I sense that you have not yet slowed down your thinking to one step at a time, you want to go straight to an answer without the requisite plodding through. That's no way to figure things out.
you have a pressure transducer that has a 24vdc power supply and a voltage of 0-5vdc to measure the change in pressure,
Okay, the transducer does not "have" a voltage of 0-5vdc, it produces one.
First you had a pressure that you wished to measure.
You used a transducer to produce an electrical signal that is proportional to that pressure.
That's step one.
with the 0-5vdc connected to an analog to digital converter.
okay, computers only deal with numbers so you placed an A/D there to convert the 0-5 volts into a number for the computer.
That's step two.
With the analog to digital converter connected to a computer.
Okay, so the A/D is correctly interfaced to the computer.
That's step three.
Control systems use input frequencies to generate an output frequency.
Say what ? That's an unfounded assumption.
Control systems measure something and try to keep it within prescribed limits.
In steps one through three you established a basis to measure pressure with a computer.
Step four should be to go back and re-examine your concept of a control system.
Is the analog to digital converter generating a sinusoidal input wave so the computer can generate a sinusoidal output?
No, that's not what A/D's do. They convert voltage into computer words.
Pressure transducers can be bought with frequency outputs that rid the need of an analog to digital converter.
Some computers can read frequency directly in as a digital word, so that's another way to turn pressure into something a computer can work with.
Frequency will never be the same in a real system due to temperature or whatever else.
That's a mental trick we use to avoid plodding through homework problems "Ohh this is just senseless and i don't want to deal with it.."
I assume you have taken a course in computer programming? Remember how hard it was to slow down and plod through your first programs, "one line at a time"? Incompatible with today's instant gratification culture...
but it's nothing new - look what Laviosier wrote years ago:
"Instead of applying observation to the things we wished to know, we have chosen rather to imagine them. Advancing from one ill founded supposition to another, we have at last bewildered ourselves amidst a multitude of errors. These errors becoming prejudices, are, of course, adopted as principles, and we thus bewilder ourselves more and more. The method, too, by which we conduct our reasonings is as absurd; we abuse words which we do not understand, and call this the art of reasoning.
Not bad for 1789, eh ?
Maybe you need to revisit what is frequency.
It is the repetition rate of anything that is reasonably repetitive.
The frequency of moonrises is roughly one every 24 hours 50 minutes
the frequency of a piano string at middle C is about 261 vibrations per second
this is not meant to insult you, but to get you to think logically instead of thrashing about.
Lavoisier continued:
When matters have been brought this length, when errors have been thus accumulated, there is but one remedy by which order can be restored to the faculty of thinking; this is, to forget all that we have learned, to trace back our ideas to their source, to follow the train in which they rise, and, as my Lord Bacon says, to frame the human understanding anew.
"This remedy becomes the more difficult in proportion as we think ourselves more learned.
What then is frequency response?
It is how something, maybe a car suspension system or a loudspeaker or a control system or your ear
responds to a stimulus. Usually we specify that stimulus as a sine wave , or a spectrum of sinewaves .
http://www.centerpointaudio.com/HowToUnderstandSoundFrequencyResponseGraph.aspx
the image won't link, so check the site..
hope this helps.