Hardware Teardown: the Withings Body Cardio weighing scale

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Nick

Co-CTO & Founder

30 Nov  ·  5 min read


This month, we're cracking the case of the Withings Body Cardio, one of the most advanced consumer body composition weighing scales on the market today.

The Withings Body Cardio is the latest in the brand’s range of smart health-centered devices, and the second Withings product we’ve tackled in this series!

It’s a smart scale with a pretty impressive set of metrics. It detects weight; fat, muscle and bone mass; hydration level; heart rate; PWV (Pulse Wave Velocity); and it shows your daily step count (if you use the app) and today’s weather.

Of course, this data collection was exactly what we were interested in: how does the scale measure these different factors?

We gathered our tools and found out…

Taking it apart

Opening up the scale was not an easy task – there was a lot of double-sided tape involved. There was also a pretty serious risk of breaking the scale while trying… So you can be extra glad we already did the hard work for you ;)

The back of the scale is fastened with four Torx screws – easy enough. The entire backplate, however, is glued to the front with double-sided adhesive cushions. Removing it simply involved pulling it firmly enough to break the adhesive, but not so hard as to break the connectors.

Backplate

With the backplate removed, you can immediately spot 4 loads cells, one in each corner. According to a patent they issued last year they arrange the load cells in such way that only two Wheatstone bridges are used. This arrangement provides two outputs being amplified by two separate amplifiers which are fed to the controller unit. Also, having two outputs gives the scale the possibility to compare the two (left and right) and detect when the user stands off-center on the scale which is indicated on the display.

Inside the scale

Having four 3-wire load cells and some kind of Wheatstone bridge configuration is one of the most common ways to design a weighing scale: open up your regular kitchen scale and you’ll probably find the same setup.

Apart from the four 3-wire load cells in the corners, you can see the main components: the main PCB in the middle, a rechargeable LiPo battery on the right, and the display on the top.

A little more details

Let’s head over to the most interesting part: the PCB!

As you can see in the image below, the main board of the scale contains a couple of large components.

Full PCB

  • in yellow, the ARM MCU
  • in red, the weighing scale and body composition front-end IC
  • in light blue, the battery charging IC (including battery protection)
  • in pink, a Broadcom Wifi/Bluetooth chip (+ antenna)
  • in green, the flash memory
  • in orange, a quad op-amp
  • in dark green, a display connector
  • in dark blue, the connector to the USB port
  • in purple, the wiring of the body composition sensors
  • and in dark red, the wiring of the load cells.

I also noticed that the right side of the PCB is covered with a protective coating. I can only assume that this is done to protect the PCB from environmental “attacks”, because this is the side of the PCB where the measurements happen. A coating like this probably serves to ensure greater accuracy.

Let’s take look at the individual PCB components.

The largest chip – the heart of the weighing scale – is the MK22FN1M0VLL12 from NXP. It’s a 120 MHz ARM® Cortex®-M4-based microcontroller, which should give more than sufficient processing power for a weighing scale.

MK22FN1M0VLL12 MCU

The second largest chip on the board is the AFE4300 from Texas Instruments. This IC is a low-cost, analog front-end incorporating two separate signal chains: one chain for weight-scale (WS) measurement, and another chain for body composition measurement (BCM) analysis.

The analog signals coming from the sensors are amplified and converted to a digital signal by an internal instrumentation amplifier and an analog-to-digital converter (ADC).

AFE4300

Moving further along the PCB, we find a battery charging IC just below the battery connector. This chip also protects the battery from overvoltage. In this case, a BQ24095 is used, also from Texas Instruments.

Battery charging

For the radio, Withings selected a BCM43438 from Cypress. Getting a decent picture here was tricky – the mirror-like top creates a lot of glare.

BCM43438 Wifi/Bluetooth radio

This model is a WiFi and Bluetooth radio in one package – it’s a popular model, also used in the RaspberryPi 3.

Antenna

The squiggly antenna trace you see in the picture above, is a fractal antenna. You don’t see this often in PCB designs, and since I’m not an expert in antenna design I can’t give you more information about it unfortunately. We would love to hear from you in case you could provide us with some useful information so we can update this blog post!

MX25L3233F flash memory

The scale has a flash memory chip as well, which I assume serves to store the user’s preferences. The flash memory is a 32Mbit MX25L3233F from Macronix.

AD8544 quad opamp

Lastly, we can see several op-amps in this circuit. One of them is the AD8544, which serves to amplify the output coming from the load cells. The other op-amps are probably used to provide extra gain and some filtering before the signals are fed to the BCM chip, although that is just an educated guess.

Block diagram

I’ve made an attempt to design a block diagram for the whole setup. This won’t be 100% accurate as I don’t know all the details but it will give you an idea on how the components are interlinked and work together.

This block diagram shows how the components are interlinked
This block diagram shows how the components are interlinked

And with that, we can wrap up our November teardown! As you can see, a weighing scale like this one requires some clever engineering, and has to house a lot of components.

If you have any comments or questions – or suggestions as to what we should tear down next – let us know!


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Nick

Co-CTO & Founder

30 Nov  ·  5 min read