Cross in and out

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DexterMccoy

Joined Feb 19, 2014
429
What is this called , when you cross the output#1 of one op amps to another input and cross the output#2 to the input of another op amp

How does this circuit work? it looks like the use of the op amps to vary the DC offset voltage depending on the output of the Op amps
 

kubeek

Joined Sep 20, 2005
5,587
Maybe, just maybe, you could redraw it into two circuits, one with the swhich in one position, and another with the switch in the other position. Then see how it is connected together and find out what it does.

Also, how come you work with such circuits when you dont have the faintest idea what it is all about?
 

Thread Starter

DexterMccoy

Joined Feb 19, 2014
429
The switch grounds the output in one position
The switch in the other position lets the output of the op amp get feed to another input of the 2nd op amps that goes back into the Non invertering op amps which controls and sets the DC offset voltage

I'm just not sure what's it called when you have that CROSS on the schematic

It's a technique to do that CROSS from output to input, but what is it called?
 

GopherT

Joined Nov 23, 2012
7,983
It seem like that circuit would be flip flopping back and forth every time something happens.
 

Thread Starter

DexterMccoy

Joined Feb 19, 2014
429
It seem like that circuit would be flip flopping back and forth every time something happens
.

Not sure , because it's not Logic gates

It's modulating the DC offset voltage to the Op amps input (+) non inverting input
It modulates the DC offset voltage compared to the op amps output pin
 

kubeek

Joined Sep 20, 2005
5,587
I'm just not sure what's it called when you have that CROSS on the schematic
It's called "two wires crossing each other" ffs.

It's a technique to do that CROSS from output to input, but what is it called?
It's no technique, it is just a way to realize some function. I don't think it is any standard solution you would find in a book. You more often than not need to know what you're doing and what needs to be done to make the circuit do what you want. And that means you need to devise your own circuits, not just blindly cobbling stuf from a cookbook together.
 

kubeek

Joined Sep 20, 2005
5,587
I think that part of this circuit is a sample and hold configuration, as denoted by the heading hold pin.

Why are you messing with aircraft circuits? Should we be worried about airline safety in the near future?
 

BillB3857

Joined Feb 28, 2009
2,493
Overall, it looks like a sampling peak hold detection. The HDG HOLD(not) signal controls the analog switch that can provide reset or sample signal to the Sample/Hold circuit U29. The output of U29 supplies a reference to U15. When the input SIN* is applied to the inverting input of U15, the output of U15, with gain, will feed down the crossed wires to the input of U25. If it is in the Sample position, U29 will sample and hold that value and send it back up to the non-inverting input of U15 which will change the comparison level. As far as the wires being crossed, it could just as well been laid out linearly with U25 and U29 being placed to the right of U15 and a longer wire looping back from the R200-C48 node to the left side of R156. Just another way of drawing it.
 

Thread Starter

DexterMccoy

Joined Feb 19, 2014
429
Thanks Bill for your time and information

As far as the wires being crossed, it could just as well been laid out linearly with U25 and U29 being placed to the right of U15 and a longer wire looping back
The wires being crossed, is this feedback or what?

Because feedback is feedbacking the output back to the input of the SAME IC op amps

These wires being crossed
1.) takes the output of Op amp#1 and goes to the input of Op amp#2
2.) Output amps#2 output goes to Op amps#1 input

See how it's different then a feedback path
 

BillB3857

Joined Feb 28, 2009
2,493
It is a form of feedback since the overall output is being controlled, in part, by comparing the overall input to the comparison of what is being fed back. Think of the cruise control on your car. You press a button to "sample" the speed you are going and that sets the comparison level used by the system. If your car tends to slow down, the system compares that slower speed to the sampled speed and says, "speed up a little." When the new speed matches the sampled speed, the system says, "try to hold this, but keep watch for change." That's what feedback us used for. It will tend to hunt a little around the set point (sampled speed) but not much if designed properly.
 

Thread Starter

DexterMccoy

Joined Feb 19, 2014
429
Thanks for your time and info

So they are using Feedback to do comparison , like using feedback to be like a comparator

The feedback is the comparison level compared to the input
 

Thread Starter

DexterMccoy

Joined Feb 19, 2014
429
The gain ratio is about 1
57.6K ( feedback resistor )
49.9K ( feedback resistor )

Why did they choose these resistor values or this type of gain?

The output of the circuit which is HDG ERROR, is a DIfference voltage, it should be zero volts if everything is good and there is no difference or differential voltage

I don't understand why they didn't use a comparator IC chip to do this? why did they use 2 op amps like this to do comparing the difference or differential voltages? can't a comparator IC chip do this? and is the difference?

I guess a comparator IC chip is only On or OFF , switches the output to a HIGH or LOW state , plus it has to have a threshold voltage

This sample and hold is variable , not just on / off states

Here is another comparator circuit that i see often , the two comparators get summed to a GATE

I'm not sure why they didn't just use this type of comparator circuit instead of that sample and hold differential comparision circuit
 

BillB3857

Joined Feb 28, 2009
2,493
For answers to those questions, you need to contact the engineers that did the original design. Possible reasons would be parts availability at the time. I've seen a lot of older circuits that could have been a lot different today with newer technology available. It was not uncommon to use operational amplifiers as comparators during that time period.
 

Thread Starter

DexterMccoy

Joined Feb 19, 2014
429
Yes true they do use op amps as comparators but see the different op amp ccomparator circuits? I posted a two channel op amp comparator circuit
 
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