Can someone explain in layman's terms with a mix of technical terms the
meaning of Input resolution regarding a building HVAC DDC control system.
I am in the process of doing a functional performance test on some new
Digital controllers and am a bit confused on this subject. Some of sensors
are Thermistors and some are using 4 to 20 MA . Any help, advice, or links
would be appreciated.
When you digitize any analog signal it will have distinct equal spaced
values. As an example a +/- 10.000V dc input or output will be represented
in the controller by a range of binary integer number values. The number
range depends on how many bits of resolution are used, which maybe low
resolution of typically 8 bits = 256 values or steps to cover the range or
increasing resolution for 10, 12, 16 or higher bits in the number of steps
for resolution for the range.
A digital signal is not a smooth line but a series of small steps.
Resolution is the amount of one step compared to the total scale. An on/off
switch is a digital device with a resolution of 1:2. A temperature
measurement system with a scale of -40 to +40 C that gives readings to the
nearest 1 C has a resolution of 1:80. You could also say it has a
resolution of 1 C. My bathroom scale reads to the nearest 0.5 pound and has
a full scale of 300 lbs. has a resolution of 0.5 lb or 1:600.
When analog signals are received by digital devices they are converted by an
ADC (analog to digital converter). Let's say that an air pressure
measurement of 0 to 10"WC is converted to a single byte of computer memory.
Then the ADC can be said to have a resolution of one byte, or eight bits, or
1:256 ( 2^8 =256) or 10"WC / 256 = 0.04"WC.
Keep in mind that any resolution exceeding the precision of the source
measurement is of no value. If your instruments are good to 1%, (1:100) any
ADC resolution greater than 8 bits (1:256) isn't worth much.
Higher resolution costs more and it is easy to pay for something you don't
Note that in Walter's 1% ( 100 parts input FULL range resolution ) example,
that the 8 bits = 256 steps. This is the correct digital range that allows
for 2 steps minimum per the 100 parts of a 1% resolution input signal range.
There should be at least 2 steps / bits in the digital range per resolution
step in the analog input or you will cut the resolution in half for some
readings that are on the border between two steps. You must also consider if
the analog range FS and Digital FS range are the same, 0-5Vdc signals are
often feed into +/- 10V ADC = 1/4 the digital range covers the input range.
A simple rule is that a 3.5 digit multimeter has 12 bits of resolution and a
4.5 digit multimeter has 16 bits.
Caution, instrumentation resolution is often based on percent FULL SCALE as
this is the low value. A better and truer resolution number is percent of
READING. Thus scaling and the linearity will also make a difference. Even
higher resolution ADC's are often used in conjunction with a calibration
table to allow for non-linear inputs in dedicated measurements.
There are 4 parameters you need to worry about.
1 Resolution. A 13 bit plus signe digital to analog convertor divides
a 0-10v singnl into 8192 steps. (ie about 1.25mV). If measuring a
load cell mass in the range 0-100kg you could resolve down to +/-12.5
2 Analog Linearity.
All that accuracy will do you not necessarily be of use you if the
analog linerity of your front end is not up to it.
An ordinary Siemens 14 bit A to D card for an S7 has a linearity of
1.0% with 0.3% losses due to temperature coefficient, linearity and
(Its generaly much much better than that but that is what they quote)
So if the card is used to measure a weight in the 0-100Kg range it
could be of by +/-1kg. Due to the high resolution it would however
detect a 12.5 gram change in weight!
A high accuracy siemens analog card has a linearity of 0.05% to 0.1%
instead of 1% and it has 15 or 16 bit resolution.
The high accuracy cards cost about 60% more.
If you are trying to control a process: eg dosing or saving of
material you may need the higher accuracy. In an oild refinery saving
0.5% material can be a lot.