AS to voltage dividers has anyone considered that an op-amp can have less than unity gain, that they make great attenuators as well and handle +/- 15 Vin as well? at ANY input impedance? 10 ohms to a T ohm are readily possible.Bob
Quote from: KeithRB on Oct 10, 2012, 05:28 pmThat is why they have a thing called a voltage divider.That's possible of course, but precision is potentially going to take a big hit even before the resolution of the ADC is factored in. From the perspective of the signal's information content a voltage divider going from 5 to 15 V down to 0 to 5 V means there will be roughly three times less usable information detectable in the output signal. For some applications that's not a problem, for others it will be. Instead, I would check to see if there is an affordable ADC chip/module that can handle the orignal input voltage and output serial communication of some sort at 5 VDC.
That is why they have a thing called a voltage divider.
Hi All, I'm trying to finish off a project I started a while ago but I've been having a bit of trouble with the Analog Inputs...What I'm trying to do...I need to be able to read the voltage of 16 individual inputs with a reasonable level of precision (12bit, maybe?). Input voltages could be anything between 5VDC and 15VDC.Can anyone provide any suggestions on a way to do something like this?Any help would be greatly appreciated.Thanks Guys,Alex
I'm afraid you are wrong in nearly every detail in that paragraph!
Firstly information is a logarithmic measure so a loss of a factor of 3 in precision is a loss of 1.6 bits of information. Given a good ADC can give 20 bits, that would be only a loss of 8% information (18.4 bits / 20 bits).
Secondly a voltage divider doesn't throw away information like that - you need to know the signal/noise ratio and bandwidth of the source and the actual resistance values and their temperature, then you can calculate degradation in signal/noise ratio. If the source is already noisy the potential divider might have almost no effect. If the source is clean and low-impedance then a high-resistance voltage divider might be injecting massive amounts of noise compared to the source.Resistors generate voltage noise proportional to temperature and to the square-root of the resistance value and to the square-root of bandwidth.Thirdly the ADC might be introducing quantisation noise that's far greater than the signal noise - in which case everything else would be academic. If you are only using a 8 / 10 / 12 bit ADC its likely to be the dominant source of error.
Where did I explicitly or implicitly suggest that the divider was significant source of noise, or for that matter even address it? My concern was always about how reducing the voltage range makes the analog signal more susceptible to externally generated noise (e.g. from EMI), not the amount of noise generated internally by the circuit.
certainly there's no automatic factor of three involved in precision.
@ zoomKat... that suggestion is a given and is discussed in the part of the thread you didn't read... Thoroughly discussed, I might add. Bob
QuoteDG408 into resistor divider into MCP3201.and to solve the Granularity issues a variable Vref (DAC) might be used to solve this:Quote "« Reply #8 on: October 10, 2012, 07:55:02 AM »"voltage divider going from 5 to 15 V down to 0 to 5 V means there will be roughly three times less 1/3 less usable information detectable in the output signalso change the Vref as required to increase the effective available information by providing a Vref suitable to the measurements.@ zoomkat... Apparently then you have an attention span issue because I made this one 2 days ago...Bob
Without know the input and output impedances and noise sources your original paragraph seemed a very specious argument for avoiding a divider circuit - certainly there's no automatic factor of three involved in precision.
a higher voltage, multi-input ADC
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