Question
According to Figure 50 in AD9613 datasheet, I want to know how to calculate the
impedance matching here. In datasheet of AD8376, Table 5 includes antialiasing
filter component recommendations for popular IF sampling center frequencies. My
center frequency and bandwidth are different from the table. I want to know how
to calculate this, how to calculate the L5 and why the paralleled resistor is
301Ω. Could you please help?
Answer
When designing the filter you will need to determine the source and load
impedance of the filter. In this case the source and load impedance was chosen
to be 300 ohms. Often times when interfacing to an ADC, especially a pipeline
ADC where the input impedance is not purely resistive, a fixed impedance is
placed across the ADC inputs. People normally put a fixed resistor between 200
to 500 ohms across the ADC inputs. Looking at the schematic below, 165 + 165 =
330 ohms. This is the load resistance of the filter. The output impedance of
the AD8376 is 16K ohms. The parallel combination of this 16K with the 301 ohms
is approximately 300 ohms. Therefore it looks like the load and source
impedance of the filter is approximately 300 ohms. With this information you
can use a filter design tool like ADS to design your filter. The first
component should be a series inductor for ease of drive-ability and not a
parallel component. Hope this helps you get started.