Simple 4 Bit ADC?

[klund at MIT.EDU]

>     Second, you can make a flash converter out of op-amps. This type of 
>     converter has a much faster conversion time, but uses a whole bunch of 
>     different valued resistors. 

All the flash converters I've ever seen use equal valued resistors...
Say we wanted to build a 4 bit 0-8v flash ADC (to make the math easy).
Take 16 equal valued resistors and string them together in a single
divider from 8 volts.  Now we have a set of 15 reference voltages, being
0.5, 1, 1.5, 2, 2.5, 3, ... up to 8 volts.  Now, with 15 comparators,
compare the input voltage to each of the refernence voltages.  This
gives a "thermometer code": each of the comparators connected to
reference voltage below the input has an output of "1" and each of the
comparators connected to reference voltage above the input has an output
of "0".  The conversion from thermometer code to a 4 bit word is
straightforward.  I'll let one of you digital weenies come up with that.

--> Kent.