I'm working on an interface to my home alarm system and using an ADC to monitor my zones. I've noticed the outputs (0v to 5v) from the alarm are high-impedance as when I connected them directly to the ADC, I got a voltage drop.
The circuit i've now implemented includes an op-amp (LMV324 - https://www.ti.com/product/LMV324) configured as a simple voltage follow. i.e. Output connected to (-) on the op-amp and the alarm feeding (+).
When the input to the op-amp is floating, the output registers at 5V. If I pull it low, then it works as expected.
Two questions.
Is the 5V output when the input is floating, expected behaviour?
For my circuit I do expect the input to be floating if the alarm zone is trigged. Should I use a pull-down resistor on the input pin of the op-amp (1M?) ?
For my circuit I do expect the input to be floating if the alarm zone is trigged. Should I use a pull-down resistor on the input pin of the op-amp (1M?) ?
Since higher impedances are prone to noise pick-up, use a lower value resistor if you can get away with it.
from the alarm are high-impedance as when I connected them directly to the ADC, I got a voltage drop.
If you are talking about the Arduino, it has very-high impedance but it is also undefined if floating.
I'm using an external MCP3008 ADC through SPI as need 8-channels.
Why is there analog in an alarm system?
The output would be at different a voltage depending on if the sensor is triggered or tampered, so could be anywhere between 0V and 5V.