Hi, I'm currently designing my own Software Defined Radio. I'm already done with most of the front end but now I have to design the ADC and VGA circuit
I decided to use a LTC2205 ADC and a LT5554 VGA, the specs seem perfect for what I want to do, however, I don't understand a thing to how I'm supposed to pair them up
First of all, I need to convert the single ended signal to a differential signal with a bias voltage for the VGA. I know this should be easy to do with a 1:1 transformer with a center tap on the secondary
But here comes the part where I'm lost:
The VGA apparently needs some sort of constant current at the output?
The datasheet says:
"A transformer with a center tap tied to VCC or a choke inductor is recommended to conduct the DC quiescent current"
What does that mean ?
Also, the output of the VGA and the input of the ADC don't have the same impedance, how can I match them correctly to get the best results possible?
And if that wasn't enough, the ADC needs a bias voltage on the input
I's also very important that the matching circuit works from as close to DC as possible, to at least 30MHz
I'm pretty much completely lost at this point
And if you couldn't tell already, I'm a complete n00b at ADCs and RF. It's my first project even with both of these
So, could someone help me understand why the VGA needs a bias current and maybe suggest a circuit?
Thanks in advance