126 CHAPTER 3 Pieces Parts Input Impedance Most A/D converters have some type of input impedance, usually significantly lower than a digital input. A digital input is often 5 to 10 M ohms of impedance, whereas an A/D may be 100 K ohms. Get to know your input impedance, and make sure it is adequately higher than the sensor output impedance so that it's not an issue. A ratio of at least 100:1 is a good place to start. That means that if your A/D is 100 K and your sensor has less than 1 K output impedance, you will have a maximum error of 1%; if that is acceptable in your design you are probably okay. Output There are numerous devices that you can output a signal to. We will cover a few of them here. Let's start with some common indicators and displays. Two that are the most common these days are the LED and the LCD. LEDs LED stands for light-emitting diode. LEDs need current to drive them. Too little and you won't get any light, too much and they will fail, so you typically need a series resistor. How much current is needed depends on the type of LED, but