LM6172QML-SP: Input capacitance questions

Kannan Sockalingam

Part Number: LM6172QML-SP

Hi guys, working with a customer on this device. Using it as a non-inverting op-amp following a DAC and R2R conversion to set a voltage level input that then feeds into a comparator. Trying to understand if we need to impedance match at the opamps inputs.

1. Is the opamp bias compensated (ie internal bias currents are well matched)?

2. Trying to understand how the input capacitance may affect the opamps bandwidth and stability. To do this could you recommend a value to use for input capacitance (CM and DM) of the opamp?

over 4 years ago

0 Hasan Babiker31 over 4 years ago

TI__Genius 15555 points

Hello Kannan,

1. The bias currents for both amplifiers are most likely matched, you can take a look at the blog post here for more info https://e2e.ti.com/blogs_/archives/b/thesignal/archive/2012/10/30/matchy-matchy-how-alike-are-dual-op-amps

2. The SPICE model suggests the part has a total input capacitance of 0.5pF, however I would expect it to be larger at at least 2pF.

Note that these are both assumptions and we do not have data characterizing either of these parameters.

Best,

Hasan Babiker

0 Michael Steffes over 4 years ago

Guru 48395 points

Hey Kannan.

The model in the TINA V11 library shows 1pF on each input as common mode caps, no Cdiff that I can find, the comments apparently follow the elements here

Here was an article on this topic,

https://www.planetanalog.com/input-impedance-extraction-and-application-for-high-speed-amplifiers-insight-9/

0 Michael Steffes over 4 years ago in reply to Michael Steffes

Guru 48395 points

I had sent this link to someone else recently, and was reminded that planet analog broke the links to expand the figures so they are unreadable - here is the original pdf,

6648.Input Impedance Extraction and Application for High Speed Amplifiers May29.pdf

0 kai klaas69 over 4 years ago in reply to Michael Steffes

Guru 140200 points

Hi Michael,

thanks for the pdf. It's much easier to read :-)

Your whole article series in the "planetanalog" is really great. I have learned a lot.

Thanks a bunch!

Kai

0 Michael Steffes over 4 years ago in reply to kai klaas69

Guru 48395 points

Thanks Kai, I have another 4 more to do on noise, but not moving too quick - they are not doing that well over there - the original editor I was working with was laid off last August, two editors later .....

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LM6172QML-SP: Input capacitance questions