28128 Parallax Inc, 28128 Datasheet - Page 94

• 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

The reason for this goes back to Ohm's Law: V = I x R (voltage equals current multiplied
by resistance). Each BASIC Stamp I/O pin can supply up to 20 mA of current. In the case
of the resistive ladder network without the buffer, the BASIC Stamp I/O pins reached
their maximum current output. Meanwhile, the resistance seen by the I/O pins stayed
about the same. In other words, I x R reached a limit because I (the current) could no
longer increase, and R is a fixed value. So the output voltage is equal to a current value
that can't get any higher multiplied by a fixed resistance. This is why the voltage stops
increasing.
Aside from isolating two circuits from each other, an op-amp in voltage follower
configuration can typically supply more current than the circuit connected to its input.
The name buffer is commonly used when a voltage follower is used to supply extra
current.
The circuit from Figure 4-8 will also be used at the beginning of Chapter #5, so do not
disassemble the circuit after completing Chapter #4.
Decimal
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
Table 4-2:D/A Converter Output with Buffer
Binary
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
DVM (Volts)
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