Updated: Sep 8
Reading an analog signal into a pc can be accomplished many ways these days. Here I show how to use an FT2232 interface to read from a TLC1549 10 bit analog to digital converter. Download the sample program in VB.net at the bottom of the page.
Only 3 wires are required to interface from the Universal USB to Serial Converter to the A/D chip. Since the USB to Serial has 9 GPIO per channel, it is possible to directly connect 9 A/D converters to the interface. However, using an 8 to 256 multiplexer, it is conceivable to have 256 different A/D converters on one channel! Since the Universal USB to Serial Converter has 2 channels, a maximum of 512 unique signals can be watched.
Below is an image of the test circuit and also a screen shot of the captured voltage.
As you can see from the images above, the measured voltage is almost exactly the same. Note that the accuracy is dependent on the positive reference voltage. This example uses the 3.3v coming from the USB to Serial Converter, which I measured and set in the software code. This is marked near the beginning of the code and should be set to your REF+ voltage. To make this a useful voltmeter, a precision reference would be needed. A 1.024V reference would be ideal and give you a 1mV resolution. Also, a way to limit over voltage by using 2 diodes in series would clamp the input voltage to about 1.4V and a current limiting resistor should be placed in series with the input. This would give you a basic voltmeter but would need further work on the input side in order to control multiple ranges. i.e. 0-1V, 0-10V, etc.
One major drawback of this design is that it can only measure positive voltages, if negative voltages are to be measured, a way of converting the input from bi-polar to uni-polar is needed. One method can be found in another posting here.
Below is a close up of the test circuit.
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