28128 Parallax Inc, 28128 Datasheet - Page 72

• Manufacturer Part Number: 28128
• Description: KIT BASIC ANALOG/DIGI PARTS ONLY
• Manufacturer: Parallax Inc

Specifications of 28128

• Accessory Type: ADC, DAC
• Product: Microcontroller Accessories
• For Use With/related Products: BASIC Stamp® 2 and Board of Education
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Page 62 · Basic Analog and Digital
amount of the voltage range covered between each of these discrete values. Since the first
value is zero, there are 255 voltage steps. The step size is given by:
5
Volts
=
=
≅
Step
Size
0.0196
Volts/step
0.02
Volts/step
255
steps
With this in mind, each time you adjust the pot, the converter comes close to
approximating the analog value, but it's not exact because of the resolution constraints.
So, there is still some uncertainty at the hundredth’s decimal place. In some applications,
the uncertainty is stated along with the measurement. Assuming the ADC0831 rounds at
the half way point, we can use this convention to read the voltage from Figure 3-9 as
"3.24 volts plus or minus 0.01 volts."
Higher resolution converters are available, such as 12 and 16 bit (and higher), but
because of their higher resolutions, they come with a higher cost as well. The
improvement in resolution is significant. As mentioned before, a 12-bit converter will
give you a resolution of 4095 steps. This results in 5 volts / 4095 steps, or one step for
every 0.0012 volts. A 12-bit converter typically costs more money than an 8-bit
converter. There is also a cost in terms of the amount of memory the measurement takes,
(12 as opposed to 8-bits) and the amount of processing it takes to get each measurement
(13 clock pulses instead of 9).
Calibration
What if the power supply on the Board of Education only supplies 4.963 volts instead of
5.000 volts? The BASIC Stamp voltmeter can be calibrated using a second voltmeter
known to be highly accurate. The difference between Vdd and Vss can be measured
using the accurate voltmeter. Developing the code to correct for this error requires more
representation of fractional values using integer math and is best left as a challenge
problem.
Another item to consider if you're shooting for a high degree of precision is that different
current draws on the power supply can also cause variations in power supply output
voltage. This is an experiment unto itself that would also require additional equipment.
As you probably guess, designing for a high degree of precision involves a number of
design challenges. For the remaining experiments, the present degree of accuracy of our
DVM is sufficient.