Signal Conditioning Circuits

Why signal conditioning?


How could we possibly measure the signal we want (from the thermocouple) when we have ten-times more signal noise??

ANSWER: WE CAN’T!

So the key is to perform signal conditioning. We’ve actually already seen this concept ...

For today, we’ll study three types of signal conditioning:
 


Filter: a circuit that allows signals of a certain frequency to "pass" while greatly attenuating (diminishing) signals of unwanted frequencies.

To see how this works in principle, let’s once again return to our voltage divider circuit:

As always, we know that the output voltage will be given by the voltage divider formula:
 
 




 











But let’s look at that formula in greater detail. Some observations:

But there are other components than resistors that have impedances that are functions of frequency. For example, there is the capacitor:

Capacitors are circuit components that are capable of storing a tiny amount of electric charge.

The unit of a capacitor is the Farad (F). However, one Farad of charge is a tremendous amount! Therefore, sizes of typical capacitors are usually on the order of microfarads (m F = 10-6 F) or picofarads (pF = 10-12 F).

The impedance of a capacitor is called the "capacitive reactance" (Xc) and is given by the formula:
 
 




 






Impedance is a function of frequency:
 


Now, let’s use a capacitor in series with a resistor to form a voltage divider circuit. This is a common circuit called an RC circuit. Let’s measure the voltage drop of our voltage divider (Eout) across the capacitor:

The voltage divider equation for this will be:
 
 




 










Consider Eout for the cases:

This circuit is called a low pass filter.

The RC circuit can also create a high pass filter.

Now, RC circuits can be used to make a filters ... but not effectively.

An ideal filter would have an output characteristic something like this:

In reality, the output of an RC circuit low pass filter would look something like this:

One of the ways to get around this is to keep adding RC circuits in parallel:

This is what’s called a passive filter.

But if we amplify the signal, we get an active filter and can overcome the problems of a passive filter.

Although there are many types of amplifiers in electrical circuits, one of the most common and useful types is called the operational amplifier, or op-amp for short:

Inputs:
 

To set gain, we add two impedance devices:

Zf is the feedback impedance
Zi is the input impedance.

It turns out, no matter what the impedance devices are, that the gain for the op-amp in this configuration will always be:
 
 



 








So, to create a general purpose amplifier with a gain of 5, we could use resistors for Zf and Zi, and all we have to do is assure that the ratio of the resistance values (Rf / Ri) is equal to 5. NOTE: high resistance values should typically be used.

Take a look at other op-amp circuits to see how an op-amp can create a low-pass and high-pass filter ...
 

Op amps are incredibly versatile and useful circuits. We’ve only begun to explore their capabilities. As you saw when we studied Wheatstone Bridges, the use of an op-amp is critical to be able to increase the voltage difference across the bridge to a measurable level. That’s a very clear case of the need for signal conditioning.

Or is it?

Another kind of signal conditioning: analog-to-digital conversion (ADC). It’s useful to microcontrollers because all computers must work on digital signals.
 


For example, a "10-bit analog-to-digital converter" creates a binary word that is comprised of ten "1"s and/or "0"s. Such a word might look something like this:
 
 


1001110101


 








But what does this mean??

each bit, moving from right to left, represents an increasing power of the number two....

29 28 27 26 25 24 23 22 21 20

1   0   0   1   1   1   0   1   0   1

Now, wherever the bits are set to 1, we simply add those powers of two together. In our example, we would add up:

29 = 512
26 = 64
25 = 32
24 = 16
22 = 4
20 = 1
______

629 = total

For this 10-bit ADC, the binary number 1001110101 represents 629 analog-to-digital counts

This is what you see on the micro ....

But that still doesn’t answer the question of how this relates to analog voltage levels.

ADC makes a comparison of an input voltage (Vin) to a set voltage level called a reference voltage (Vref). The ratio of analog-to-digital "counts" to the maximum possible counts is then set equal to the ratio of Vin to Vref. NOTE: the maximum possible counts is given by 2N, where N is the number of bits in the ADC. In equation form, this is:
 
 



 






or, put another way …
 
 




 






which is the ADC equation you have used in lab. In lab:
 

So what does this have to do with signal conditioning?
  In that sense, analog-to-digital conversion is a very, very effective method of signal conditioning.

Lastly, the resolution (R) of an analog-to-digital converter is given by the formula:
 
 




 






This provides us with a measure of what a difference in one analog-to-digital count means in terms of differences in analog voltage amplitude.

So let’s say that we had a much higher bit ADC, and used a smaller Vref. For example, 24-bit ADCs are commercially available. According to the resolution formula, with Vref = 1 volt, the resolution for this would be about 59.6 nanovolts … that’s less than 60 billionths of a single volt!

Now, just food for thought, if we could digitize a small signal using this kind of ADC, where the resolution of the signal is so infinitesimally small, how important is an amplifier? In some applications, it turns out that they’re no longer very important at all. One case of this is a device called the Digital Bridge Analyzer, which was invented by Kettering’s Mechanical Engineering Professors Stokes and Kowalski. In this device, an amplifier is no longer needed with the signal differences of a Wheatstone Bridge used for strain gage measurements. They’ve successfully used a 24-bit ADC to read the signal and send it’s digital equivalent to a microcontroller for analysis!