28119 Parallax Inc, 28119 Datasheet - Page 110

• Manufacturer Part Number: 28119
• Description: KIT UNDERSTANDING SIGNALS
• Manufacturer: Parallax Inc

Specifications of 28119

• Lead Free Status: Contains lead
• Accessory Type: Oscilloscope
• Interface Type: USB
• For Use With/related Products: BASIC Stamp® 2 and Board of Education
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

AN OP-AMP USED AS A BUFFER
Sometimes, when connecting two circuits that were designed to each perform a specific
function, the connection allows an interaction that makes the new combined circuit
behave in unwanted ways. To prevent this unwanted interaction, a buffer circuit can be
inserted between the two circuits, connecting them yet allowing them to operate as
originally intended. Op-amps are commonly used to quickly and easily fashion buffers,
which isolate and buffer circuits in a larger network. An example of an op-amp buffer
circuit is shown in Figure 8-4.
Functionally, a buffer circuit aims to generate an output that is identical to the input it
receives. To use an op-amp as a buffer, make sure you have proper supply voltages
(discussed below) and a fast enough slew rate. For all BASIC stamp applications, the
LM358 has fast enough slew rate for the job.
For example, let’s say we want to buffer a signal out of the BASIC Stamp that swings
from 0 V to 5 V. To prevent the LM358 from clipping the output, Vcc should be
supplied with a voltage that is 1.5 V higher than 5 V to prevent the signal from clipping.
Also, the power supply may fluctuate, so to be on the safe side, increase Vcc by another
10-20% (depending on the precision of your power supply).
Likewise, the supply voltage at Vee needs to be 20 mV below the lowest possible voltage
value of the input signal. Since the input signal is expected to drop as low as 0 V, the
Vee supply must be at least -20 mV. Again, you may also want to adjust downward by
another 10-20% in anticipation of power supply fluctuation.
Signal
Vss
Vcc
Vee
Vss
Vout
Figure 8-4:
An op-amp used
as a buffer