CURRENT ISSUE
Contests
FEATURE ARTICLE
|
|
Issue #214 May 2008
Where Analog And Digital Collide
An Easy-To-Use LCR Meter
Third Place Microchip 2007 Design Contest
by Miguel Rusch
Start | LCR Meter | Back to Basics | The Big Picture | Creating a Wave | Analog Stages | Signal Conditioning | User Interface | Firmware | Take a Measurement | System Performance | Further Development | What's Next? | Sources & PDF
THE BIG PICTURE
 |
| Figure 3 — The dsPIC30F4012 core communicates with five devices in the analog stages and the user interface via a SPI. User input is made by switches in a resistor ladder that is digitized by the ADC module. |
Figure 3 shows the main parts of the system. The core device is a Microchip Technology dsPIC30F4012 microprocessor, whose system clock is derived from an external 6-MHz crystal feeding an internal 16× PLL, resulting in 24-MIPS operation (see Figure 4). The oscillator frequency was chosen to allow ADC sample rates that were exact multiples of the test frequency.
The analog measurement block and a user interface block support the core microprocessor. Extensive use of the SPI port was made to maximize functionality from the 28-pin dsPIC device. For example, the LCD’s 8-bit parallel port is controlled by a Microchip Technology MCP23S08 SPI GPIO. The analog block also consists of several SPI devices, including two Microchip Technology MCP6S91 PGAs, an Analog Devices AD9833 DDS signal generator, and a Microchip Technology MCP41010 10-kW digital potentiometer. The chip select (CS) signal between the digital potentiometer and the GPIO were able to be shared due to the instruction set bit masking, further reducing the required pins. User input requires only a single pin because control buttons are interfaced to the ADC with a resistor ladder.
The prototype system was built around Microchip’s 16-bit 28-pin development board. I also built three other boards (see Photo 1). One board carries the user interface, which consists of a graphic LCD and control buttons. Another custom-etched board contains the analog measurement components. The antialiasing filters are constructed on a separate protoboard. Jumper wires connect the boards.
The DUT is connected via a set of test leads to the analog board. The prototype used RCA connectors between the four-wire test leads. The PCB cable shielding is connected to the analog ground plane by a pair of screw terminals. RCA plugs were used because they are inexpensive and perform well at the test frequencies. In the final design, four separate RCA plugs will be employed, each with their shield connected to the outer terminal to enable better noise immunity. The DUT is connected via a pair of parrot clips, which enable hands-free use for axial components. A tweezer test lead will be made for SMD parts.
Previous | Next *| Subscription Deals |*
|
