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Topic: 12v to 5v - Voltage divider issues (Read 3690 times)previous topic - next topic

Divinitous

#15
Mar 28, 2013, 07:17 pm

Any possible alternative to a voltage divider....

In theory: 10 times more resistance in your divider should mean 10 times less voltage drop (more or less).

You could try putting bigger resistors in there, eg. in the megaohm range. The Arduino ADC input has 100 megaohm resistance so there's still some margin to work with.

I'm experiencing the problem before the voltage divider even gets to the arduino.  With the arduino removed from the circuit the Vin still loses ~1v.  I think the best solution would be the opamp as someone has mentioned.

fungus

#16
Mar 28, 2013, 07:23 pm

You could try putting bigger resistors in there, eg. in the megaohm range. The Arduino ADC input has 100 megaohm resistance so there's still some margin to work with.

Yes, but the analogRead() function works best and is recommended being driven by a voltage source of 10K ohms or less

That would suggest there's some capacitance in the input stage of the chip. If that's the case then a small resistor could be needed for sampling (eg.) a 10kHz signal. For a low frequency signal though, it would make no difference.

I'd say "try it and see" before adding opamps (or whatever). There's nothing to lose except two resistors.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

retrolefty

#17
Mar 28, 2013, 07:28 pm

You could try putting bigger resistors in there, eg. in the megaohm range. The Arduino ADC input has 100 megaohm resistance so there's still some margin to work with.

Yes, but the analogRead() function works best and is recommended being driven by a voltage source of 10K ohms or less

That would suggest there's some capacitance in the input stage of the chip. If that's the case then a small resistor could be needed for sampling (eg.) a 10kHz signal. For a low frequency signal though, it would make no difference.

I'd say "try it and see" before adding opamps (or whatever). There's nothing to lose except two resistors.

Certainly, the AVR single ADC converter uses a sample and hold capacitor that is switched to the pin to be measured by the input pin multiplexer. The 'charge time' for the S/H cap is effected by the source impedance of the voltage trying to charge the cap. This is unrelated independent of the DC input resistance of the analog input pins.

Lefty

fungus

#18
Mar 28, 2013, 07:36 pm

Certainly, the AVR single ADC converter uses a sample and hold capacitor that is switched to the pin to be measured by the input pin multiplexer. The 'charge time' for the S/H cap is effected by the source impedance of the voltage trying to charge the cap.

Sure, but for slowly changing signals the charge on the capacitor will be about the same between each sample. You don't need much source current unless you're trying to change that charge by large amounts on each sample.

This is all just pontificating of course, but it's easy/cheap to try it and see what happens.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

dc42

#19
Mar 28, 2013, 07:38 pm
If the voltage source really does have a very high source resistance, then you can increase the values of the resistors in the voltage divider, provided that you take appropriate precautions when using the ADC to read the voltage, and that you don't need to measure a rapidly-changing voltage. I've used two 4.7M resistors in one of my designs to monitor the voltage of the 9V battery powering a circuit that had no on/off switch. However, it seems strange to me that at voltage or 9.5V should have such a high source resistance.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Divinitous

#20
Mar 28, 2013, 08:13 pm

If the voltage source really does have a very high source resistance, then you can increase the values of the resistors in the voltage divider, provided that you take appropriate precautions when using the ADC to read the voltage, and that you don't need to measure a rapidly-changing voltage. I've used two 4.7M resistors in one of my designs to monitor the voltage of the 9V battery powering a circuit that had no on/off switch. However, it seems strange to me that at voltage or 9.5V should have such a high source resistance.

Well, it seems you get the prize... if there was one.  I put a 4.7M and a 10M resistors in there and the voltage drop is very minimal.  All that's left is to see if I'm getting correct values with the arduino calculated properly.

dc42

#21
Mar 28, 2013, 09:35 pm
You need to take precautions either in hardware or in software to get reasonably accurate readings when using such a high resistance source. The simplest is to connect a 0.1uF capacitor between the analog input and ground. Also remember that the input resistance of the ADC is only 100M ohms typical, so you are likely to get about 3% error using 10M and 4.4Mohm resistors.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

tocpcs

#22
Mar 28, 2013, 11:14 pm
I'm trying to consider why one wouldn't use an opamp buffer in this design?
It makes sense to me - accurate readings, no load on source circuit.

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