• Gain: The amplification of the input signal. This is done to make it the correct level for connection to the next stage, be it a load, data converter, etc.
  

• Attenuation: The reduction in amplitude of the input signal.
  

• Buffering: A unity gain buffering of the input signal - almost always done to convert a high input impedance to a low impedance to drive a subsequent device.

• Non-Inverting: The output signal occurs in phase with the input signal.
  

• Inverting: The output signal occurs 180^o out of phase with the incoming signal. Op amp stages come in inverting and non-inverting varieties, which may or may not be important for your design. Performance wise, there is little difference between the two configurations. If you don't care about the polarity, you can choose either non-inverting or inverting gain configurations - based on which results in a simpler implementation. Flexibility is almost always a good thing! Look at both cases that fit your design, and choose the simplest. If you do care about polarity, the solution for that need is supported with the best possible fit for your design.
  

• Offset: The addition or subtraction of a DC potential to a signal. This is not required in applications that are AC coupled - that have a capacitor isolating the input and the output of the stage. Offset can be related to single-supply operation - which is covered in other Knowledgebase FAQ's. Most applications are AC coupled, so the offset can be ignored. DC coupled applications, however, MUST take DC gain on offsets into account.
  

• Filtering: The removal of some frequencies from the input signal while others are passed. This is the subject of other Knowledgebase FAQ's, and is also handled by Texas Instrument's Filter Pro software.

• Gain Plus Offset: An op amp stage that contains both gain and offset. The gain can also be unity or attenuation. It can also be inverting or non-inverting. Offset may be positive or negative - which leads to many possibilities or "cases" (see next definition).
  

• Case: Used in this Knowledgebase section to indicate combinations of gain and offset. The concept of "cases" comes from the book "Op Amps for Everyone", edited by Ron Mancini, individual chapters of which are downloadable from the TI web site. Chapter 4, Supply Op Amp Design Techniques, introduces four different cases. It became evident that these are only four cases out of several more that are possible - from the trivial to the quite complex. There is a table showing combinations of gain and offset on the utilities page that linked you here. Clicking on different "cases" sends you to calculators to find the best solution.

Why All The Cases?

Take, for example, gain with no offset (AC coupled):

• Gain = - Rf/Rin for inverting
• Gain = 1 + Rf/Rin for non-inverting

These are two of the "cases" in the table. So, you ask - why use a calulator for gain when the equations are so simple? I counter with a question: do you know the best set of resistors for your gain? If you need a gain of 5.75, for example, do you know off the top of your head to use 1.15k and 200 Ohms? I sure don't! Real resistors have tolerances, of course, so even a calculator that gets you the best set of resistors does not guarantee a perfect gain. But it is good to get as close as you can to begin with, and it is easy to buy precision resistors. A quick check of several vendors shows 0.1% and better are available in both the E24 and E96 sequences. The added cost is small compared to the cost of additional A/D bits to compensate for range lost due to imprecise interface circuitry.

Now, consider the cases that add offset to the design. For many years, I would just use two op amps to do it, particularly if the answer was not self-evident. In the vast majority of cases, though, a very good - perhaps ideal solution exists which can be implemented in a single op-amp. The schematic may or may not be easy to draw, but the design equations may not be obvious at all. In one case, the derivation of one deceptively simple case consumed three days, including false starts and "blind alleys" of simultaneous equations. We did the work - so you don't have to!

How to Determine the Case

You got here by clicking on this image:

The pieces of information in the boxes represent the minimum amount of information you need to figure out what design case you have. You need to know your input range, output range, and a value of reference voltage for cases with offset.

The input range is determined by knowing:

• Vin Zero Scale: The lowest voltage level to which your input will go.
• Vin Full Scale: The highest voltage level to which your input will go.

Similarly, the output range is determined by knowing:

• Vout Zero Scale: The lowest voltage level that you want your output to go.
• Vout Full Scale: The highest voltage level that you want your output to go.

Vin zero and full scale are usually dictated to you by an input device or stage. Similarly, Vout zero and full scale are probably determined by the subsequent stage. Your op amp interface is the "glue" between the two.

BUT WAIT!!! All I know is the gain! --- you might be saying. OK, fair enough. Your application is probably AC coupled. But you better know the amplitude of your input signal - or you might end up hitting the voltage rails of your op amp if you give it too much gain! DO YOUR HOMEWORK - find that amplitude, do your calculations based on the maximum. Once you have the amplitude - say 1 Vpp - then center it on zero volts. For 1 Vpp:

• Vin Zero Scale = -0.5 Volts
• Vin Full Scale = +0.5 Volts

Next - center your output amplitude on zero volts. You do know your output amplitude, don't you? It is the input amplitude multiplied by the gain. Let's say you want a gain of ten on the signal above. You output amplitude, then, is 10 Vpp, and:

• Vout Zero Scale = -5 Volts
• Vout Full Scale = +5 Volts

These are the values you enter in to determine the case. Because this example was AC coupled, you probably don't have a Vref. If you do, it is only used internal to the stage to set the DC operating point - and won't affect the selection of case.

A Little Parting Advice

In the example above, the output amplitude range is +/-5 Volts. Hopefully, it goes without saying that you cannot get this range from a single 5 Volt supply. What is more annoying is that you also cannot do it from a +/- 5V supply, although some rail-to-rail devices will come close.