Here at Analog Wire, we want to bring you some of the most useful resources and advice to apply to your next design. Whether engineers by profession or hobbyists, we all love hearing about new projects, conquered challenges or shared frustrations. That’s why we thought you might enjoy this peak at a project from the community of engineers.
Keith Wakeham is a nuclear design engineer at Babcock and Wilcox in Ontario, Canada. We connected with Keith via Twitter and found out he’s chronicling some of his coolest hobby projects on his blog “Keith Hack.” His current build: a cycling power meter, a device which measures the output power of a bicycle created by the rider.
Check out what Keith told us about his current project and how the 24-bit ADS1247 delta-sigma analog-to-digital converter (ADC) revolutionized his design…
“Bicycle power meters are the hot topic in the cycling industry right now. Just about everyone is looking to provide a solution to train and race with power. To provide that solution, one needs to build a load cell with strain gauges and a wheatstone bridge.
Skipping over how strain gauges and wheatstone bridges work, you’re left with a very small differential voltage from the load cell. My previous experience had me using an instrumentation amplifier combined with a SAR ADC. My design requires two load sensors necessitating four ICs.
Having built two prototypes, I was still unhappy with noise performance of my 16-bit SAR setup, its physical size and power consumption. A visitor to my blog emailed me about a dual-input, 16-bit delta-sigma: the ADS1147. I knew my 16-bit SAR was decent, but it was operating at about 16ksps-per-channel to keep the noise low. I wanted to reduce this rate in order to reduce power consumption but I wanted to keep the noise low– the ADS1247 at 24 bits looked ideal. I figured a few dollars to get one and a breakout board would be worth finding out if I could use it with the strain-gauge-based load cell. It has a programmable gain amplifier, doesn’t require any voltage dividers and has two inputs, all in a footprint a quarter the size of what I’m currently using.
Within a couple of hours, I had two load cells wired into the ADS1247 with a microcontroller and was reading back data. It was this moment that I realized I didn’t need 16ksps just to drive down the noise, what I had needed was a better design. I was completely blown away by the performance. I barely believed my little proto-board setup was producing so many noise-free bits.
I’m only starting to break the ice with the ADS1247, as it contains some great features. For instance, its precision on-board reference can power both load cells, and it can power down the reference when idle. Keeping everything powered down as much as possible helps battery life in coin cell applications. I’m looking forward to seeing my consumption drop now in my next design with improved sensing. And the best part is that it cuts the cost of my analog to digital setup by about 70 percent.”
-Keith Wakeham
Here’s the latest prototype, next to the ADS1247 broken out on the proto-board, which is wired to strain gauges on a test tube next to it.
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