about simple flash ADC

[liusupeng]

for the simple flash ADC， the voltage reference for each comparator is generated by injecting a current into a series of resistors. suppose one LSB is R*I. The bottom resistor should be of value R or R/2?

 

[pannaguma]

It is R/2 AFAIK.
Lets suppose its a one bit ADC... you now need two take off points from the resistor ladder.
Recall that the top of the resistor ladder is connected to Vcc and bottom to Gnd.
So for two take off points, you effectively need three resistors, say A, B and C so that you get two take off points, ie between A-B and the other between B-C.
Now since the overall quantization is just around two levels in this case, it is customary to have sum of resistors in ladder = 2R.
Hence for symmetry the top and bottom resistors will be R/2, while central one is R.

You can extend the same logic for 2bit, 4 bit etc...
for 2 bit:R/2,R,R,R,R/2
for 4 bit:R/2,R,R,R,R,R,R,R,R/2

Hope it helps...

 

[liusupeng]

It is R/2 AFAIK.
Lets suppose its a one bit ADC... you now need two take off points from the resistor ladder.
Recall that the top of the resistor ladder is connected to Vcc and bottom to Gnd.
So for two take off points, you effectively need three resistors, say A, B and C so that you get two take off points, ie between A-B and the other between B-C.
Now since the overall quantization is just around two levels in this case, it is customary to have sum of resistors in ladder = 2R.
Hence for symmetry the top and bottom resistors will be R/2, while central one is R.

You can extend the same logic for 2bit, 4 bit etc...
for 2 bit:R/2,R,R,R,R/2
for 4 bit:R/2,R,R,R,R,R,R,R,R/2

Hope it helps...

In the one bit case, the output thermal code will be 00, 01, 11. How should i translate this to binary code of 0 and 1? 00 correspond to 0? bot 01 and 11 correspon to 1?

 

[pannaguma]

could you please rephrase your question?
I don't understand your question..

in one bit caseutput will be either 0 or 1.... NOT 00, 01, 11
in two bit case: it will be 00,01,10,11

 

[liusupeng]

could you please rephrase your question?
I don't understand your question..

in one bit caseutput will be either 0 or 1.... NOT 00, 01, 11
in two bit case: it will be 00,01,10,11

Hi,
As you mentioned, there are 3 resistors in series for the 1 bit case. So there will be 2 comparators. As the input voltage goes from low to high, the 2 comparator output should be thermal code starting from 00 to 01 and finally 11. Next the thermal code need to be converted to binay code. So how to convert this thermal code to binary code?

 

[jgk2004]

If you have a ladder of all R's then the ADC will be a mid tread flash, this means the mid point will not be at Vdd/2 but above and below by LSB/2. If you have R/2 on top and bottom you shift your ADC to be a mid rise flash, this means your mid point will be Vdd/2

Hope this helps,

Jgk