Electronics > Beginners
Voltage Divider Power Question
suicidaleggroll:
--- Quote from: bryancostanich on September 26, 2017, 09:18:56 pm ---I don't know what the sampling frequency is set to. It's a Netduino with an STMF4 running at 168MHz. As I understand, the sampling frequency is set in software; but I don't know what it's been set to.
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What do you mean "what it's been set to"? Aren't you writing your own firmware? You're the one that sets the sample rate.
--- Quote from: bryancostanich on September 26, 2017, 09:18:56 pm ---It's so strange to me that this has been such a rabbit hole. There are all these tutorials out there [here is one](https://learn.sparkfun.com/tutorials/voltage-dividers) that say, "hey if you have a sensor with a 5V output and you need 3.3V; no problem, just use a voltage divider."
But in digging in, that doesn't seem to hold the scrutiny of the folks here. :-//
After reading and studying, I get a lot of what's been said here, but it's totally contradictory to what I'm reading in these tutorials.
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I don't see any contradictions, only simplifications. A lone voltage divider is easy, and as with all easy solutions, it has a number of drawbacks. It will divide the voltage, but it will force you to slow down your sample rate due to the divider's output impedance, and it will dissipate excess power. Sometimes these drawbacks are important, sometimes they're not, YOU are the one that has to decide. YOU are the one that came here concerned about excess power draw from a resistor divider. If a simple resistor divider does not work for YOUR application, then you need to find an alternative. That does not contradict anything in the sparkfun tutorial. All the members here have been doing is pointing out what the drawbacks of a simple voltage divider are, and ways you can work around them if those drawbacks are problematic in your application.
As rstofer pointed out above:
--- Quote ---the voltage divider ratio changes with the operating conditions. So, not knowing the operating conditions means you can't solve the equation and, therefore, you can't come up with an appropriate divider.
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You are not adequately defining the problem, which means it's impossible to tell you which approaches will or will not work or with what level of accuracy. Come back to us with some actual requirements, like "I want to sample an analog voltage at 100 Hz using an STM32F4's built-in ADC with <0.1% error, without wasting more than 1mW of excess power, for less than $2 in single digit quantities", and we can turn around and recommend a circuit that will do that. With an unknown sample rate, unknown accuracy requirement, unknown power dissipation requirement, unknown price limit, unknown size limit, etc. there are an infinite number of solutions, some better in certain aspects, and worse in others.
bryancostanich:
Thanks folks.
I think I'm going to steer folks clear of this kind of setup. Especially given that you can get level shifter ICs and circuits for hardly any $. I was hoping that this would be simple for prototyping, but it's not. And the advice out there in the tutorials seems basically worthless. Actually, worse than that, because it gives people a false sense of something like this working and being accurate.
suicidaleggroll:
Level shifters, like the ones I imagine you're referring to (eg: SN74LVC1T45) are for digital signals, not analog. There's no such thing as a general purpose "do everything" analog level shifter that will work for all applications. A voltage divider may work just fine, or it may not. Being an electrical engineer means knowing the advantages and drawbacks of all potential solutions to a problem and deciding which is the best fit given the application, "given the application" being the key phrase in that sentence. If you don't know the application, you can't recommend a circuit with any level of confidence in its performance.
LukeW:
"I'm using it to hook a 5V sensor to a 3.3 ADC input."
I assume you mean a 0-5V sensor signal, and a single-ended ADC with a 3.3V reference. If the output was always 5V it wouldn't be a very good sensor.
If we assume the ADC input impedance is high, then yes, a voltage divider will work. It won't be infinite, but it should still be reasonably high.
Lower resistances lower the effect of this finite input impedance, at the expense of greater power dissipation, greater thermal noise and greater sensitivity to the non-zero resistance on the input side.
If you know the ADC true input impedance, then you can calculate the corrected resistor value to use to give you exactly the right division, with this resistor (the bottom half of the divider) in parallel with the internal ADC input impedance.
Basically, a voltage divider is only practical when we assume the source impedance is zero and the load impedance is infinite.
You can model this as a source resistance in series with the top divider resistor and a load resistance in parallel with the bottom divider resistor.
If in doubt, you can use an opamp voltage follower on the output side.
VanitarNordic:
What about if the original signal comes from an opamp output (naturally very low impedance) and has a high voltage, for example 5V or greater and must be injected to the ADC input (3.3V limit)
Then a resistor divider or a potentiometer is enough or still another opamp as a follower would be necessary?
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