# Successive approximation help!

#### simon.cathy

Joined Dec 18, 2006
2
I am constructing binary words from a given voltage, using successive approximation. What happens when the voltage is the SAME as a bit value(ie not greater, or less than).Hope that makes sense! Thanks.

#### Ron H

Joined Apr 14, 2005
7,014
I would think that the comparators in most successive approximation A/Ds have hysteresis (Google), which precludes an indeterminate logic output level at the expense of a small amount of error. If no hysteresis is present, the succeeding latch (or flip-flop) can possibly go into a metastable state (Google).

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

If you get the spec sheet for one, the manufacturer usually does a good job of explaining the theory behind the A to D's operation. Analog Devices makes a zillion of them.

#### simon.cathy

Joined Dec 18, 2006
2
Hi,

If you get the spec sheet for one, the manufacturer usually does a good job of explaining the theory behind the A to D's operation. Analog Devices makes a zillion of them.
A quick look at Analog Devices tutorials shows a greater than or equal to sign, which solves my problem! Thanks for the pointer.
Simon

#### Dave

Joined Nov 17, 2003
6,969
I am constructing binary words from a given voltage, using successive approximation. What happens when the voltage is the SAME as a bit value(ie not greater, or less than).Hope that makes sense! Thanks.
Having the two inputs of a comparator the same is pretty difficult to achieve in practice still one must account for the possibility. Ron H has hit the nail on the head with the assessment of the comparator being in terms of a hysteresis characteristic. The comparator has two thresholds VTL and VTH which define the low and high input voltages respectively required to trigger a switch at the output. In fact in the context of ADCs a comparator without this hysteresis characteristic is pretty useless. Consider the situation with high-frequnecy noise (say from interference) imposed on the low-frequency signal you are comparing to some datum signal. If there were no hysteresis characteristic (i.e. VTL = VTH), then at the the theshold point the output of the comparator would fluctuate wildly in line with the high-frequency noise as it fluctuated around the threshold point, thus making the output reading null. In this sense the comparator is performing a high-frequnecy filtering function.

Dave