130-28176 Parallax Inc, 130-28176 Datasheet - Page 172

• Manufacturer Part Number: 130-28176
• Description: KIT PARTS PROCESS CONTROL
• Manufacturer: Parallax Inc

Specifications of 130-28176

• Accessory Type: Education Kit
• Product: Microcontroller Accessories
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With/related Products: Board of Education Full Kit
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

You may note the voltage increasing slightly over time. Very small leakage through the
non-inverting input is causing the capacitor to charge further. While not critical, if not
refreshed regularly (occurs when StampPlot is not connected), the voltage at the output
will rise to maximum over several minutes.
What occurs as the PWM is adjusted to maximum? Do you still get 5 V out? Chances
are, you see a maximum voltage around 3.5 V. The output of the op-amp cannot be
higher than the voltage supplied to it, and, in fact, will be 1 to 2 volts less than the rail or
supply voltages. To overcome this problem, the op-amp can be supplied from a higher
voltage – Vin on our boards.
Another name for the combination of filter and op-amp buffer is an active low-pass filter.
If we could adjust the input frequency, the maximum frequency (cutoff frequency)
allowed to be passed before being dramatically attenuated is 1/(2πRC).
In our example, attenuation is good because it is being filtered to a DC level leaving no
ripple or AC component. The PWM at 112 kHz is well above the cutoff frequency.
A high-pass filter, or one that blocks low frequencies while allowing high frequencies to
pass, has the resistor and capacitor positions in the circuit reversed but uses the same
formula for cutoff frequency.
ACTIVITY #5: OP-AMP NON-INVERTING AMPLIFIER
The buffer has a gain of 1, or unity gain. For each 1 V change in the input, there is a 1 V
change in the output. With an input of V
balance the inverting and non-inverting inputs.
√
√
√
Adjust the drive, which should produce stable voltages.
Connect pin 8 of the op-amp (NOT P8 on the BASIC Stamp!) to Vin. Ensure
this voltage is not connected to any other devices that may be damaged by it.
Retest the maximum voltage achieved.
f
CUTOFF
= 1/(2πRC) = 1/(2 x 3.14 x 10 kΩ x 0.68 µF) = 23.4 Hz
I
= 2 V, the output has to drive at V
O
= 2 V to