Isolated, 0.1% Accurate, 8 Channel, 12bit A/D Converter with PC Interface
This circuit uses the parallel port on the PC to interface to a MAX147 12bit, 8 channel A/D converter. By using the National LM4040AIM-4.1 reference, an accuracy of 0.1% is obtained. Each of the input channels to the A/D is buffered with a rail-to-rail op-amp to isolate and buffer the analog inputs. These inputs also have input protection diodes to protect the op-amps. At 25C, these diodes have a reverse leakage current of 25nA so keep the source impedance less than 10K ohms to keep the offset error les than 1 LSB. Power is obtained by following the 6-9 volt battery with a low dropout linear regulator. The HPCL-2730 opto-isolators were used for their high current transfer ratio and to electrically isolate the PC from the harsh analog realities of the outside world.
The following table maps the MAX147 A/D converter signal to the matching signal on the PC parallel port:
| PC Pin / Signal | MAX147 |
| 15 / S3 | Dout |
| 9 / D7 | See Text |
| 13 / S4 | Spare Output |
| 2 / D0 | SCLK |
| 3 / D1 | CS\ |
| 4 / D2 | Din |
| 5 / D3 | Spare Input |
Since the Parallel port of the PC does not have any power coming out to it, it's necessary to drive the pull-up resistors with the D7 output. Therefore, ensure that D7 is high if you want to see the Din signal from the A/D. A spare, isolated output and input are available for additional I/O.
At this time it will left as an exercise for the student to write the code necessary to perform the I/O function between the MAX147 and the PC. However, the following link should contain more than enough information to perform this task whether you're writing in C, C++, MS BASIC or Visual Basic:
In addition, click the following link to download the MAX147 datasheet in PDF format:
MAX147 Schematic