Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Voltage divider

Status
Not open for further replies.

Anniyan_x

Member
Hi refering to circuit attached, let say the Vin is 8.84volts, how do i calculate the exact vout, first of all i need to calculate the totall impedance(e.g Zi) of R212 and C212, meaning R212 // C212 => Zi, so
Zi = [R212] / (1+[R212]jwC), and then Vout should be, Vout = Zi/(Zi+R211). is it right, this what i think?? and how about the 1.4offset voltage being fed in, how do i take this offset volatage into considerations?? so i need to take into consideration of the cap reactance value also?? and how about the (jw) ??
 
Since you only specify a single value for the input voltage, I can only look at it as a DC value. So the capacitor does not show up in the calculation for Vout.
Just treat it as a straight resistor divider with an offset.
First subtract the offset from Vin, then apply the result to the two resistors to get the voltage across R212. Add the offset back in to get Vout.

The capacitor will add a delay to the Vin to Vout timing, but only when Vin changes.

Assumptions:
The leakage current of C212 is negligible.
There is no current being drawn from Vout.
 
Hi refering to circuit attached, let say the Vin is 8.84volts, how do i calculate the exact vout, first of all i need to calculate the totall impedance(e.g Zi) of R212 and C212, meaning R212 // C212 => Zi, so
Zi = [R212] / (1+[R212]jwC), and then Vout should be, Vout = Zi/(Zi+R211). is it right, this what i think?? and how about the 1.4offset voltage being fed in, how do i take this offset volatage into considerations?? so i need to take into consideration of the cap reactance value also?? and how about the (jw) ??


Hi there,


It looks like you have the AC part of it correct. Once you calculate your Vout you'll have the expression for the AC part of the response.

For the DC part because of the offset, you will also want to calculate the offset at the output Vout. To do this, you have to know the resistive part of the input source impedance or else assume it is zero, and the input offset if it isnt zero. You then do the resistive divider 'backwards' with the source being 1.4v and Vout being the node you are solving for. That will give you a DC value for Vout. This means you'll end up with BOTH an AC value and a DC offset for the output Vout. "jw" doesnt get affected except by the frequency of the input source because w=2*pi*f.

For an example if the two resistors were both 10k and there was 0v input offset and the input source impedance resistive part was 0 ohms then the DC offset at the output Vout would be 0.7 volts (half of 1.4v). This means your AC signal will be riding on a 0.7v DC offset.

For another example with some source resistance, say the upper resistor is 5k and the lower resistor 10k, and the input source resistance part of the impedance is 5k. 5k plus 5k equals 10k so we end up with 10k on the bottom and 10k on the top so the output DC is again half of 1.4 volts.

If you have any input source offset Vios, you would have to calculate the voltage divider as:
Vdc=(Vios-1.4)*R212/(R211+R212)+1.4

and that gives you the DC output offset.

There is one more little thing to consider. That's the transient start up period. Because of the capacitor, the DC output offset will not be present as soon as power is first applied but will take some finite amount of time to reach its final DC value. This is found by calculating the time response to a step change in the offset voltage (and the input offset if present). If the solution is not needed until after 5 time constants however then you can assume that the cap is charged up to its final DC value as long as none of the DC offsets change. Many times the output does not get 'used' (sampled) until after some time after the power has been applied so in those cases the transient response wont matter for the DC value. One time constant is R*C212 where R is the parallel combination of two resistors where one resistor is R212 and the other resistor is R211 plus any input source resistance.
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top