looking for a certain type of ADC - not sure if it exists, what it would be called, or the best way to achieve the desired output...

Aug 08, 2005 4 Replies
P

Hi. Hope its ok to just pop in and ask a question like this.



I'm looking for something similar to an 8-bit ADC, except that instead of having the normal bit output of an 8-bit binary number, 0-255, I want only one bit to be high at a time - essentially for the output to be quantized to 1,2,4,8,16,32,64 and 128 (00000001, 00000010, 00000100,



00001000, 00010000, 00100000, 01000000, 10000000).

The objective is to control a series of analog switches and unfortunately due to the design of the circuit I'm interfacing with here, I can only have one switch open at a time, therefore only one bit of the ADC output high at a time.



Someone else out there must have needed this type of output in the past but I haven't been able to find an IC that does it. Any suggestions would be much appreciated.



Thanks picobot



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J

What you need is a priority encoder followed by a 3-line to 8-line (or a dual 2-line to 4-line) decoder/demultiplexer. You could use either an XX147 or XX148 for the priority encoder, and either an XX138 or XX238 for the 3-line to 8-line decoder, depending on whether you wanted inverted or true inputs. If you wanted to use a dual 2-line to 4-line decoder you could use either an XX139 or XX239, (along with an inverter) again depending on whether you wanted an inverted or true output. The priority encoders have active low inputs and outputs, so you'll have to invert the ADC's outputs and reorder the decoder outputs so they sequence properly.
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P

I think a 138 decoder will do the trick as you suggested.

Looks like I could also use a 3 bit ADC instead of the priority encoder for the input going to this decoder.

Thanks much for the info.

picobot

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J

Yes, excellent idea! Scale the input to the ADC so that full scale volts in equals 111 out and zero volts in equals 000 out, and the '138's outputs will give you just what you want. Very nice.
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P

Yea I was thinking that too last night...after realizing that a 3 bit adc may be hard to find. I have some 8bit ADCs here already so I will try to scale the analog input down to the range of the 1st 3 bits of output.

` . |-`~`|_ | | | | |_|

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