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AD8376 Filter Matching

Hi, actually I'm working in an interface between the ADL5380 and the AD8376 chip. The ADL5380 specfifies that using a high impedance load like for example 500 Ohm you'll get some  extra gain.

The AD8376 has internal termination of 150 Ohm differential.

The design decision comes now. I need to tune a filter between the ADL5380 and the AD8376.

From this point of view, the first alternative is

- To use a 150 Ohm diff like the load (directly using the input impedance like load) and to match it with a filter that adapt the 50 Ohm differencial in the source with this 150 Ohms differential in the load.

- A second option is to add a resistor of 37.5 Ohm in each terminal in order to get 25 Ohm SE in the load or 50 Ohm differential. In this case we'll have a filter between the 50 Ohm source and the 50 Ohm load.

Q1: The first alternative presents more gain in voltage. Does the second-one presents any advantage over the first-one?

Q2: There is any advantage using filters with the same load and source impedance?

Q3: Where is the limit of Impedance in the load? if the gain improves when it grows, why not use 1KOhm, 10K...?

Q4: When we make the conversion over the load resistance, if we increase the voltage, does the current decrease? It it's like this, does it have any implication?

Thanks in advance!

  • Thank you very much for your answers!

    About the first part, you are right, after the amplifier we have the ADC. We have here an AAF in this interface. The purpose of this filter between the demodulator and the amplifier is to avoid the spurious and non-linearities coming from the mixing product to came into the amplifier in the bandwidth between the 80MHz and the 380MHz.

    Just one more question:

    The ADL5380 takes power inputs and return the outputs in voltage, isn't it? Or it works in other way?

    How is this power to voltage gain conversion?

  • Hello,

    If my understanding is correct, the line up goes ADL5380, filter, AD8376, and then an ADC?  If there is an ADC following the AD8376, then the filter should go between the amplifier and ADC for the purpose of anti-alias filtering.  The demodulator already has a low pass filter profile with a 3dB cutoff at 380 MHz.  If your application and line-up is completely different and the filter should go between the demod and amplifier, then just ignore my comments.

    A1: The best configuration for a filter between the demod and the amp is to design a filter with a 50 ohm source and 150 ohm load.  This will provide you with minimal signal loss compared to the second option.  Another important note is the ADL5380 linearity performance will be much degraded when driving a 50 ohm load.  As the datasheet mentions, the demod should be loaded with a high impedance load. 

    A2: It is easier to design a filter with the same load and source impedance.  However, given the impedance mismatch of the load impedance of the demod and the input impedance of the amp, you would want to design a filter with different source and load impedances in order to maximize on signal amplitude.

    A3. Impedances greater than 500 ohms shows minimal gain improvement.  You can think of it like this.  The output impedance of the ADL5380 forms a voltage divider with its output load.  The output impedance of the ADL5380 is 50 ohms.  If you have a load of 500 ohms then 500/550 is 0.9.  If you continue to increase the load, the ratio increases very slowly.

    A4. The device has a limited current drive capability so you will need to simulate your circuit and verify it does not exceed the current limitations.  The current limitation is 12 mA.  You will want to design the filter with series inductors as the first components the demod sees and not a parallel capacitor.  It is more difficult to drive large capacitors.


  • Hello,

    Please refer to the attached document.  It should answer your question.