how much current going to pin?

[sweethomela8]

If I have 12 V going to a resistive divider series of (1.5MOhm) and parallel to ground (1.0MOhm), and I take that voltage divided output and route it to a pin...how much current is being driven to that pin?

 

[IanP]

That depends on what is the input resistance of that pin ..
Without knowing it it's a guesswork ..

IanP

 

[sweethomela8]

What if the input resistance was 10.0k...how would it be calculated? Do you interpret input resistance as a parallel resistor to ground?

 

[IanP]

What if the input resistance was 10.0k...how would it be calculated? Do you interpret input resistance as a parallel resistor to ground?

You are on the right track - input resistance can be seen as a perallel reistor to ground - see attached picture ..

Formulas and calculations (incl. calculator) are here:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/voldiv.html

IanP

 

[sweethomela8]

So to calculate the amount of current going into the pin,

12 V-----> R1 --------------------------------Input resistance to ground
|
|
R2 to ground

to calculate the current into the pin, would it just be

12 V / total resistance?

total resistance = R1 + R2 || Input resistance


Thanks

 

[IanP]

.. to calculate the current into the pin, would it just be

12 V / total resistance?

total resistance = R1 + R2 || Input resistance

Not exactly, your approach would calculate total current that flows from the power supply, your aim is to calculate that part that goes to the pin, so:

I[pin] => {U * (Rin || R2)} / {Rin * [R1+(Rin || R2)]}

 

[sweethomela8]

What is U? and can you explain how you came up with that equation?

thanks so much, im just trying to understand clearly

 

[IanP]

Equation for standard voltage divider may look like this:

[1] U[pin] = U * (Rin || R2) / [R1+(Rin || R2)]

Also:
[2] I[pin] = U[pin] /Rin ====> U[pin] = I[pin] * Rin

[1] and [2] together:
I[pin] * Rin = U * (Rin || R2) / [R1+(Rin || R2)]

And finally:

I[pin] => {U * (Rin || R2)} / {Rin * [R1+(Rin || R2)]}

where U is the input voltage, in your case 12V ..


IanP

 

[sweethomela8]

For input resistances, you represent that by a parallel resistor to ground.

How do you represent output resistance/impedance? Is that a series resistance?

 

[IanP]

How do you represent output resistance/impedance? Is that a series resistance?

That depends on what type of source do you have in mind:

The real voltage source, such as an opamp or a battery, is molded by an ideal voltage source in series with a resistor R ..

The real current source is molded by an ideal current source in parallel with a resistor R ..

IanP

 

[sweethomela8]

What if the output is of lets say a IC, for instance the data outputs of a ADC? The data bit output can either be 3.3 V or 0V, with a output resistance X ohms. Would that be a ideal voltage source at 3.3 with a series resistance of X ohms?