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LMH5401: Question on imput impedance matching

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