# Help with voltage divider

1. May 1, 2010

### jeuhyis

Hi first time poster. I'm a little stuck, through trial and error on Mentor graphics I was able to create the attached circuit. Using +&- 9V supply rails I managed to output -10mV, which I required. Now I have come round to try and work it out by hand i'm struggling to remember how! I know that the first part is basically a potential divider but how does this work with a positive and negative voltage?

Any help greatly appreciated

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2. May 1, 2010

### willem2

Treat it as a divider between 18V and 0V and then subtract 9V from the output voltage.

3. May 1, 2010

### Phrak

The two supplies on the left and the two resistors can be replaced with an equivalent single supply and single resistor.

R = 10*8.87/(10+8.87) KOhms

V = (9+9)8.87/(10+8.87) - 9 Volts

4. May 1, 2010

### jeuhyis

So therefore using this
R = 10*8.87/(10+8.87) KOhms = 4.7KOhms

V = (9+9)8.87/(10+8.87) - 9 Volts = -0.53895V

So then is there some calculation required using the 500K and 1K utilising the -0.53895V to reach the 10mV?

Thanks for your help so far

5. May 1, 2010

### Phrak

That's an odd combination of components. What's it for?

Vout = -0.539[10/(500 + 4.7 + 10)]

not 10mV, but -10mV

6. May 1, 2010

### jeuhyis

Its for an input offset voltage on the non-inverting input of an op-amp to give a zero voltage output. I spent ages designing an RMS circuit for a sound level meter and then I hit this problem. Managed to figure out what offset was required simulated that circuit but then I could remember how to hand calculate!

7. May 1, 2010

### Staff: Mentor

Input offset voltages vary with temperature, and from part-to-part. What opamp are you using? There may be a more reliable way to deal with the input offset voltage. Can you post your schematic?

8. May 2, 2010

### jeuhyis

Here's a screen shot of what I've been designing. Im happy with the overall performance of the circuit, its being used within a group to create a sound level meter, and i have this working as a Type 0 with a 60dB dynamic range.

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