28128 Parallax Inc, 28128 Datasheet - Page 77

• 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

Chapter #4: Basic Digital to Analog Conversion
BUILD A RESISTIVE LADDER NETWORK
Digital to analog conversion (D/A conversion) is, for the most part, the reverse of A/D
conversion. With A/D conversion, we started with a continuous voltage range at the
converter's input. The A/D converter rounded to the nearest voltage step and sent a binary
output indicating which step it measured.
D/A conversion starts with a binary number as the input, and the output is a voltage step.
While the A/D process starts with an analog input and ends with a binary output, the D/A
process starts with a binary input and ends with a voltage step for an output. It's not a true
analog value that varies continuously; it's a discrete voltage that varies in steps.
The term resolution was introduced at the end of Chapter #3. Since a D/A converter's
output always going gets rounded to a voltage step (a discrete voltage value) it's
important to pick the right resolution for your D/A converter. Remember that with higher
resolution comes higher precision, but it typically comes at the price of greater expense,
more memory, and more processing steps.
The number of voltage levels a D/A converter can produce is given by how many
counting numbers you can get from the number of binary bits in the resolution. We can
use the combinations equation again to figure this out.
The D/A converter we will use in this experiment has 4-bit resolution, so the number of
output voltage levels for the converter will be:
In Chapter #3, we used an integrated circuit, which performed the A/D conversion. In this
experiment, we'll build a D/A converter using resistors. It's called a resistive ladder
network, and adding or removing resistors can be done to change the resolution of the
converter. With a resistive ladder network, if you start with a 4-bit converter, and you
want to increase the resolution by 1-bit, all it takes is two extra resistors added to the
network.
In this chapter, we will build a resistive ladder network and program the BASIC Stamp to
make the network do D/A conversion. PBASIC will be used to program the BASIC
combinations = 2
combinations = 2
bits
bits
= 2
4
= 16