I am looking for the impedance value of a pin on the electron when configured to pinMode (INPUT).
I was able to find information like this on the Arduino.
Pins configured this way are said to be in a high-impedance state. Input pins make extremely small demands on the circuit that they are sampling, equivalent to a series resistor of 100 megohm in front of the pin
The relevant measure here would be the input leakage current which is stated with max. +/- 1µA for 3.3V signals and max 3µA for 5V signals on 5V tolerant pins.
For more elaborate info consult the STM32F205RGT6 datasheet (linked in the docs) page 102/178
@ScruffR 's advice is good here but I wanted to add two things:
If we pick the most popular Arduino board, the UNO, with an ATMEGA328, it has an input leakage spec of 1uA for 5.5V both high and low so around 5.5Mohm equivalent, so that information from the Arduino doc is not totally correct.
The most important question to ask is, why do you care?
You should not design a circuit that depends on any particular value, other than to assume it is larger than say 1Mohm.
My reason for wanting to know what the input impedance was is so that I could confirm that the current draw through the system is very low. @bko Yes, the impedance value is not a core part of the design.
Thank you for the quick responses!
Follow up question – I’m looking at a current transformer that provides a 0-5VDC signal, but specifies the burden impedance should be >=1Mohm. I realize I’ll need to scale the output for the Electron’s input (3.3) likely via some series resistors, but don’t know how to assess the impedance situation.
Can you provide some more background on the intended use?
What (primary) signal are you intending to measure?
Why do you seek for a 0-5V output when you know you’ll need 0-3.3V in the end?
Aren’t you usually looking for burden impedances to be as low as possible?
@ScruffR, @bko has already provided some guidance via a private message, but for the sake of the community, I’ll (attempt to) re-iterate it here. The device I’m looking at is: http://www.crmagnetics.com/current-sensors/cr9580 and the documentation (and tech support) stress the need for a burden impedance >= 1 Meg ohm. The device provides a 0-5vdc output and I’ll scale it to 0-3 with a couple of precision resistors totaling 1 Meg ohm and input this to a voltage following op-amp which will then be connected to the Electron’s analog input. It would appear this configuration would provide the required high impedance burden with proper scaling suitable for the Electron’s AI input.
What I need to so is accurately measure the current in a circuit and break the circuit if the current gets too high (say, 3A). I plan to do this with a relay/contactor of some persuasion on a digital output. The end goal is a remotely resettable circuit breaker.
You guys are amazing and any thoughts will be greatly appreciated as always.
@ctmorrison, The input impedance of the STM32F205 used in the Electron is quite low. You can use resistor divider (to scale the 0-5VDC CT output) with high values but you will most likely need a unity gain op-amp (high input impedance) between the divider and the ADC input of the Electron in order to maintain that high impedance on the CT output.
@peekay123, yes, that’s what @bko had recommended. I appreciate everyone’s feedback and have ordered the parts to give it a whirl!
To continue on…I see the output of the current transformer is 0-5VDC and I believe this is if the amperage stays below the rated 10A. However, the specs indicate the max output could be 12VDC and I’m assuming in my circuit that could happen if there’s a short in the 24VAC circuit somewhere – the condition I’m trying to detect and stop. Therefore, I was contemplating adding a 3.3 ohm zener diode from the input (recall the resistor divider output) of the op-amp to ground to limit the input and therefore the output to 3.3 volts. Does this make sense?
@ctmorrison, zeners don’t have a perfect drop-off so you may want to use a 3.6v zener. Also, powering the op-amp with 3.3v will prevent an over-voltage on the output. Be sure to get an op-amp with an output that can go rail-to-rail (ie 0v to 3.3v).