how to make proper voltage divider

[garg29]

hi friends,
i just want to know solution for a small problem, how can I make proper voltage divider. As in attachment i have shown 3 different circuits, all of them will provide same voltage, but which one is correct & why? (sorry if it's to simple question, as i'm not a electronic student)

Thanks a lot in advance,

with regards,
Garg

 

[mohankumarv]

the first circuit with resistors 1k is correct.

 

[manu]

The one with 1k resistors is immune to noise but isn't "power saving" in low-consumption circuits. The one with 100k resistors is but it will work bad in noisy environments.

 

[garg29]

Welcome Mr. Mohan to edaboard. & Happy Diwali. Regarding circuit as manu & u say 1k circuit is correct, keeping noise problems in mind, I'll try out with this.

Thanks once again

Regards,
Garg

 

[LvW]

May I try to give an answer which is a bit more detailed ?

Well, all three circuits are "correct". This is an indication for the fact that - in theory - every circuit synthesis leads to a variety of infinite solutions.
Now, what shall we do with infinite options to design something ?
We have to choose the alternative which gives the best result under real world conditions.
And this leads to the question: Is a resistor a resistor ?
The answer is NO. Instead it is a combination of R´s and L´s and C´s (as can be shown in an equivalent circuit diagram).
Fortunately, in some or most cases, this is only a theoretical question - as far as all the parasitic effects play a negligible role.
Therefore, what are you doing with the voltage divider ? Is the working frequency 10 Hz or 100 MHz ? Or is noise an important issue ? Or power dissipation ?
In summary: The desired application together with some other considerations (power, cost, noise) dictate the choice. Under ideal conditions all divider circuits can be used - however, the world is not always ideal....sorry

 

[stabby21]

Like as LvW said, all are true.

Why not include the noise as it is not a DC source but a broad spectrum of frequency.

It is a matter of what will you do to the output, which in your example is 2.5 volts.

The impedance makes the three different. In your example, the impedances are 500 ohms,5k and 50 k respectively.

Using Thevenin's approach, it's like a source with 2.5v input with its impedance.

 

[garg29]

Thanks everybody once again.
Regarding stabby21, I'm trying to learn basics of analog circuits, considering about the 'where the output will be used', most of the time i use these dividers for some analog voltages measuremnts. I may feed this signal to opamp. but, again, my basic aim is to learn.
one thing more, still very basic, why does the circuit with higher value resistors become more prone to noise?

Thanks.

With best regards,
Garg

 

[LvW]

Thanks everybody once again.
................
one thing more, still very basic, why does the circuit with higher value resistors become more prone to noise?

Starting with noise POWER DENSITY and using the relationship between thermal and electrical power you can derive the following equation:

Noise VOLTAGE Un=sqrt (k*T*B*R) with k=Boltzmann constant, absolute temperature T, B=noise bandwidth, R=resistor.

That the relation between R and noise.

 

[kingto0609]

1k resistors should be correct

 

[garg29]

thanks kingto0609 for replying. 1k combination has already been suggested, but still, i want to know your reason behind this.

 

[Siniša]

When considering voltage divider, you have to consider load as well.

If your microcontroller or amplifer that requires voltage from such voltage divider you have to consider this:

What is maximum ammount or error in application in question?
Is there a low power consumtion requirement?
Is high frequency involved and how high?
What is load current draw/ input impedance?
Does load have spikes in current draw ?
Does power supply has interference or noise that needs to be supressed?
What is bandwidth of signal needed?
What is the temperature range of this circuit?
.... and probably many more that I forgot...

If you address all of these questions you could write a book about just a simple voltage divider, as LvW already pointed out infinity of possible solutions. So to limit our questions, let's assume that is voltage divider is to work only on dc and without temperature consideration.

If that is the case than only thing you have to consider is power consumption and voltage change under load current draw. These two requirements are completely opposite. For low voltage change you need to reduce resistor values as low as possible, and for power consumption is opposite. Obviously, you have to make compromises. In your case if we have ±1µA of load current draw with 1k version voltage variation will be only ±1mV, but current draw from power supply will be 2.5mA. Version with 100k will draw from power supply 100 times less current, but will have error 100 times bigger.

±0.1V for 100k case would represent ±4% error on 2.5V and in some applications that might be good enough. Low consumption of power might makes this choise much better in such applications. Well, this was simple?

Unfortunatelly this is just the begining of error calculations. After this comes bandwidth of circuit, noise, Tc effects, ...

Read OpAmps for Everyone, free book available from Texas Instruments web site, it will help you in understanding sources of error in analog circuits.