Fancy measurement gear is often expensive to buy, but some bits of kit are entirely DIY’able if you’re willing to put a little work into the project. [Christer Weinigel] needed to get some measurements of a differential clock signal that was ticking away around 500 MHz. El-cheapo probes aren’t going to cut it here. They won’t have the bandwidth and most off-the-rack probes are single-ended, that is they’re referenced to ground. [Christer] needed the difference between two balanced signals, neither of which is grounded. In short, [Christer] needed a high-frequency active differential oscilloscope probe, and they’re not cheap. So he built one himself.
The circuit in the probe is really just an instrumentation amplifier design with a modified input stage and a 50 ohm output impedance. (See this article on in-amps if you need to brush up.) With higher frequencies like this, it’s going to be demanding on the op-amp, so [Christer] spent some time simulating the circuit to make sure it would work with his chosen part. Then he made up a bunch of PCB designs and had them made. Actual results matched fairly well with the simulation.
With some minor tweaking on the input damping resistors, he got a tool that’s dead flat up to 300 MHz, and totally usable up to 850 MHz. If you tried to buy one of these, it’d set you back the cost of a few hundred lattes, but this one can be made for the price of one or two if you get the PCBs done cheaply. Of course, the design files are available for your own use. Kudos [Christer].
Edit: By total coincidence, Bil Herd just posted a video intro to differential signals. Go check it out.
And thanks to [nebk] for the tip!
In the first article about measurement systems we looked at sensors as a way to bring data into a measurement system. I explained that a sensor measures physical quantities which are turned into a voltage with a variable conversion element such as a resistor bridge. There will always be noise in any system, and an operational amplifier (op-amp) can be used to remove some of that noise. The example we considered used an op-amp in a differential configuration that removes any disturbance signal that is common to both inputs of the op-amp.
But that single application of an op-amp is just skimming the surface of the process of bringing a real-world measurement of a physical quantity into a digital system. Often, you’ll need to do more work on the signal before it’s ready for sampling with a digital-to-analog converter. Signal conditioning with amplifiers is a deep and rich topic, so let me make it clear that that this article will not cover every aspect of designing and implementing a measurement system. Instead, I’m aiming to get you started without getting too technical and math-y. Let’s just relax and ponder amplifiers without getting lost in detail. Doesn’t that sound nice?
Continue reading “Beyond Measure: Instrumentation Amplifiers”
These days there a large number of sensors and analog circuits that are “controller friendly” meaning that their output signal is easily interfaced to the built-in Analog to Digital Convertors (ADCs) often found in today’s micro-controllers. This means that the signals typically are already amplified, often filtered, and corrected for offset and linearity. But when faced with very low level signals, or signals buried in a larger signal an Instrumentation Amplifier may be what’s needed. The qualities of an Instrumentation Amplifier include:
- A differential amplifier with high impedance and low bias current on both inputs.
- Low noise and low drift when amplifying very small signals.
- The ability to reject a voltage that is present on both inputs, referred to as Common Mode Rejection Ratio (CMRR)
Continue reading “Instrumentation Amplifiers and How to Measure Miniscule Change”
The title of [Nuclearrambo’s] post says it all, “Android based wireless ECG monitoring (Temperature sensor and glucometer included).” Wow! What a project!
The project is built around the HC-06 bluetooth module and the Stellaris LaunchPad from TI, an inexpensive ARM developer kit. Building an ECG is a great way to learn about instrumentation amplifiers, a type of differential amplifier used for its extremely high common mode rejection ratio (CMRR). Please be sure to keep in mind that there are a myriad of safety issues and regulation concerns for medical device, and there is no doubt that an ECG is considered a medical device. Sadly, [Nuclearrambo’s] post does not include all of the code and design files you need to build the system, which is understandable considering this is a medical device. That being said, he provides a lot of information about building high-quality ECG instrumentation and the web interface.
It would be great if [Nuclearrambo] could post the Android application code and Stellaris LaunchPad code. Even with these omissions, this post is still worth reading. Designing medical devices requires a lot of know-how, but who knows, maybe your next project can save your life!