[sdiy] voltage ladder with hysteresis

[Steve Lenham]

----- Original Message ----- 
From: "mark verbos" <mverbos at earthlink.net>

> Hi there,
>
> I built a ladder of 16 comparators on a sequencer. The CV goes  through 
> and inverting stage and thresholds are set with a string of  resistors 
> from ground to -15 volts, going to the non-inverting input.  I found the 
> transition points were blurry, and it needed some  hysteresis. I added 1M 
> resistors from the comparator outputs to the +  inputs and this works fine 
> on the lower stages, but I find that when  I sweep the input with a saw 
> wave, now the lower stages go slow and  it moves faster as it goes to the 
> right. All the way on the highest  stages wouldn't even switch right until 
> I upped the feedback resistor  to 2M. Is there some standard way to figure 
> this problem out?  Should  I just put a 1M on stage 1 and a 2M on stage 16 
> and divide equally  between?

Hi Mark,

You are using inverting comparators, so your hysteresis resistors are all 
injecting current into the string of reference resistors, but at different 
points. Therefore the effect that they will have on the reference voltage at 
that particular node on the string will vary. Imagine the hysteresis 
resistor and the "bottom" part of the reference string as a potential 
divider between the output of the comparator and ground.

For the "lower" stages, the resistance to ground is fairly low, so the 
comparator output voltage will be divided down significantly and the change 
to the reference voltage will be small.

However, the higher up the chain you go, the higher the resistance to 
ground, with the voltage being divided down to a smaller degree and the 
effect on the reference voltage being correspondingly larger. The highest 
stages will require increasingly large input voltages in order to turn on 
(remember that as well as making the comparator stay on more easily once its 
reference has been exceeded, hysteresis also makes it slightly harder to 
turn on when it is off).

The calculations for your circuit are complicated by the fact that several 
comparators can be on at the same time and so the effect of all 16 
hysteresis resistors needs to be taken into account at all times.

I would suggest that you look at using non-inverting comparators (Google for 
it), where the reference string feeds the inverting input whule the input 
and hysteresis feedback are applied to the non-inverting input. This 
isolates the hysteresis for each stage, allowing you to easily set the same 
amount of hysteresis for each with no interaction between stages and no need 
to consider different hysteresis resistor values for each stage.

HTH,

Steve L.