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Weight-Scale and Body Composition Measurement
Reference Design
TI Reference Designs
TI Reference Designs are mixed-signal solutions created by TI's experts. Verified Designs offer the theory, complete PCB
schematic & layout, bill of materials and measured performance of the overall system.
Circuit Description
This document discusses the analog circuit design of weight-scale and body composition measurement applications. Two
separate signal chains are discussed in this document: one for weight-scale (WS) measurements and the other for body
composition measurement (BCM) analysis.
Design Resources
Design Archive (ZIP File)
AFE4300
1.
Design Summary
This design takes a block level approach for designing a Weight Scale and Body Composition system.
1.1 Design Goal
Provide a Weight Scale and Body Composition system reference example.
All design files
Product Folder
AFE4300
Foot BCM
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Summary of Contents for Texas Instruments tidu131
- Page 1 Weight-Scale and Body Composition Measurement Reference Design TI Reference Designs TI Reference Designs are mixed-signal solutions created by TI’s experts. Verified Designs offer the theory, complete PCB schematic & layout, bill of materials and measured performance of the overall system. Circuit Description This document discusses the analog circuit design of weight-scale and body composition measurement applications.
- Page 2 2. Theory of Operation 2.1 Background on weight measurements Weight-scale and body composition measurements are two separate signal chains, so weight-scale application will be discussed first. While there are multiple techniques for converting weight into an electrical signal, perhaps the most common is to use a resistive load cell configured as a Wheatstone bridge.
- Page 3 Figure 3 Bio-impedance measurement model To measure the bio-impedance, a high-frequency AC current is injected into the tissue through the drive electrodes (refer Figure 3). The AC current causes a potential voltage difference of the two receive electrodes (refer Figure 3). This potential voltage difference is related to the resistivity of the tissue between the voltage-sensing or receive electrodes.
- Page 4 An internal reference source provides a constant voltage of 1.7V at the VLDO output to drive the external bridge. As the Wheatstone bridge has high source impedance and the output common-mode voltage is relative high, so the output of the bridge is connected to an INA (first stage).
- Page 5 Typically, a high-resolution analog-to-digital converter (ADC) then follows the PGA to digitize the amplified voltage. Given that the signals being measured usually are changing slowly, and that very high resolution is required, delta-sigma topologies are often used to implement the ADC. As with the gain stage, the stability of the ADC overtime and temperature is very important so as not to limit overall performance.
- Page 6 At the same time that the current is injected, a second set of multiplexers connects a differential amplifier across the same body impedance in order to measure the voltage drop created by the injected current: Where Z and θ are the module and phase of the impedance at , respectively.
- Page 7 Where: Z= the module of the impedance at θ= phase of the impedance at The I/Q demodulator takes the v(t) signal and outputs two dc values. These two values are used to extract the impedance module and phase with a single frequency measurement. Figure 6 shows the block diagram of the implementation. Using the I/Q demodulator helps reduce power consumption while yielding excellent performance.
- Page 8 Device control: The area on the screen of the app that controls a device. Figure 8 Health Hub PC GUI Interface with highlighted Device Controls 3.3.1 Common Operations Common operations apply to all of the demos with the Health Hub app. 3.3.1.1 Find Devices The first step in initiating a connection to a Health Hub demonstration device is to have the PC finding the device.
- Page 9 3.3.1.2 Connection The second step is to form a connection. Before finding a device by using Discover Devices the device controls appear as in the figure below. Figure 10 BLE enabled device not found After a device has been found the device control will appear as in the figure below. Figure 11 Device found –...
- Page 10 Figure 12 Desired Device selection from the list When the desired device is selected from the list, the app will form a connection to the device over Bluetooth low energy. After connection the device control will open fully and control over the device can begin. 3.4 Measured Results The device pictured below is the foot BCM device.
- Page 11 Figure 14 Foot BCM Calibration The “Scale Ready” button indicates that the foot BCM device is ready to measure the user’s weight. Ask the subject to step on the scale and remain still. Press the measure button. Figure 15 Foot BCM Measuring The following figure shows up some measurement results Figure 16 Measurement results...
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Page 12: Important Notice
IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest issue.
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