I'm an electronics and communications engineering student on my 3rd year, I'm currently working on a group project trying to make an industrial digital relay, whatever, it deals with current signals through a DSP, but before introducing the analog signal to the DSP there's a main block I should use called "Signal Conditioner", I think it works on filtering the signal of any noise probably, all I know till now that it consists of two stage RC filters and we're given the values of these components, the current signal is about 50~60 Hz, so I want to know how these values where determined and what are the cutoff frequencies..

These are the given values

And I've no idea what these zener diodes doing there also..

 

hi MT,
They are not Zener diodes, they are Schottky Barrier diodes, used for clamping the Input lines to Vdd+ 0.4V and 0V - 0.4V.
E

 

It is not clear from your schematic which side is the input and which side is the output.
You say the input signal is 50-60 Hz. Is there a DC component to the signal?

Since this is a school question, we will not just give out the answer.
Use what you know. Start at the beginning. What is the equation for the corner frequency of a single-pole R-C filter?

ak

 

hi,
I am assuming that the right hand side is the Output marked with the Yellow boxed arrow. pointing Out.?? Please confirm.

Look at these two LTSpice plots, #3 is the AC response and #4 shows the clamping diodes limiting the 5Vac input signal.
E

 

The right side is the input actually and the left side is the analog output after conditioning

I think it is a second order low pass filter to pass the 50~60 Hz signal and remove the noise, but I don't understand how..

 

I am assuming that the right hand side is the Output marked with the Yellow boxed arrow. pointing Out.??

If that is true, then it is only a single-pole filter. Right is the input.

ak

 

I can't find any available material about analyzing the circuit elements and the cutoff frequency

 

I can't find any available material about analyzing the circuit elements and the cutoff frequency

You're a 3rd year student studying electronics and you can't identify the symbol for a schottky diode, look up the datasheet given a part number, or analyze a low pass filter??

 

You're a 3rd year student studying electronics and you can't identify the symbol for a schottky diode, look up the datasheet given a part number, or analyze a low pass filter??

I've never seen these in the past 3 courses actually, and I know that it's a low pass filter but I don't understand how a second degree one works. Thanks anyway.

 

I've never seen these in the past 3 courses actually, and I know that it's a low pass filter but I don't understand how a second degree one works.

Lucky for you that it's only a single order. From the value of the first capacitor, you can tell it's for high frequency decoupling.

 

Lucky for you that it's only a single order. From the value of the first capacitor, you can tell it's for high frequency decoupling.

Disagree. Starting at the right and working toward the left, it is two single-pole low pass filter stages in series. Because the network impedance of the second is greater than 10x the impedance of the first, it doesn't load the first stage output (to a first order approximation).

Back to the TS: ignore the term "second order" for now, and concentrate on the first R-C stage. What is its cutoff frequency? What is its attenuation at 60 Hz?

ak

 

Disagree.

I stand corrected. I wasn't paying enough attention and didn't notice that the flow was from right to left. Another stupid backwards schematic...

 

I stand corrected. I wasn't paying enough attention and didn't notice that the flow was from right to left. Another stupid backwards schematic...

Kids these days ...

 

hi MT,
LTSpice re-run with the Vin , Vout corrected.
If possible, it is conventional to draw a circuit with the Input on the left side and Output on the right side, signal flow left to right.
Also note the LTS direction labels for Inp/Out
The second plot shows a 15Volt sine wave Vinp, which is just clipped by the diodes.
E

 

I was hoping the student would do the work.

ak

 

I was hoping the student would do the work.

hi ak,
I agree, that is why I have been careful to only post the LTS plots, which he can use to check his written calculations.

I hope he will post back his work.

E