[SI-LIST] Re: question about voltage dividers
- From: DAVID CUTHBERT <telegrapher9@xxxxxxxxx>
- To: Roy M <roymesi@xxxxxxxxx>
- Date: Wed, 4 Mar 2009 06:58:46 -0700
Roy,
the resistor value is driven by the power dissipated, the frequency response
needed, and the circuit loading that is allowable. The circuit loading
includes both the source and the receiver. That is, is the divider designed
to drive a 50 ohm circuit or a 1M ohm circuit?
A high voltage divider usually drives the design to high resistor values to
keep the power dissipation low enough. High frequency response drives the
design to low resistor values if a pure resistive divider is desired. A high
frequency, high resistance divider can be built if it is an RC divider.
Here is an example of a 10:1 voltage divider with an input resistance of 10M
ohms. 9M ohm resistor on top and 1M ohm resistor on the bottom. Let's say
the bottom resistor is driving an op amp having an input capacitance of 3
pF. The frequency response of this divider will be 1/(2.2RC) = 167 kHz. Note
that R is the Thevinin equivalent of 9M || 1M. To obtain a higher frequency
response the resistive divider can be paralleled with capacitors. In this
case a 0.3 pF cap across the top resistor does it. For a practical divider
there is more to it than this. We are concerned about stray capacitance, the
inductance of the bottom capacitor, and damping the resulting resonant
circuit.
For the 10M/1M divider shown we might make the top capacitor 4.7 pF and the
bottom capacitor 47 pF (including the Op Amp input C). With an SMT cap and a
good layout the bottom cap may have an inductance of 3 nH. Now our divider
is resonant at around 400 MHz. A damping resistor of sq rt L/C can be placed
at the top of the divider. The value is about 10 ohms. At high frequencies
the divider input Z looks like 10 ohms. Not such a high Z divider after all,
is it?
Designing accurate high frequency, high R, high fidelity voltage dividers is
a challenge. The receiver is another challenging design issue. Want to drive
a long coaxial cable and you introduce another layer of design
considerations.
Having designed many such dividers I know the challenges well. If you look
at the commerical oscilloscope probes you will see that hi-Z probes go no
higher than 500 MHz. And this number is achieved with the probe plugged into
a custom 25 ohm fixture. In the real world one would never obtain the
specfified bandwidth.
Dave Cuthbert
On Wed, Mar 4, 2009 at 1:36 AM, Roy M <roymesi@xxxxxxxxx> wrote:
> 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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