A few weeks ago I wrote a blog titled “Behind the Tilted Triangle,” which introduced a new JFET amplifier by way of a brief history of all amplifiers, from tubes to semiconductors. Today I’m going to talk about another new, but very special kind of amplifier. First, here’s a riddle:
When is an amplifier not an amplifier?
When it is three amplifiers.
Engineers among us already know that today’s blog is about the type of amplifier known as an Instrumentation Amplifier, or in-amp. In-amps have several characteristics that make them well-suited for test equipment and laboratory devices. These are places where extreme precision is not just desirable but necessary. (We all want the machine the doctor is using to be accurate, right?) Among other qualities, in-amps also possess the ability to reject noise, especially electromagnetic interference. Back to the health care provider’s office where nearby MRIs, CT Scanners, and other equipment are emitting lots of interference which, without the noise rejection capability, can mess with a diagnosis. Factory floors and power plants are other likely noisy places.
Exactly how an in-amp works is beyond the scope of this blog, but there’s a pretty good Wikipedia page if you want to dive into the details. However, I’ve explain enough about the in-amp to explain the riddle. Although in a schematic it is represented by the same single, tilted triangle as a regular op-amp, the in-amp is almost always made up internally of three op-amps. The schematic below, from the same Wiki, shows a basic in-amp design. And now, you have a riddle you can tell your engineer friends.
As with any product Analog Devices sells, our designers are always looking for an edge, an advantage over competing products. Such is the case with the LT6370, a new instrumentation amplifier.
You have probably heard the expression “football is a game of inches.” Given that a football field is 100 yards long, non-fans often ask, “what difference can a few inches make?” Plenty, when you’re close to the goal line and your team needs those six points. This is similar to the challenge of processing small signals in noisy environments, such as those MRIs and factory floors. We measure and quantify the ability of an in-amp to reject noise and pass along the desired signal using decibels (dB).
The LT6370’s ability to reject noise is measured at 94 dB. The next-best noise-rejection number from the competition is 90 dB. Four decibels doesn’t seem like much but they are like those inches on a football field. This higher level of precision can be critical to a healthcare diagnosis or the resolution of an industrial problem.
Another inches-to-the-goal-line measurement for in-amps is how much the output of the in-amp changes as things heat up. Engineers call this value Gain Drift, and they measure it by cranking the heat up or down one degree Celsius at a time. Then they measure that drift in parts per million (abbreviated as ppm.) The LT6370 has a Gain Drift of just 30 ppm/°C and our nearest competitor has a Gain Drift of 35 ppm. Mere inches, you say? Think about that during your next visit to the doctor. How precise do you want his equipment to be?
For more information on the LT6370, and to order samples or a test evaluation board, you can visit the LT6370 product page on the Analog Devices website. We also have a page on all of our industry-leading instrumentation amps.