Most instrumentation amplifier (in-amp) users are familiar with the concept that the output swing of an in-amp can be reduced for high or low common-mode voltages because of the saturation of internal nodes. This gives rise to a very common graph in in-amp data sheets called Input Common-Mode Voltage vs Output Voltage, also nicknamed the Diamond Plot. It is an essential design consideration and one of our most common in-amp questions as you can see from this FAQ and the many related questions.
There are two problems with the Diamond Plots shown in in-amp data sheets. 1) Not all of the possible configurations are covered and 2) it is difficult to accurately apply the information in the plot to an actual circuit. With our new in-amp web tool, we plan to solve both of these problems and turn designing with in-amps from a headache into a cakewalk.
Access our tool at http://www.analog.com/designtools/en/diamond/ , and please leave feedback for us.
The first problem can be solved if we can generate a Diamond Plot based on user input. That way we eliminate all of the irrelevant information and only present you with the graph that fits your circuit. There are only a handful of different types of in-amps in the ADI portfolio. With the input from a few experts, we have plotted all of the equations and are able to generate a diamond plot. Here is a side-by-side comparison of a Diamond Plot from the AD8422 data sheet and a Diamond Plot from our prototype.
AD8422 Diamond Plot from datasheet,
AD8422 Diamond Plot from web tool,
Because the prototype can generate this graph for arbitrary conditions, you’re not locked into this configuration and it’s easy to see what happens if one of the supplies or the REF pin voltage changes.
The second problem is a matter of determining the area where your circuit operates on the graph. Typically, the area of operation can be drawn as a box with limits at the circuit’s minimum and maximum output voltage range used and the minimum and maximum common-mode voltage seen at the inputs. Keep in mind, however, that the definition of Common-Mode Voltage is VCM = (V+IN + V-IN) / 2. Therefore single-ended input signals, in which the voltage at one input is fixed and the other is varied, will change the common-mode voltage along with the differential voltage, as shown below:
The prototype tool is designed to account for all of this by calculating the operating range and plotting it directly on the Diamond Plot. Using all of this information, the tool prototype can generate an error message if your circuit is out of bounds for the chosen in-amp. It will also warn you if the supply voltage, gain, or REF pin voltage are out of bounds, so you know that your circuit will work.
In the past, selecting an in-amp has been a difficult process, especially with small supplies or a single supply, where the Diamond Plot becomes a serious limiting factor. The tool prototype can calculate whether all of an in-amp’s voltage ranges are in bounds, therefore it is straight-forward to generate a list of suitable in-amps for any chosen configuration. This list allows a designer to go forward and select an in-amp based on its specifications, without wasting time trying to determine if the headroom limits are satisfied for every in-amp.
Here’s a pro tip: transistor saturation is a ‘soft’ limit, therefore linearity decreases a few hundred millivolts away from the actual limit. If you want to ensure that you will always be in a nice linear range, you can reduce the supplies in the tool and use that voltage instead to generate your list of products. For example: If you have a single +5V supply, consider setting –Vs to +0.2V and +Vs to +4.8V to ensure a little bit of extra headroom. This is also a good way to account for power supply droop and tolerance.
This is a work-in-progress and if there are any bugs or issues, we want to know about them. But there are also a few limitations to keep in mind. The limits shown in the prototype are verified by the in-amp designers and by characterization data, but they represent typical behavior, and not a guarantee. Furthermore, these limits are at 25°C and the behavior over temperature is not represented.
Please visit the tool at http://www.analog.com/designtools/en/diamond/ and let us know what you think. Use the feedback form on the tool page, or in the comments section below.