[SI-LIST] FW: Re: question about voltage dividers
- From: Richard Jungert <r_jungert@xxxxxxxxxxx>
- To: si list freelist <si-list@xxxxxxxxxxxxx>, si list <si-list-bounce@xxxxxxxxxxxxx>, roy mesi <roymesi@xxxxxxxxx>
- Date: Wed, 4 Mar 2009 17:49:53 -0800
From: r_jungert@xxxxxxxxxxx
To: roymesi@xxxxxxxxx
Subject: FW: [SI-LIST] Re: question about voltage dividers
Date: Wed, 4 Mar 2009 17:44:21 -0800
Roy.
You might want to consider a TEE network or a PI network to do this. That way
you get good impedance control, the network will attenuate the signal and also
do the impedance matching. We call these types of networks pads in the RF
world.
These networks are very common in the RF world to do attenuation Ie: -10db pad
or a -20 db pad etc etc. If your interested search TEE network on the Net and
there are calculators out there just for this to help you get exactly the
values you need.
One can do 100 ohm to say 50 ohm impedance matching or 150 ohm to 75 ohm for
example.
Does this help?
Richard Jungert
> To: dbrooks7@xxxxxxxxxx
> CC: roymesi@xxxxxxxxx; si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: question about voltage dividers
> From: wolfgang.maichen@xxxxxxxxxxxx
> Date: Wed, 4 Mar 2009 09:41:28 -0800
>
> As a small refinement to below considerations - the effective (Thevenin)
> output impedance of a voltage divider is
> 1/Rout = 1/R1 + 1/R2,
>
> i.e. the load sees the two resistors of the divider in parallel as far as
> source impedance is concerned. (this formula assumes the impedance of the
> source driving the divider is much smaller that R1+R2). So a divider
> consisting of 1k + 1k has an impedance of 500 Ohm feeding the load. This
> impedance then should be much smaller than the load's input impedance
> (Doug's 100x is about the right ballpark if you want accuracy of around
> 1%; in many cases - e.g. termination or threshold voltages - it's often
> possible to loosen the requirement to e.g. 5 or 10%, so around 10x is
> sufficient in that case).
>
> Another consideration can be power consumption - if the divider is used to
> generate a bias or threshold voltage, with the divider sitting between Vcc
> and GND, then the shunt current through the divider is effectively wasted.
> Larger resistance values help in that case. Of course that only works if
> the divider doesn't have the additional task of providing signal
> termination and needs to have e.g. 50 Ohm Thevenin resistance (like in the
> case of some PECL termination schemes).
>
> Wolfgang
>
>
>
>
>
>
> "Doug Brooks" <dbrooks7@xxxxxxxxxx>
> Sent by: si-list-bounce@xxxxxxxxxxxxx
> 03/04/2009 09:10 AM
>
> To
> "Roy M" <roymesi@xxxxxxxxx>
> cc
> si-list@xxxxxxxxxxxxx
> Subject
> [SI-LIST] Re: question about voltage dividers
>
>
>
>
>
>
> In the GENERAL CASE, the answer is pretty straightforward. When you say
> that the *ratio* is the determining factor, you are making two implicit
> assumptions:
> 1. The resistor divider is not loading down the driver circuit, and
> 2. The load is not loading down the resistor divider.
>
> For the first assumption to be true, R1+R2 must be greater that about 100X
> the Zout of the driver, or R1+R2>100*Zout (Some will quibble over whether
> 100X is the right number!!)
>
> For the second assumption, the "bottom" resistor, R1 or R2, must be less
> than about one percent of the input impedance of the next stage, i.e.
> Zin>100*(R1 or R2).
>
> IN GENERAL, any set of values for the voltage divider that meet these two
> assumptions are fine.
>
> There may be SPECIFIC cases that limit the R values. Some possible
> examples:
> a. If the Zin is *so* high that parasitics might come into play
> b. If the frequencies are *so* high that parastics may come into play (or
> SI issues effect the selection)
> c. Noise issues are *so* extreme that special materials might be used
> d. Power requirements are *so* extreme that power must be minimized at all
> costs....... (etc.)
>
> But these SPECIAL cases do not change the GENERAL case, only possibly put
> boundaries around it.
>
> (My book has a section specifically devoted to resistive voltage dividers.
>
> Doug Brooks
> http://www.ultracad.com
>
>
> > 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 R1000k and
> R200K
> > 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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