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AD9257 Expected VIN Load

afoote
on Jul 11, 2014

Hello,

I am wondering what the recommended load (50Ω, 100Ω, 1kΩ, etc.) is for the VIN+/- input pins of the AD9257. We will be potentially using the ADA4930 as a single-ended to differential driver before the ADC, and I'm not sure what load to put on the outputs of the driver.

Examples from the AD9257 datasheet on page 21 show an expected 100Ω load for the CLK+/- input pins, but I cannot find similar examples for VIN+/- pins.

Thanks for your assistance.

  • 0
    •  Analog Employees 
    on Jul 12, 2014 7:29 AM over 6 years ago

    Hello Mr. Foote,

    The 100Ω terminations for the clocks are to comply with the requirements of the respective signaling standards (e.g. for LVDS, etc.).

    The analog input pins of the AD9257 are relatively high impedance, so the load that the signal source sees will largely be a function of components external to the ADC, and not the ADC pins themselves. If you are using a passive analog input network, the termination impedance close to the ADC inputs is “seen” by the signal source through the transformer, so the desired termination resistance close to the ADC inputs will depend on what the source impedance of the signal source is (commonly 50Ohms). Examples of this type of configuration are shown in Figure 46 and Figure 47 of the AD9257 datasheet.

    In the case of using an amplifier/driver, the output load of the driver is isolated from the input, so in this case the choice of the load value is not influenced by the source impedance of the signal source. In the case of the ADA4930, I believe it is designed to work over a fairly large range of output impedances. Often there will be a low pass filter between the amplifier and ADC inputs, which will determine the output impedance that the amplifier sees. The ADA4930 datasheet has some examples of this in Figure 59 and Figure 60.

    I am hoping that the having the keyword “ADA4930” in this exchange will prompt a more complete response from the amplifier people. If I don’t see any additional reply in a few days, I’ll ping them on my side and ask them to have a look at this.

    Perhaps you have already seen this, but there is a Diff-Amp-Calculator that is available free from ADI. I have found this useful in playing with amplifier configurations. It can be found at http://www.analog.com/en/amplifier-linear-tools/adi-diff-amp-calc/topic.html .

    Please take care.

    Doug