What is an analog input?
Analog inputs on a Remote Terminal Unit are used to precisely monitor continuously variable values (like temperature, relative humidity, and fuel tank levels). Unlike discrete inputs, which are binary, analog inputs can tell you "how much?" rather than just "yes/no".
Traditionally, analog input circuits are expecting to receive either a DC voltage in the 0-5v range or a current in the 4-20mA range. Some analog inputs have a shunt that allows you to toggle between these 2 input modes.
What can you connect to an analog input?
Any device that outputs data through physical voltage/current (as opposed to a protocol) can be wired into an analog input. There are just a few limitations.
First, the output range of the device must fit within the input range of the RTU's analog input circuit. For example, imagine a temperature sensor that outputs a minimum of 0 volts (representing -40 degrees) and a maximum of 5 volts (representing 170 degrees). An analog input that can accept -90 to +90 volts DC easily contains the 0-5 volts DC range of the sensor output. You could also use that same analog input to track the voltage of a -48 VDC power supply.
Second, the RTU's firmware must be equipped to handle the scaling pattern of the output. In most cases, output is linear. If 0 volts represents 0% relative humidity, while 5 volts represents 100%, then 2.5 volts ("halfway") on a linear scale would represent 50% relative humidity. RTUs can easily accommodate linear scaling when you input just 2 anchor points, usually listed in the sensor documentation. Other scaling types (logarithmic, etc.) would require a more complex RTU configuration to ensure proper interpretation.
Commonly measured via analog input:
How many analogs do I need?
Your required number of RTU analog inputs is completely dependent on the equipment and environmental levels you need to monitor at each remote site. If you want to monitor both generator and battery string voltages, you'll need two inputs. If you want to monitor temperature at three locations, that's three more.
To do this right, you need to first understand what's possible. Study online or talk to an industry expert. Learn what common-sense things you should be monitoring.
Next, conduct a site survey using your new knowledge. Count up how many analog inputs you'll need on an RTU.
Finally, find an RTU that has enough analogs to handle your site (and perhaps a few more to handle site growth in the foreseeable future).
How do I physically connect to analog inputs?
Just like discrete and other input types, there really is no definitive standard connector for analog inputs.
Much of the time, amphenol connectors are used for RTUs with dense analog capacity. They're a common standard in telco environments, and 50-pin capacity means that manufacturers can squeeze a lot of analogs onto one connector for you.
"Piano style" or screw-down terminals are used for lighter capacities. These require few (or no) tools to install, so they're most appropriate for light-capacity RTUs.
Some RTUs even use a combination of connector types. There's no sense in adding another 50-pin amphenol connector to the back of the RTU for the sake of a few additional analog-input pins. In that case, a properly designed RTU will simply have a small 4-pin (or similar) connector that's appropriately labeled so you'll understand what to wire into it.