# Voltage divider networks should be linear, right?

Discussion in 'The Projects Forum' started by Camz, Jul 22, 2010.

1. ### Camz Thread Starter New Member

Feb 15, 2010
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Little background:

I'm working on a project with the TI MSP430 microcontroller. Ultimately I'll have to monitor my battery voltage with the chip. There are 2 ways to do this, one using the a/d converter and another using the chip's built-in SVS feature that will raise a flag if the voltage falls below a user set threshold.

My problem is this:

I'll be using a 12v battery and the chip can take a max of 3.6v to it's input pins. No problem, I'll use a voltage divider to knock that down to that ballpark. So, I'm going to represent 0-12v with 0-3.6v. If I want to alarm on 10.4v; i would just use this equation to get my resulting threshold right:

12 : 3.6 , 10.4:x >>> which ultimately ends up being 12x = 10.4(3.6) >> x = 3.12v.

So in order to alarm on 10.4v; I'd just set my chip to alarm when the a/d converter "sees" 3.12v coming in on the input.

I set up the circuit with the chip but the voltage values coming off the resistor don't really coincide with my calculated values. Obviously they won't be exact; but i'm getting discrepancies of .2-.3v which is too much when I'm working on this small scale. Any tips, advice? If it doesn't make sense please ask questions.

Thanks!

2. ### Markd77 Senior Member

Sep 7, 2009
2,803
596
Any current coming out of the centre of the divider changes the voltage. Smaller value resistors would reduce this change.
Using an op amp buffer would reduce the current coming out and thus the error.

3. ### Camz Thread Starter New Member

Feb 15, 2010
15
0
thanks Mark; been out of the circuit design game for a while now; things are kinda fuzzy

4. ### Camz Thread Starter New Member

Feb 15, 2010
15
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So, using a general purpose 741 opamp coming off the divider would work?

5. ### Papabravo Expert

Feb 24, 2006
11,761
2,487
Except there are modern alternatives to the "should be obsolete by now" 741.

6. ### someonesdad Senior Member

Jul 7, 2009
1,585
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You probably don't need lots of resolution to detect a low voltage condition. I recommend not using the full 3.6 V if that's the upper spec. Use e.g. 3 volts. The reason is that systems can be more reliable if you design more margin in. If that 3.6 V limit is valid, then a rare voltage glitch on the power supply might result on the voltage exceeding the maximum, potentially damaging the chip. If you have other information you didn't give that makes it so you don't worry about this, fine. But it's a good general design tip for both software and hardware.

7. ### Camz Thread Starter New Member

Feb 15, 2010
15
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heh; something like the LF353n?

8. ### Camz Thread Starter New Member

Feb 15, 2010
15
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good point someone'sdad. The chip has a max voltage of 4.2v probably for situations in which you describe, but it's recommended that the input be in between 1.8-3.6v.

9. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
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Change one of the resistors to a trim pot, and then you can adjust the trip for where you want it.

10. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
The LF353, TL07x, TL08x families of opamps will not be suitable because they cannot "see" within 3v of the negative rail.

Don't forget that when being charged, the battery voltage may exceed 14.5v.

If an automotive environment, you may get rather high voltage transients. You'll need to make certain that the input can't exceed your power rails.

11. ### Camz Thread Starter New Member

Feb 15, 2010
15
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Sgt,

I'm looking on the datasheet of the LF353 and don't see anything mentioned about the the limits of what it "sees" within the negative rail. What exactly am I looking for that would tell me this?

12. ### windoze killa AAC Fanatic!

Feb 23, 2006
605
24
Having looked at the datasheet for that micro the current on the ADC I/P pin is less than 1mA so I don't think a voltage drop would be too much of a problem. I would uses a 10K and a 2.2K resistor as the voltage divider. You could also use a trim pot if you want to be able to adjust it.

13. ### Camz Thread Starter New Member

Feb 15, 2010
15
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Would this be something more suitable?

http://www.national.com/ds/LP/LPV521.pdf