the input circuit. At higher frequencies the parasitic components will start to effect the input impedance and the impedance will roll off. The magnitude of the impedance is shown on the left side of figure 81. At 50 MHz the magnitude of the impedance is 426 ohms and from the smith chart the equivalent circuit is a 62.2 ohm resistor in series with ~8 pF of capacitance.

    Regards,
    Matt Guibord
  • 0 in reply to Matt Guibord
    Prodigy 40 points

    Hi,

    In my design, 62.2 ohm resistor with 8pF capacitance external input circuit is needed? 

  • 0 in reply to user4293780
    TI__Genius 17290 points
    Hi,

    I don't understand your question... You do not need an external 62.2 ohm resistor and 8 pF capacitor. That is simply the equivalent impedance looking into the ADC at 50 MHz.

    Regards,
    Matt Guibord
  • 0 in reply to user4293780
    Prodigy 40 points
    Hi Matt，

    What is the requirement of PCB impedance for these input signals? Shall i set the differential impedance as 50 ohm or 100 ohm?
  • 0 in reply to user4293780
    TI__Genius 17290 points

    Hi,

    You need to follow normal transmission line rules. I'll give you a few examples. Please take a look at the EVM schematic for an example input circuit.

    1. A differential amplifier driving the ADC - two options
      1. If termination is placed at the ADC input (i.e. 100 ohms differential) then we typically use 100 ohm differential traces and use 100 ohm source impedance at the amplifier output (i.e. 50 ohm in series with each output)
      2. If no termination is placed at the ADC input, then the impedances aren't going to be matched anyways. So it doesn't matter what you use. This is perfectly okay for lower speed signals and when the distance from the amplifier to the ADC is very short (e.g. rule of thumb for lumped vs distributed components).
    2. A 1:1 transformer driving the ADC - This is the typical circuit used on our EVMs. The source is usually a 50 ohm signal generator and we use 50 ohm differential termination at the ADC. In this case, the trace before the transformer should be 50 ohms and the traces after should be 25-ohm signal-ended or 50-ohm differential. Again though, if the traces are relatively short from the transformer to the ADC, the trace impedance may not contribute significantly. You may find that 25-ohm single-ended traces become too wide.
    3. A 2:1 transformer driving the ADC - In this case the source is still a 50 ohm source, but the ADC termination is set to 100 ohm differential. The input trace to the transformer should still be 50 ohms, but the output traces should be 100 ohm differential (or 50 ohm single-ended).

    Regards,
    Matt Guibord