My dear friend, Neil Albaugh—a veritable analog ‘Wizard’, wrote an often unnoticed handbook in January 2000 at Burr-Brown which stands the time 16 years later and will be still relevant in the future as well—Analog basics are “evergreen”. The title was The Instrumentation Amplifier Handbook and I would like to share the top 12 most important aspects of designing a functional and robust circuit using the instrumentation amplifier. You can access it online here. All images and references are attributable to this excellent handbook and there are far more details and tips about INAs in the handbook that will make you an expert in design.
Click on the slideshow image below to see some insightful tidbits about INAs or IAs, as they are sometimes called, that might give you a feel for proper usage and design using this long-time useful tool for Analog designers:
Here we see a poor choice in the use of an INA in a high impedance transducer application. The designer needs to create a low noise pre-amp for the piezo accelerometer source at its input. Well, the INA103 has under 1 nV/ √ Hz and low distortion with excellent common mode rejection. Also, a very low noise amplifier will want to have a low source impedance. That 1 MΩ resistor, needed for extending the low-end frequency response of the transducer/amplifier combination will not bode well for this design. The INA103, chosen here, has an input bias current of 2.5uA typical and 8uA max which will pass through the 1 MΩ resistor creating a whopping 2.5V input offset at the inverting input of the INA—-with the INA’s gain of 100—the output saturates at the rail. In these types of high impedance sources, a better choice of amplification would be a very high impedance input JFET or CMOS amplifier.