Discussion in 'Electronic Basics' started by aman, Apr 23, 2005.

1. ### amanGuest

I have seen in some analog circuits that if we need a 2.5 V supply
there is way to do it that generate a 5V square pulse with duty cycle
1/2 and pass it through a low pass filter to get a DC 2.5V.

I was thinking of another way of generating 2.5V. Use a voltage divider
and then use an opamp buffer.

Which of the above 2 ways is a recommended way of generating 2.5V DC?

2. ### John PopelishGuest

This method has problems with additional voltage drop through the low
pass filter when the load on the 2.5 volt output uses current.
As long as the load doesn't upset the opamp (less current than its
output can deliver without current limiting, and capacitance to ground
that does not make the buffer unstable) this can work fine. You can
also add a capacitor from the divider output to ground to clean up any
high frequency noise that was present on the 5 volt supply. The 2.5
volt output will be less accurate than the 5 volt supply (must include
divider tolerance and drift and opamp offset voltage).
I prefer the second for most low current needs. There are also fixed

For example:
http://www.national.com/ds/LM/LM117.pdf
http://www.national.com/ds.cgi/LM/LM4140.pdf
http://www.national.com/pf/LM/LM4120.html#Datasheet

These may improve on the voltage accuracy of the 5 volt supply.

If the load current is more than a few tens of milliamps, you may want
to look into a switching buck regulator, because of its higher efficiency.

3. ### amanGuest

I just need 2.5V to be at one of the inputs of the opamp. I will be
using the virtual ground property of opamp getting 2.5V at the other
input of the opamp also. I dont need practically any current to be
drawn by the load because after opamp theren will be only a schmitt
trigger(comparator). So that you know this is part of an water overflow
detector. Still you think the regulator or voltage divider are good
ideas for this.

4. ### John PopelishGuest

A simple two resistor divider may be all you need.
If only an opamp input is connected to it (no current load except the
opamp bias current) a pair of 100k resistors may be stiff enough.

5. ### JamieGuest

normally a circuit like that is used to generate - voltages from a
+ source like in battery operated equipment etc. .
if you already have a + supply and that is all you need, then using
a simple Voltage regulator from the main source should work.
something like a resistor and zener combo or a fixed reg. etc..

6. ### JamieGuest

thats what she said!