Analog Module Resolution

[New_To_PLCs]

I am little embarassed to ask this question but I thought this would be the place since the people here are so receptive to people new to the field. How do I determine the resolution of an Analog Input Module. For example, lets say I have a 16-bit Analog Input Module scaled to 0 - 10v connected to a pyromter that is capable of measuring temperature from 1000 to 2400 C. What would be the maximum value the module could read and how many decimal points to the right accurate would it be? Any other examples or info would be appreciated.

Thank you for the input!

 

[Steve Bailey]

Let's start with the theoretical. 16-bit A/D conversion is one part in 65536. That's because when you have 16 bits, each of which can take a value of one or zero, there are 65536 possible combinations of ones and zeros.

For your example of a 0 - 10 volt range, that span can be divided into 65536 steps of 0.0001526 volts. That's the theoretical increment size for the analog input module. Be aware that your analog input module's specifications might actually be +/- 10 volts, in which case the span is 20 volts divided into 65536 steps of 0.000305 volts.

For the pyrometer, you need to know how it is calibrated. Is it set to output zero volts when it senses 1000 degrees C and 10 volts when it senses 2400 degrees? If so, that 1400 degree span would theoretically be divided into 65536 steps of 0.0214 degrees. In reality, it may not be capable of such fine resolution. Also in the real world, you'll probably find some electrical noise riding on your analog signal, so your actual performance will fall short of the theoretical best.

 

[Tom Jenkins]

Remember, resolution is not the same as accuracy. If your primary measurement device is ±1% accuracy, then your answer can't be relied on to be more accurate than that, regardless of resolution.

For any unit conversion on any PLC you can use four function math with the following, which asumes integer math:

DataOffset = Data in input register at 4 mA (or zero analog signal of whatever range - for example 0 VDC on a 0-10 VDC transmitter)
DataSPan = Data at 20 mA - Data @ 4 mA
FACTOR = arbitrary factor (multiple of 10) needed to get proper resolution and accuracy
EU = Engineering Units x FACTOR
EUOffset = Engineering Units @ 4 mA x FACTOR
EUSpan = (Engineering Units @ 20 mA - Engineering Units @ 4 mA) x FACTOR
DATA = Actual data reading in input register

EU = ((EUSpan x (DATA - DataOffset))/DataSpan) + EUOffset

ProcessValue = EU / FACTOR

Example, 0-60 psig from 4-20 mA with resolution of 0.1 psig:

FACTOR = 10
Data @ 4 mA = 6240
Data @ 20 mA = 31208
DataOffset = 6240
DataSpan = 31208 - 6240 = 24968
EUOffset = 0 x 10 = 0
EUSpan = (60 - 0) x 10 = 600

DATA = 18975

EU = ((600 x (18975 - 6240)) / 24968) + 0 = 306

Process Value = 306 / 10 = 30.6 psig

Plug in 0 data and max data to get the range of values you can read.

 

[jcp]

hi;
Data @ 4 mA = 6240
Data @ 20 mA = 31208
how u calcualte it?
regards

 

[Mickey]

hi;
Data @ 4 mA = 6240
Data @ 20 mA = 31208
how u calcualte it?
regards

WOW!! a 9 year old thread?

The raw data above is just an example, it would depend on the analog card used and how it is configured to give you those values.

 

[leitmotif]

WOW!! a 9 year old thread?

The raw data above is just an example, it would depend on the analog card used and how it is configured to give you those values.

Yes nine years old but a good reminder and refresher.

Dan Bentler

 

[Gil47]

hi;
Data @ 4 mA = 6240
Data @ 20 mA = 31208
how u calcualte it?
regards

31208 multiply by 4 divided by 20 equals 6241.6

Those value look very much like flex analog scaling values.



Mickey and Lietmotif the post might be 9 years old,
but it was new to you,
when comparing your joining dates

 

[hilbramarya]

yes nine years old thread. but it still good to all of new practical in this field. thank plc talk