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I am designing a signal chain that includes a 4 pole cascade Sallen-Key filter based on the AD8034. The circuit will be used to shape tail pulses (~10ns rise time, ~50us decay) into gaussians. The filter design is taken directly from page 20 of the AD8034 datasheet. I would like to include two AD8009 gain stages before the filter as shown in the schematic below. Is a buffer stage separating the AD8009 from the AD8034 necessary in this case?

schematic

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  • \$\begingroup\$ I don't see why you would need one? Pin 3 of U7A draws no current, so what is the buffer for? \$\endgroup\$
    – user253751
    Aug 3 at 18:37
  • \$\begingroup\$ Some of your resistor values are very low. My rough rule: If an opamp is driving less than 1k impedance, I make sure that the opamp can drive the output to the expected voltage. \$\endgroup\$
    – Mattman944
    Aug 3 at 19:30
  • \$\begingroup\$ @Mattman944 Are refering to the feedback resistors on the AD8009s? I got these values from the datasheet for G = +10 (page 2 of datasheet). \$\endgroup\$
    – earl
    Aug 3 at 19:49
  • \$\begingroup\$ The AD8009 is OK, it is a video amp, much more powerful than a normal opamp. The AD8034 is more of a normal opamp and it has 100 ohm loads that I don't see any reason for. \$\endgroup\$
    – Mattman944
    Aug 3 at 21:32

1 Answer 1

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The purpose of a buffer stage is to prevent the current drawn by the next stage from affecting the voltage output of the previous stage, i.e. to bridge a high-impedance output to a low-impedance input.

The inputs of U7 AD8034 have very high impedance - at least at DC. For AC, I guess working it out would require some complex calculations I'm not familiar with. The input capacitance as specified as "a few pF". Adding a buffer stage would have the same capacitance in the buffer stage so it wouldn't improve anything, anyway.

In fact, if you look at how the circuit is constructed, U7A is a buffer stage!

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  • \$\begingroup\$ Wow thanks for the quick answer! Very helpful. \$\endgroup\$
    – earl
    Aug 3 at 19:11

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