06-05-2011, 12:14 PM
More and more experimentsare now ‘PC-assisted.’ Alsoconventional acquisition systemsare very expensive. Since portablePCs are today common and a USB linkis a better solution than an old ISA bus,here we present an oscilloscope usingUSB port of the PC that operates at upto 10 kHz with ±16V input voltage. Ithas much more improved features thanthe PC-based oscilloscope and LEDscope for audio servicing published inEFY Dec. 2002 and Electronics ProjectsVol. 8, respectively. The oscilloscopeuses IC PIC18F2550 from Microchipas the main controller, which makesthe oscilloscope compact as there is noneed of additional power supply forthe entire circuit board. The prototypeof the circuit along with the enclosureis shown above.
Circuit description
At the heart of this oscilloscope isUSB2.0-compliant microcontrollerPIC18F2550 from Microchip. Youcan also use PIC18F2445 in place ofPIC18F2550. Specifications of this miitspins 5, 6 and 7.The MCP6S91 amplifier is designedwith CMOS input devices. It is designedto not exhibit phase inversionwhen the input pins exceed the supplyvoltages. The maximum voltage thatcan be applied to the input pin is –0.3V(VSS) to +0.3V (VDD). Input voltages thatexceed this absolute maximum ratingcan cause excessive current into or outof the input pins. Current beyond ±2mA can cause reliability problems. Applicationsthat exceed this rating mustbe externally limited with a resistor tothe input pin.Vref (pin 3), which is an analogueinput, should be at a voltage betweenVSS and VDD. The voltage at this pinshifts the output voltage. The SPIinterface inputs are chip-select (CS),serial input (SI) and serial clock (SCK).These are Schmitt-triggered, CMOSlogic inputs.The only disadvantage is that theseamplifiers accept only positive signals.That’s why voltage-shifting amplifiersLF353 (IC4Aand IC5A) are used,one each for eachchannel input (seeFig. 1). The LF353is a JFET input operationalamplifierwith an internallycompensated inputoffset voltage. TheJFET input deviceprovides wide bandwidth,low inputbias currents andoffset currents. Thisvoltage-shifting amplifierresults in ahigh input impedanceand an attenuationfactor of 1:4.5.A ±16V input signalis then shifted to thecrocontroller are given here in the box.This microcontroller has a USB2.0-complianttransceiver and a CPU runningup to 12 MIPS.Fig. 1 shows the circuit of thetwo-channel PC-based oscilloscope.MCP6S91 from Microchip Technologyis an analogue programmable gain amplifierthat is well suitedto driving analogue-todigitalconverters (ADCs)and an analogue input toa PIC microcontroller.Two MCP6S91 programmablegain amplifiers(IC2 and IC3) makeit possible to choose theinput ranges for each ofthe two channels, by selectinga gain from 1:1 to32:1. The amplifiers aresmall, cheap and easy touse. A simple three-wireserial peripheral interface(SPI) allows the PICto control them through
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http://hbeonlabsEFY%20Projects/PC%20based%20Oscilloscope.pdf
