The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: "Tenho Hatonen"
Subject: Re: How do i measure signals without affecting them ?
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Tue, 19 Nov 2002 14:00:42 GMT
NNTP-Posting-Date: Tue, 19 Nov 2002 16:00:42 EET
Organization: Sonera corp Internet services
> I want to measure the pulses from various devices, all of which put out
> sort of pulse. Ie, hall effect device that has already been conditioned to
> be a square wave.
> How would i interface a PIC with the same wire the sensors main signal is
> without affecting the signal ?
> I have been told i can use a rather large resistor (in the megaohms) in
> conjunction with a schmitt trigger to do it cleanly, but i dont quite
> understand how the voltage going through the resistor would be enough to
> activate the schmitt?
> If anyone knows of some ways to do this please post them.
> Also, what is the formal name for this type of interfacing so i can do a
> proper search for more info ?
as far as you are measuring digital (logic 1/0) signals, the maximum load
you can put on the signal is smaller than the one that causes the
receiving gate NOT to recognice the change of the logic state. The different
logic families have different sink/source ability, i.e how many milliamps
they are able to sink/source at the output. For example a CMOS might (varies
from type to type) have 2.5mA sink and -3.0 mA source while a basic 74xx
sinks 16mA and sources -0.8 mA.
Obviously, the load connected to the output of the logics must not overcome
it's sink/source ability.
If you can connect the PIC directly to a signal source depends on the type
of signal you are monitoring. If it is digital, has its sink/source
compatible with the PIC, the voltages (PIC Vcc and measured pulse) match and
the frequency range is ok, you can connect the PIC directly to the signal.
If the signal to be measured is analog, you have to know the output
impedance of the circuit that creates it and you must also know how much you
are allowed to "disturb" the signal. For examle : suppose the output
impedance is 100 kohm and 1% maximum voltage drop due to the measuring
induced load is allowed, the input impedance of the measuring circuit must
be at least 9.9Mohm. The reactive input/output impedances complicate the
calculations required a bit, but the principle remains the same: the source
and load impedances must be known and the descision about the need for some
additional buffering must be made according to the required accuracy.
You may find some info by searching the data sheets of the devices you have
in mind and looking at the input/output impedances and sink/source currents.
Hope this helps,
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup