Calculating Voltage Divider Voltage Tolerances

[danner123]

Hi everyone, I have a basic question. I am trying to figure out how to calculate circuit tolerances based on the resistors used in a voltage divider circuit. See the circuit below.

The tolerances are as follows:

3.3V Supply = 3.3V +/- 10mV
R1 = 1000 Ohms 1%
R2 = 1000 Ohms 1%

By the voltage divider equation, the voltage at Va = 5V and Vb = 2.5.

What would be the voltage tolerances at Va and Vb based on the overall circuit tolerances?

Va = 5V +/- ?? %
Vb = 2.5V +/- ?? %




Thanks so much!

 

[ashugtiwari]

If u r supplying 3.3V then voltage at Va output can never reach 5V. And since the Va is directly connected to Vcc, the tolerance of Va is equal to tolerance of Vcc.
Considering ideal conditions where the values are ideal, the Vcc = 3.3V and so Va=3.3V
then Vb=3.3*1000/(1000+1000) = 1.65V

Now, taking all the values to their lower side of tolerance, Vcc = Va = 3.3-0.01=3.29V, R1=R2=990 i.e. -1% tolerance
then Vb=3.29*990/(990+990) = 1.645V

Now, repeating for higher tolerence we get, Vcc = Va = 3.3+0.01=3.31V, R1=R2 = 1010 i.e. +1% tolerance
then Vb=3.31*1010/(1010+1010) = 1.655V

So, Vb is 1.65V +/- 5mV

Hope that helps.

 

[Kral]

For worst case low output you must use R1 at its high extreme (1010), and R2 at its low extreme (990). Similarly, for the worst case high output, you would use R1 at its low extreme (990), and R1 at its high extreme (1010).

 

[Audioguru]

The load on a voltage divider reduces its voltage. The load resistance was not mentioned.