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How can I match 100K om to 12K om - RF Cafe Forums

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cpeter
Post subject: How can I match 100K om to 12K om
Unread postPosted: Wed Mar 09, 2005 11:32 pm
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Joined: Wed Mar 09, 2005 11:20 pm
Posts: 2
I am doing a design:

AD9854 -> differential & quadrature signal -> 80MHz filter & match network? -> AD8346

And:
output impedance of AD9854: 100k om
input impedance of AD8346: 12k om

how can I design the filter & match circuit?

Thanks.
Peter


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Guest
Post subject: Impedance matching?
Unread postPosted: Sun Mar 13, 2005 10:18 pm

Peter,
Your question leaves out some important information - such as the frequency range involved.

The impedances you cite are only accurate at very low frequencies. There is significant reactance present at any reasonable RF frequency. (There's both bond wire inductance and bonding pad capacitance to ground).

The implication is that you probably don't want to match 100k to 12k, since you really won't have it anyway! Digital-to-Analog converters are often driven into the summing junction of an operational amplifier - which is a "virtual ground". From the classical RF viewpoint, that's weird - but the DAC output is often essentially a current, which must be converted to a voltage.

Check out Analog Devices' application notes on the AD9854 - I'm sure that they don't show the kind of impedance-matching network you're asking for.

Good Luck!


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cpeter
Post subject: Thanks.
Unread postPosted: Wed Mar 16, 2005 1:47 am
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Joined: Wed Mar 09, 2005 11:20 pm
Posts: 2
You answer is very helpful, thanks a lot!
:D







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