ADS8320: With APA363, Aol Zero Formation

William Mcknight

Part Number: ADS8320

Hello Community,

On page 33 of the document attached, there are two equations with respect to a pole and zero being produced. Since this forms an LPF RC filter, the 1/ReqC makes the pole understandable, but I don't understand where the zero comes from. Could I please have a detailed analysis of how this zero is formed?

Thanks,

Will

7384.4478.PA-001 Optimize_SAR_converter_design REV b.pdf

over 5 years ago

0 Joseph Wu over 5 years ago

TI__Guru* 94055 points

Will,

If you have a purely capacitive load, the result is a pole added at the frequency determined by the output impedance of the op-amp and the load capacitance. As this is often unstable, you can add an external resistor, that adds zero to the load.

You can think of it this way: with no compensating resistor, Aol continues to drop as the equivalent load drops to 0 as the frequency gets higher. However, with the inclusion of the resistor Rflt this adds a zero. Over frequency the equivalent load drops, but stops dropping over frequency and levels out to the Rflt resistance of 100Ω.

There's a basic write-up in this design note:

www.ti.com/.../tidu032c.pdf

Joseph Wu

0 William Mcknight over 5 years ago in reply to Joseph Wu

Intellectual 370 points

Thanks Joseph for the resource and the explanation I really appreciate it! But what I was hoping for was a line by line derivation of the transfer function since for some reason I can't end up at the same solution.

0 Joseph Wu over 5 years ago in reply to William Mcknight

TI__Guru* 94055 points

Will,

Here's an alternate explanation from TI Precision Labs. It's in the Op-Amps Stability series, and discusses the use of an isolation resistor.

training.ti.com/ti-precision-labs-op-amps-stability-5

There's a video at the top of the page, but further down, you can find the slides separately. You can just download the slides here:

training.ti.com/.../1335 - Stability 5 - slides.pdf

Joseph Wu

0 William Mcknight over 5 years ago in reply to Joseph Wu

Intellectual 370 points

Joseph Wu you are my hero! thank you!!!

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ADS8320: With APA363, Aol Zero Formation