Use of MAX4090 with reconstruction filter

For a standard video signal, the video passband is approximately 6MHz and the system oversampling frequency is 27MHz. A 9MHz BW low-pass filter (LPF) is usually used as the reconstruction filter. The following discussion discusses how to build simple 2nd- and 3rd-order passive Butterworth low-pass filters with a 9MHz cutoff frequency, and explores the use of these filters with the MAX4090 ( Figures 1 and 4 ).

Implementation of 2nd order Butterworth low-pass filter

Table 1 gives the nominal component values ​​for a 2nd order Butterworth low-pass filter at 1 rad/s with a source/load impedance of 1Ω.

Table 1.

According to the following formula, L and C can be calculated when the cutoff frequency is 9MHz. Table 2 gives the approximate L and C values ​​as well as the -3dB bandwidth and attenuation at the 27MHz frequency point. It can effectively attenuate the aliasing noise caused by sampling, and the attenuation at the 27MHz frequency is 20dB. The MAX4090 requires low input impedance for stability, and the input impedance cannot be reactive. If R1/R2 is greater than 100Ω, a 20Ω to 100Ω resistor R _IS (Figure 1) needs to be connected in series between the filter and the input capacitor (C4) to avoid oscillation.