Creating a high impedence probe

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Ron Hayes
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Creating a high impedence probe

Post by Ron Hayes » Wed Feb 12, 2003 3:51 pm

Hello, I'm trying to make a logic analyzer using my parallel port but I don't want to load down the target circuit, what would be the best way to do this?
My first thought would be to use a MOSFET since they have a high input resistance.<p>Any ideas?<p>RonH

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Edd
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Re: Creating a high impedence probe

Post by Edd » Wed Feb 12, 2003 3:59 pm

Ron:
How about starting with a 10x scope probe and its ~10 meg isolation. I don't experience any problems except on the feeblest....millivolt level.....probings in even RF circuitry.<p>73’s de Edd
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[email protected] (Firewalled-Spam*Cookies*Crumbs)<p>[ February 12, 2003: Message edited by: Edd Whatley ]</p>

russlk
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Re: Creating a high impedence probe

Post by russlk » Wed Feb 12, 2003 8:03 pm

Since you are using the parallel port, you must be looking at logic signals. The solution is to use CMOS logic buffer such as 74AC244 or 74HC244.

Ron Hayes
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Re: Creating a high impedence probe

Post by Ron Hayes » Wed Feb 12, 2003 8:25 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by Russ Kincaid:
Since you are using the parallel port, you must be looking at logic signals. The solution is to use CMOS logic buffer such as 74AC244 or 74HC244.<hr></blockquote><p>I actually did think about using that, I have a couple 74HC244's here but I wasn't sure if it was the right way to go or not, I guess I'd have to use resistors on the inputs to limit the current draw?<p>Scope probes would work but since I'm making an 8 input logic analyzer I didn't think that would be cheap enough for me, I'm acutally making this to see what is happening in another circuit that I'm making( neccesity is the mother of invention ), a PS/2 keyboard emulator.

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Re: Creating a high impedence probe

Post by natcsparky » Thu Feb 13, 2003 10:52 am

Also, consider that your parrallel port will be extremely slow (real time) in comparison to logic analyzers which have local memory then transfer results in non-realtime. I have played with the parrallel port with a pentium class (mmx 200) and was able to handle about 100KHz aggregate (meaning that 8 channels would be approx 12 KHz). If this is fast enough then a logic level buffer (the HCT is my favorite for input capacitance and impedence). A resistor won't hurt, but may not buy you anything, since the input bias current is so low on these things anyway. Have fun!
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Ron Hayes
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Re: Creating a high impedence probe

Post by Ron Hayes » Thu Feb 13, 2003 3:05 pm

Thanks for all of your help<p>I used a 74LS244 and it's working like a charm, the probram that I', using was written in ASM and is fairly fast, I'm not too concerened about realtime being in windows I never expected that, I just wanted to see the relationship between the lines. I'm running a P3/733 and it's doing a good job, I can get 10us without a problem, that's fast enough for me, now I just wish I didn't have a stupid 65K barrier!<p>Now I can work on my original project a keyboard emulator, if anyone has had any luck breaking the PS/2 protocol I'd love to hear from you.<p>Thanks

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Re: Creating a high impedence probe

Post by Dean Huster » Thu Feb 13, 2003 6:20 pm

Resistance is rarely the culprit when it comes to loading of circuits by things like scope probes .... it's the capacitance. While an X10 attenuator probe for a scope presents a 10M ohm load to the circuit, the capacitance can be as low as 227 ohms of Xc at 100MHz for a 7pF capacitive load. That's why active probes are so popular for reducing the load. Yes, their resistance is high, but their capacitance is usually less than a picofarad.<p>Don't forget that the use of an attenuator probe also means a loss of sensitivity!<p>Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

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