question about fig 52 in ADL5569

Hi Ramin,

I moved your query to its proper community RF and Microwave.

Regards,

Goz

Are you sure? I’m puzzled because ADL5569 is a differential amplifier meant to drive ADCs, hardly an RF&MW part?

Regards/Ramin

Hi Ramin,

The biasing on the input and output, with the 650 ohms on the input and 750 ohms on the output are to set the common-mode voltage in that particular application, fig 52.  Depending on your application supply voltage rails and desired common mode voltages on input and output, these resistor values will change.   

Thanks,

Darrell

Hi Darnell,

I figured out that much, but my question is really this: Why you need all these biasing to set the output common mode voltage when all you have to do is to set the desired voltage on the Vcom pin which by the way you do as well in addition to all the biasing. By the way what voltage appears on Vcom as a result of that voltage divider between +3.3V and -2.0V? And one more thing don’t you lose some gain because of the these input and output biasing?

Regards/Ramin

Hi Ramin,

The reason for the external bias resistors is due to "internal feedback paths" within the device, that effect bias common mode points.  I'm attaching a draft discussion paper that helps to  illustrate further. The Vcom that you see in fig 52 is 1.4 volts, as a result of the external voltage divider and internal device impedance.  Finally, the 650 and 750 ohm resistor values have minimal effect on gain, with input and output impedance of 100 ohms. 

Thanks,

Darrell

PDF