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Re: "Best op-amp for analogue filter?"

Posted by Jacques (A) on October 12, 2006 at 15:15:40

In Reply to: "Best op-amp for analogue filter?" posted by lovekylie on October 12, 2006 at 11:54:03:

You need a somewhat fast amp in order not to jeopardize the sinx/x lowpass filter curve through too small a gain-bandwidth product of the amp (the effect is that the values of the filter passive component don't fit anymore to give the required filter's parameters)
A newbie in this matter (note that the datasheet is from Sept'06 !) is National's LM4562.

Seems pretty nice, and would fit easily in your design.

Beware of its power supply, however.
It should need correct decoupling. I mean:

• series resistor 10ohm on each rail,
• 0.22uF25V ceramic SMS cap paralleled with:
• 10uF25V Ta between each rail and the return of the load -not the ground plane!-

Some caveats:

• Ceramic caps at less than 0.5cm from the chip's supply pins.
• Series resistor can be remote say several cm.
• Useless to use plastic caps instead of ceramic for the HF decoupling, the high dielectric absorption and high variation of capacitance with voltage are not drawbacks for decoupling highspeed ICs. On the contrary, the advantage of very low series inductance is more important in this use

Note that the distorsion curve shows a class B or AB ouput stage, with distorsion grpowing below some level.
If you want to work in class A, you should add some very high impedance current source at the output. That's another topic, if interested, it will be for another post, it's late here.
Good luck and good music !

• LM4562   (Open in New Window)

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