ADL5580 provided S-parameter

Question

Hello, 
 I am simulating the ADL5580 with the ADI provided S-parameters. 
 I have started to simulate the ADL5580 with input differential drive (100 Ohm differential source impedance). In this configuration, the differentially driven low frequency ADL5580 single-ende input impedance is very close to 50 Ohm and the voltage gain is 9.6dB. (consistent with the datasheet)

I then simulated the ADL5580 in the single-ended to differential configuration (see schematic below). The source resistance is 2 x 50 Ohm single-ended. I have noticed that additional termination resistances RT of 420 Ohm are mandatory to get a low frequency input impedance of 50 Ohm. 
 
 Questions about SE to Diff configuration: 
 
 without RT=420 Ohm, the low frequency SE input impedance is 55 Ohm. Is it something expected? Do you have a mean to calculate the optimum RT? 
 with or without RT, the SE input impedance "peaks" to 60 Ohm around 100MHz. Such behavior does not happen with the input differential drive. Do you have an explanation for this behavior? Is it an input common mode interference? 
 the low frequency differential input impedance is 100 Ohm. What is the min-max specification? 
 
 In advance, thank you for your answer. 
 Nicolas

Nao · Accepted Answer

Hi Nicolas 
 I ran S-parameter simulations in ADS at the single-end input condition, and got same results. this device was designed for differential amplifier so we don't have the optimum RT values today. Around 60 ohms peak at the single-end input mode might be something due to common mode interference as you mentioned. we don't have Min and Max values of input impedance today. S-parameter is the one for typical performance. to get 50ohms input impedance, RT seems to be required but the gain is degraded to due to RT. I could duplicate it at S-parameter simulation.)

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ADL5580 provided S-parameter