So I have a new project I'm working on. Nothing fancy, but requires I measure a 0-12V signal on the ADC, as well as a few 0-5V signals. The kicker is, I want to be a little more thorough about my protection of these pins, as the possibility exists of them getting transient voltages up to 30V, along with the pins being inherently "exposed" to things like finger touches (read: ESD).
In reading the datasheet, it seems the source impedance is suggested to be < 10k.
My first thought was just a voltage divider (24K, 12K) which would give a 0-15V measurement range, and an output impedance of 8K, just letting the protection diodes do the rest if it sees any transients.
For the 5V channel, I was just going to use a 8.2K resistor in series with the input.
Am I going to run into capacitance issues/sample time problems? I am running the ADC clock at 1MHZ (fosc: 16MHz; ADC clock set: fosc /16). Assuming 120pF and 0 input impedence, I come up with an RC of 960ns. Which seems very appropriate, but I realize I am not accounting for leakage, input impedence, etc.
Any rules of thumb or good reference material for these calculations?
In reading the datasheet, it seems the source impedance is suggested to be < 10k.
My first thought was just a voltage divider (24K, 12K) which would give a 0-15V measurement range, and an output impedance of 8K, just letting the protection diodes do the rest if it sees any transients.
For the 5V channel, I was just going to use a 8.2K resistor in series with the input.
Am I going to run into capacitance issues/sample time problems? I am running the ADC clock at 1MHZ (fosc: 16MHz; ADC clock set: fosc /16). Assuming 120pF and 0 input impedence, I come up with an RC of 960ns. Which seems very appropriate, but I realize I am not accounting for leakage, input impedence, etc.
Any rules of thumb or good reference material for these calculations?
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