[SI-LIST] Re: question about voltage dividers
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: "'Roy M'" <roymesi@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 04 Mar 2009 08:54:20 -0800
You are forgetting about a few small details, the input impedance of
whatever loads the output of the divider. This in many cases will drive the
order of magnitude of the resistors in the divider. For example if the
input impedance is 50 Ohm you can forget about any high R values. You would
only need to use a single 150 Ohm resistor feeding the input directly in
your example. A 1K input impedance will limit the resistance to a max of
3K. You can always reduce the value of the equivalent R2 (R2//Zin) to lower
the R1 value. For example if the input Z is 1K you can parallel it with a
52.6 Ohm resister to get an equivalent input of 50 Ohms. Then you can use
the 150 Ohm R1.
Next is the loading on the source you are measuring. If the output
impedance of the source is 100 Ohms then loading it with 200 Ohms (150 +50)
will reduce the measurement to 2/3.
Lastly is the AC component of the input Z. How much C and the source drive.
What is the RC time constant of the system?
Regards
Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
503-430-1285 FAX
tom@xxxxxxxxxxxxx
www.teraspeed.com
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Roy M
Sent: Wednesday, March 04, 2009 12:36 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] question about voltage dividers
HI,
I have a general, rather basic, question concerning voltage dividers.
We all know that when it comes to voltage dividers, the *ratio* between the
resistors is what matters to the Vout value, and not the resistors'
*value*itself.
But if I know for instance, that the ratio is 1:3 (i.e, R1=3*R2), so I can
choose R1 to be 3k and R2 to be 1k, OR I can choose R1=3000k and R2=1000K or
(well, you got my idea...).
Now, I understand that a very low value means high power (cause there's alot
of current going to gnd) but what if I choose very high value? (like 1M and
3M),. it doesn't sounds good, but I can't explain why..
Help anyone???
Thanks,
Roy
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