# ISA Interchange

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# AD Converters Quiz: convert an analog signal to a digital one

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AD Converters are necessary to convert an analog signal to a digital one that our computers can understand. The output of an AD Converter is a binary number. Digital instruments most commonly use 12, 14 and 16-bit converters, although there are much higher bit converters available.

The following is a practice question from one of ISA’s classes:

1. Calculate the incremental steps for a 12-bit A/D converter

2. If the sensor connected to the A/D converter was ranged for 0-1000 F, what is the smallest increment of temperature that can be seen on the digital side of the A/D converter?

3.  If I wanted to control the temperature to within 0.1 F can I do this with a 12-bit A/D converter in the sensor module?

4. What effect would a 14-bit A/D have?

5. Can you think of further improvements?

• Answer to 1.

LSB represents = 100% / 2^12 = 100%/4096 = 0.0244%

• Answer to 2.

LSB (or smallest increment of temperature on the digital side):

= 1000 F / 2^12 = 1000 F/ 4096 = 0.244 F

• A 12-bit A/D converter in the sensor module is not accurate enough.
• A 14-bit A/D converter would give a resolution or LSB of :
• = 1000  F / 2^14 = 1000  F / 16384 = 0.061  F
• 14-bit A/D converter would be better for control to within 0.1 F, however a 16-bit A/D converter would be even better because the resolution would be 0.0153 F.

Some points to remember are that although a higher bit converter allows for highly accurate instruments, we want our test equipment that we use to calibrate them to be at least 4 times more accurate. So for example if we had a transmitter with an accuracy of .02% FS, then our test equipment would have to have an accuracy of .005% FS. Also if the AD Converter on the host system’s analog input card (when the transmitter’s digital signal is being converted to 4-20) is not equal or higher in bits then the transmitter’s, the resolution that you gained is lost. ###### Mark Weisner
Mark Weisner is a technical specialist at ISA.