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AD9467 : Passive input drive

Question asked by Aot on Feb 24, 2012
Latest reply on Aug 24, 2012 by Aot

Hi there,

 

As I mentioned in the title, I have an AD9467 adc and I want to drive it passively.

My first approach was like the Figure 51 on page 20 of the Datasheet.

http://www.analog.com/static/imported-files/data_sheets/AD9467.pdf

 

But when I analyzed the schematic I saw that the parallel 33Ohm resistors (for the 50Ohm match) consume around 11dB signal power.

At the adc itself only around +1.7dBm signal power is necessary to drive it to full-scale.

So this circuits generates less noise than the actively driven variant but needs an horrific amount of "unused" signal power. So it generates around 22dB more IM products than needed! (in respect to the leading amp)

So it's not an option for me.

 

My question:

Did anyone tried another passively drive circuit successfully? (and with less or non disandventages)

Why are there 2 Baluns necessary and why do you connect them so that you match the ADC to 50Ohms?

Wouldn't  it be better to use an 1:4 Balun and transform the input 50Ohm match to 200Ohm? So the matching circuit doesn't have to waste so much signal power?

What are the disadvantages when I use 1 balun instead of 2 baluns?

 

In addition:

No it's not an option to switch to an actively driven variant. Because at the ADC input I have an input Signal around +3-4dBm. With the actively driven variant, I would have to attenuate the signal first and the following differential amp ADL5562 (NF~8dB) would increase the noise too much.

And on the other hand I could use the adc passively driven with an input signal power of +12.5dBm but in this case I would have to use a low noise amplifier with an minimum gain around 12-15dB. And of course the low noise amp have low IP3 characteristics so I have to much IM products and I have also to attenuate the signal first.

An general purpose amp with a great IP3 usually have a to high NF.

So if I manage to decrease the amount of "unused matching" power, so that it fit directly to my +3-4dBm input signal, it would give me the best performance.

 

Thanks for your help

Sincerely

Alexander

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