We’re often asked how to best measure a discrete input with an analog data acquisition system. Many of our products provide discrete (digital) input ports that can be read with WinDaq data acquisition software, but sometimes it’s better to acquire the discrete input along with the analog data through a spare analog input channel so that critical timing elements can more easily be correlated. If the discrete originates from a dry contact switch, special considerations are required which are often overlooked.
The nearby graphic shows two ways that the state of a dry contact switch can be measured by an analog input channel. Note that both require the use of either a pull-up (top) or pull-down (bottom) resistor to a DC voltage or ground respectively to ensure that the amplifier input sees a stable level regardless of the state of the switch. The choice of a pull-up or -down resistor depends upon the logic state desired at the output of the analog amplifier as shown.