diode work at Hi-freq.

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david753
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diode work at Hi-freq.

Post by david753 » Tue Jan 04, 2005 3:15 am

I am looking for a diode that works at hi freqency(40mhz).
I'd like to put it in front of analog input pin of AD9057 to limit the input voltage.
Does anybody can tell me? which brand? and model NO.?

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Edd
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Re: diode work at Hi-freq.

Post by Edd » Tue Jan 04, 2005 8:50 pm

That is already taken care of with the clamp diodes designed within the input stage of the
analog input on pin 7.
http://www.analog.com/UploadedFiles/Dat ... 9057_d.pdf <p>73's de Edd
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Ron H
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Re: diode work at Hi-freq.

Post by Ron H » Wed Jan 05, 2005 6:58 am

I doubt that the input diodes are designed to handle more than a few milliamps. See p.32 of this document from Analog Devices<p>[ January 05, 2005: Message edited by: RonH ]</p>

david753
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Re: diode work at Hi-freq.

Post by david753 » Wed Jan 05, 2005 8:38 am

indeed, there is a pair of diodes embeded in AD9057 in front of analog input.
But, in the real situation, the output is not correct when the input signal is over voltage (about 2-3Vpp).

Ron H
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Re: diode work at Hi-freq.

Post by Ron H » Wed Jan 05, 2005 11:16 am

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by david753:
indeed, there is a pair of diodes embeded in AD9057 in front of analog input.
But, in the real situation, the output is not correct when the input signal is over voltage (about 2-3Vpp).
<hr></blockquote><p>If you want to limit the input to the full scale range of the A/D, have a look at
AD8036 or AD8037.

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haklesup
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Re: diode work at Hi-freq.

Post by haklesup » Wed Jan 05, 2005 2:36 pm

Wether you need additional diode clamping comes down to what type of overstress you anticipate. Most ICs have input protection that can withstand several hunderd mA for a few hunderd ms but less if it were continuous.<p>As for the speed of the clamp diode, it probably dosen't matter when you consider it is in parallel with the one in the device's input pin. The current would be split between the two. The appnote RonH referenced recommends Schottky diodes perhaps because they have a lower Vthreshold (roughly equivelent to lower resistance in this case) which would cause it to share a greater load of the overstress.<p>In any case, if you are sampling voltages within a circuit, you probably don't need extra protection. However, if the A/D is connected to the outside world via a probe of some sort, extra protection is a good idea. Around here we usually just use 1n4148 but probably just about any diode would work out.

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