BCD Switch to Voltage Converter

[galaxytech]

I would like to be able to dial in a voltage value (millivolts) output using a BCD switch as the input. There would be 4 BCD switches arranged from LSB on the right to MSB on the left. The highest BCD input would be "9999", and would generate 9,999 millivolts (or 9.999 volts). The lowest, of course, would be "0000" and 0mv would be the output. A "1" on the LSB digit would output 1mv, "2" would output 2mv, etc. This is for providing a feedback signal to a thermocouple chip developing a setpoint for the chip and an output to drive a relay.

I can't seem to find any chips that would provide this functionality. Any help would be greatly appreciated.

 

[crutschow]

Unfortunately there doesn't seem to be a lot out there to do what you want. You could use a DM74184 BCD to Binary converter to drive a binary D/A converter. The problem is, I think the DN74184 is a obsolete device and may be hard to find.

You could also program a 16-bit microcontroller with a built-in D/A to do the BCD to Binary conversion and output the voltage.

 

[duffy]

You could do it with 16 binary weighted resistors (trimpots), buffers, opamps, and a fair amount of patience. Each set of 4 resistors would be adjusted to give a maximum voltage 10x the previous set, this would handle the BCD to binary conversion. You would need the buffer because the switches probably aren't SPDT, and you are going to have to actively pull these resistors low and high.

The buffer probably isn't going to go rail-to-rail, and even a millivolt of offset will throw the low-order inputs - but if you adjust the resistors to match the characteristics you ought to be able to compensate for offset and span in the final stage.