AD8361 Output Offset Voltage

Document created by enash Employee on Apr 9, 2013
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Question:  I have an evaluation board for AD8361.  With the board powered and no RF input connected the output is greater than 150 mV eventhough the specifications call out a maximum Output Intercept of 150mV.  Is there something wrong with his part since its output is 157mV?


Answer: Don't confuse Output Intercept with output voltage. The intercept is an extrapolation of the transfer function to the point at which it hits the y-axis. Or to put it another way,  it is the output voltage that the part should provide for 0V in if the transfer function was fully linear, even at very low input levels. In practice, the transfer function becomes non-linear at low input levels (e.g. below 20 mV rms or -21 dBm). For some devices, the transfer function flattens out, yielding what looks like an output offset; for other devices, the output voltage will collapse to 0V.


Take a look at Figure 50 in the ADL5501 Rev. B datasheet.  This will give you a better sense for what happens with these devices. Incidentially, ADL5501 is probably a better choice than AD8361. It's not that AD8361 is not recommended for new designs. ADL5501 is just a newer better part.