PC Based Oscilloscope

        Now a day more and more experiments are now 'PC Based'. Also conventional acquisition systems are very expensive.Since Portable PCs are now common and USB link is better solution than Other ports.Here we present an Oscilloscope using USB Port of the PC that operates up to 10Khz with ±16V input voltage

         The oscilloscope uses IC PIC18F2550 from Microchip as the main controller, which makes the oscilloscope compact as there is no need of additional power supply for the entire circuit board. The microcontoller has an USB2.0 compliant transceiver and a CPU running up to 12 MB/s.It has a 10Bit ADC module with up to 13- channel multiplexer.

• A user-interface softwar  is used for output.

• IC LF-353 is used as a voltage-follower.It is a JFET input opamp with an internally compensated input offset voltage.The JFET input device provides wide band-width,low input bias currents and offset currents.


• IC MCP6S91  is an analogue programmable gain amplifier that is well suited to driving ADC converters and an analogue input to a PIC microcontroller.Two MCP6S91 programmable gain amplifiers make it possible to choose the input ranges for each of the two channels,by selecting a gain from 1:1 to 32:1.


• Major advantages of the USB is the ability to drive 5V,100mA devices.All the data is transmitted on the D+/D- symmetrical pins of USB using a variable bit rate.The position of a resistor on D+/D- allows to choose the full speed (12 Mbps) and low speed(1.5 Mbps) modes.


• A user-interface software written in VISUAL BASIC 6 used as user -interface unit.


• PC monitor is used as the output unit. USB and User software are connected by driver software mcchp.sys.


• All the data is transmitted on the D+/D- symmetrical pins using a variable bit rate. The position of a resistor (R13) on D+or D- in Circuit allows you to choose between the full-speed (12 Mbps) and low speed modes (1.5 Mbps).The VUSB pin may be used to pullup D+ or D- .Thepull-up resistor must be 1.5 kilo-ohms(±5%) as required by the USB specifications.


• 


• COMPONENTS LIST
  IC1 - PIC18F2550 microcontroller
  IC2, IC3 - MCP6S91 programmable gain amplifier
  IC4(A, B),
  IC5(A, B) - LF353 dual operational amplifier
  IC6 - ICL7660 switched-capacitor voltage converters
  Resistors (all ¼-watt, ±5% carbon):
  R1, R7 - 1-mega-ohm
  R2, R8 - 82-kilo-ohm
  R3, R9 - 33-kilo-ohm
  R4, R10 - 220-kilo-ohm
  R5, R11 - 150-kilo-ohm
  R6, R12 - 1-kilo-ohm
  Capacitors:
  C1, C2 - 2.7pF ceramic
  C3, C4,
  C5, C6 - 68nF ceramic
  C7 - 0.022μF polyester
  C8 - 150nF ceramic
  C9 - 10μF, 16V electrolytic
  C10 - 47μF, 16V electrolytic
  Miscellaneous:
  X1 - 4MHz crystal oscillator
  CON1 - USB socket (type-B) for PCB


• Programming description: The program for the microcontroller is written in ‘C’ language.All operations are initiated by the host (the PC), which sends a 16-byte command.The first command byte determines the chip actions. The four possible actions are:
  1. Command 80h: Clears the EEPROMcalibration memory
  2. Command 81h: Receives parameters,and stores the gain-error compensation for the two channels.
  3. Command 83h: Initiates a zerocalibration sequence of the two channels.The two inputs have to be grounded.
  4. Command 85h: Initiates measurement of the input signals.
  The PIC software consists of eight ‘C’ program source files, 14header files
  and a linker file all under‘aquis.mcp’ project


• REFERENCE: 1.Electronics For You December 2006
  2.www.wikipedia.org
  3.www.alldatasheet.com
  4.www.microchip.com
  5.www.electronicsforu.com
  Govt.


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