Hi
I have a question about protecting a microcontroller's analog inputs. I have a PIC that I'd like to use to measure an analog signal up to +5VDC. I was planning on using a standard voltage divider, consisting of two resistors in series, with the input going in the top of the upper resistor, a ground on the bottom of the lower resisotor, and a 3.3V zener in parallel with the lower resistor. The input to the controller would come from the node between the resistors:
**broken link removed**
Since the PIC can take a max of 3.6V on an analog input, and I've heard a lot of talk of the soft knee of most zeners, I was thinking of making the resistors equal in value, so the max the PIC would see in normal operation would be theoretically 2.5V (half the max input). It might serve to effectively lower the precision of the ADC's result a little, but would hopefully add a little protection to the circuit.
Now the question: I have seen a few app notes from Microchip that show a "typical" resistance to a PICs input in the range of 470K. However, I've seen many zener datasheets that show a "test current" in the neighborhood of 20mA. I'm assuming that is the amount of current it takes for the zener to kick in, and notice that many datasheets don't promise much unless you operate the zener at the test current.
If so, that would mean I'd need at least 20mA available from the input through the top resistor to divert through the zener to ground in a protection scenario, which, if I'm doing my math right, means half the input voltage drops over the top resistor, half the bottom one. This means at max input, 3.6V / 20mA = 180 Ω, which just seems dangerously small for a processor's input to me. Am I missing something? Would I want a further resistor to the right of the middle zener node, between the node and controller to limit current? If so, what would a safe range for it normally be?
Also, I've heard about these TVS devices as an alternative to a zener. But, I can't seem to ascertain whether they are made for sudden, ultra-high voltages (as in static spikes), or for continued slightly-higher than normal inputs (like someone hooking up +12V to an input designed for +5). Is a TVS made for sustained overvoltage conditions? Is that usually a concern in microcontroller input protection, or is static the main worry usually?
Thank you for your help!
-Josh
I have a question about protecting a microcontroller's analog inputs. I have a PIC that I'd like to use to measure an analog signal up to +5VDC. I was planning on using a standard voltage divider, consisting of two resistors in series, with the input going in the top of the upper resistor, a ground on the bottom of the lower resisotor, and a 3.3V zener in parallel with the lower resistor. The input to the controller would come from the node between the resistors:
**broken link removed**
Since the PIC can take a max of 3.6V on an analog input, and I've heard a lot of talk of the soft knee of most zeners, I was thinking of making the resistors equal in value, so the max the PIC would see in normal operation would be theoretically 2.5V (half the max input). It might serve to effectively lower the precision of the ADC's result a little, but would hopefully add a little protection to the circuit.
Now the question: I have seen a few app notes from Microchip that show a "typical" resistance to a PICs input in the range of 470K. However, I've seen many zener datasheets that show a "test current" in the neighborhood of 20mA. I'm assuming that is the amount of current it takes for the zener to kick in, and notice that many datasheets don't promise much unless you operate the zener at the test current.
If so, that would mean I'd need at least 20mA available from the input through the top resistor to divert through the zener to ground in a protection scenario, which, if I'm doing my math right, means half the input voltage drops over the top resistor, half the bottom one. This means at max input, 3.6V / 20mA = 180 Ω, which just seems dangerously small for a processor's input to me. Am I missing something? Would I want a further resistor to the right of the middle zener node, between the node and controller to limit current? If so, what would a safe range for it normally be?
Also, I've heard about these TVS devices as an alternative to a zener. But, I can't seem to ascertain whether they are made for sudden, ultra-high voltages (as in static spikes), or for continued slightly-higher than normal inputs (like someone hooking up +12V to an input designed for +5). Is a TVS made for sustained overvoltage conditions? Is that usually a concern in microcontroller input protection, or is static the main worry usually?
Thank you for your help!
-Josh