Internet PLC Forum
General => Technical support => Topic started by: ssiewert on May 14, 2012, 05:38:12 AM
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Chapter 5 discusses the analog inputs, of which there are 2 types. In addition, there are two analog converters, each handling 4 of the eight. Section 5.2.2 mixes discussion of points 7&8 and 5-8. Are points 5&6 some how different than points 1-4, other than they are on a different converter? Please explain or if it is a typo, make a note so it may be corrected in the next revision. (I'm reading Rev 1, Nov2011).
5.2.2 A/D #7 to 8
The DC input impedance of Analog inputs #7 & 8 are all 20.00K ohms (0.1%). This is not a problem when connected to a 0-5V low impedance analog source.
However, if you need to connect to 0-10V inputs or 4-20mA analog source to A/D #5 to #8, you have to take into consideration the low A/D input impedance in your design. The following figures show how to connect 4-20mA current source signals and 0-10V signals to the A/D inputs #7 to 8.
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The fact that analog inputs 1 to 4 and 5 to 8 are converted by two different analog converters simply means it work faster than an MCU with only a single converter. However, this point is completely transparent to the user since the PLC O/S automatically handle the two converters.
Analog input 7 & 8 do not have high impedance buffer, whereas analog input 1 to 6 has high impedance input buffer. This is independent from the fact that they are on different analog converters and there is no mix up.