Simple 4 Bit ADC?

[gstopp at fibermux.com]

     Actually the Polyphony design uses 4 op-amps plus some kind of divider 
     networks that change the thresholds of the comparators depending on 
     the current code value - that's why all the different resistors. Plus 
     the output is already binary so there's no decoding to do. So the 
     parts count would be much less than the 16-resistor 16-comparator plus 
     decoding scheme. However the latter circuit would certainly be easier 
     to debug if you made a soldering mistake!
     
     - Gene


______________________________ Reply Separator _________________________________
Subject: Simple 4 Bit ADC?
Author:  klund at MIT.EDU at ccrelayout
Date:    2/5/97 12:22 PM


>     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.