# How do I calculate the impedance matching values of R1 and R2 for the ADL5561..?

Document created by analog-archivist on Feb 23, 2016
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### Q

How do I calculate the impedance matching values of R1 and R2 for the ADL5561
in Single-ended mode using a gain of 11.1dB?

### A

In this case, RG=100ohms and RF=400ohms and according to Table 1 in the
datasheet the input impedance =179ohms when Rsource=50ohms
This means that the ADL5561's +ve input sees 179ohm impedance. Therefore,
R2//179ohms = 50ohms and solving this you obtain the following:

(179 x R2)/(R2 + 179) = 50

179R2 = 50(179 + R2) => 129R2 = 8950, where R2=69.38ohms and rounded-down to
69ohms

The net impedance seen by the noninverting input is:

RF + R2//Rsource, where R2//Rsource = R1

= 400ohms + 69.38ohms//50ohms

R1 = 69.38//50 = (69.38x50)/(69.38+50)] = 29.06ohms and rounded-up to 29ohms

These R1 and R2 values correctly correspond to the values shown in Table 6 of
the datasheet (Rev. F).

The following Application Note, AN-0990, also show how to calculate the
matching resistors for a single-ended and differential input for a differential
amplifier:
http://www.analog.com/static/imported-files/application_notes/AN-0990.pdf