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Post subject: AD8309
Unread postPosted: Tue Apr 13, 2004
Our board has includes a limiter. The application
30-500MHz. We use the AD8309 device. This device has
an internal resistance of 1KOhm. Our source is 50Ohm. We therefore need
to match between those 2 impedances. According to the datasheet the
recommended way is to use a flux transformer. The impedance ratio is
16:1, the maximal bandwidth available for such impedances ratio is up
to 130MHz (according to the numerous number of transformers we checked
so far). To use a pad will dramatically reduce the SNR. Can anyone suggest
an alternative way (besides cascading two 4:1 transformers)?
Thanks in advance
Post subject: Just some thoughts...
Unread postPosted: Tue Apr
13, 2004 11:43 pm
Sun Aug 03, 2003 2:02 pm
Location: Erie, PA
I misunderstand your specs, the impedance ratio is actually 20:1, not
16:1. That means your transformer turns ratio should ideally be sqrt(20)
= 4.47:1, or about 4.5:1. This can be implemented with a 9:2 transformer
using integer turns.
Finding that transformer will be, as you
stated, difficult. However, if you are going to need a large quantity,
companies like Sprague Goodman and others can make a custom part for
you. Alternately, if you only need a few, you could wind your own (keeping
low IL across the band will be a chore).
Another approach might be a tradeoff of overall insertion loss where
you implement part of the impedance transformation with a resistive
pad and part with a transformer.
Yet another might be to use
a high speed buffering opamp that can be set for a 50 ohm input and
1 kohm output and still maintain the dynamic range that you need.
Just some thoughts...
- Kirt B. :mrgreen:
Wed Apr 14, 2004 10:10 am
Thanks a lot Kirt,
Engineer of Analog Devices suggested us a way to overcome this problem
(actually it is mentioned in the data sheet) - To connect resistor of
52.3 ohm in series with 4.7nH inductor, this gives a flat match of 50
ohm up to a frequency of 1GHz.
I was thinking like you of using
a fast op-amp (amazing how fast these devices are becoming)...but the
solution appears to be much simpler.