Raspberry Pi I2C Analog-to-Digital Converter

The original plan for the Analog-to-Digital Converter was to use the Dallas 1-Wire system and use the DS2450 1-Wire Quad A/D Converter which was advertised as being a 16 bit device but after buying two of the chips which are around £5 each I found that near the end of the spec PDF they say that “In the shaded areas the accuracy is less than the resolution. The conversion results may include random noise.” Which meant that it is really a 8 bit AD converter with the higher quality settings not being reliable.

Raspberry Pi I2C Analog-to-Digital Converter

Raspberry Pi I2C Analog-to-Digital Converter

11. 07 2012 23:24 by brian

The original plan for the Analog-to-Digital Converter was to use the Dallas 1-Wire system and use the DS2450 1-Wire Quad A/D Converter which was advertised as being a 16 bit device but after buying two of the chips which are around £5 each I found that near the end of the spec PDF they say that “In the shaded areas the accuracy is less than the resolution. The conversion results may include random noise.” Which meant that it is really a 8 bit AD converter with the higher quality settings not being reliable.

Further research found the MCP3428 16-Bit, Multi-Channel Analog-to-Digital Converter with I2C™ Interface and On-Board Reference by Microchip which has 4 input channels and as it communicates via the I2C interface should be a lot faster than the 1-Wire IC’s

The I2C inputs on the Raspberry Pi needed to be buffered so a simple buffer was added to the circuit and the two MCP3428 IC’s are set to use different addresses by their address pins.

Each input is connected via a low pass filter to remove any high frequency noise and this now gives me 8 0-5V ADC inputs with a 16 bit resolution.

Raspberry Pi I2C Analog-to-Digital Converter boardWe spent today finishing the design and then building the new ADC board for the Raspberry Pi.

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The photos below show the build process starting with designing the PCB using Diptrace which allowed us to generate a 3D mockup of the board. The design was then printed onto transparency and then using a UV light box it was exposed , developed and then etched to produce the finished PCB board.

 

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