LMH5401: Question on imput impedance matching

Vincent GLEYZES

Part Number: LMH5401
Other Parts Discussed in Thread: THS4541

Dear Madam, Sir,

I am designing a digitizer with LMH5401 wired in SE/DE 4V/V on the input of my channels.

I am trying to calulate the impact of the tolerance of the resistors RT, RF, RG1 and RG2 that I choose on my equipment input impedance.

Thus, I have recalculate the input impedance of my equipment using Eq7 from the datasheet (see§ 8.3.2.4). For the moment I use datasheet values for my resistors:

- RG1 = 22.6 ohms

- RG2 = 66.5ohms

- RT = 357 ohms

- RF = RFinternal+RFexternal = 25+127=152 ohms

From Eq 7 I find ZA=54.58 ohms and thus from from §8.3.2.4 I can find my input impedance which is ZA//RT

This gives me Zin=47.27 ohms.

I am a bit surprised to find this value as we should find something closer to 50 ohms. I have measured on my board the input impedance which is definetely closer to 50 ohms than 47 ohms.

Could you help me figure out what is wrong on my calculation as I need to do be able to calcule Zin from RS, RT, RF, RG1 and RG2

Thank you in advance for your answer,

Regards

Vincent

over 2 years ago

0 Jon Alejandro over 2 years ago

TI__Prodigy 547 points

Hi Vincent,

There seems to be an error in the equation (7) in DS. With correct equation, I am getting 49.9 ohm instead of 47 ohm. Let me double check with my team.

Thanks,
Jon

+1 Michael Steffes over 2 years ago

Guru 48395 points

Well Vincent, I want to thank you for spotting this basic error I made when I mapped my original equations over to include in the TI FDA datasheet.

The solution equations giving you the R values are correct. I went back to my original work from Aug 2011 and found these expressions,

If I then try to manipulate these into what shows up in the TI FDA's (starting with the THS4541 datasheet and on forward from there, I actually get this expression for Za the active input impedance looking into Rg1,