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TI’s AFE4300 is a low-cost scale and body composition (BCM) measurement analog front end (AFE) that integrates two separate signal chains: one for weight measurement and one for body composition analysis using a 16-bit 860SPS ADC. The three BCM paths of the device allow partial body impedance analysis (BIA) between the hands and feet of the body, resulting in a full picture of body composition. The device operates from 2 V to 3.6 V and is mainly used for weighing instruments and body composition measurements. This paper introduces the main features of AFE4300, block diagram, front-end block diagram of weighing instrument and front-end block diagram of BCM and evaluation board AFE4300EVM-PDK block diagram, circuit diagram, bill of materials and PCB component layout diagram, and introduces a typical body composition analysis diagram.

TI introduces the industry’s first fully integrated analog front end for both weight and body composition measurement (BCM). The AFE4300 is an easy-to-use, precision, low-power front end solution, which allows engineers designing weigh scales that incorporate body composition meters, body impedance analyzers, and impedance measurement equipment to get their products to market faster than by implementing existing, more complex multi-chip solutions.

The AFE4300 incorporates two separate signal chains, one targeting weight measurement and the other targeting body composition analysis, with a 16-bit, 860-sps analog-to-digital converter (ADC) multiplexed between them. The device’s three BCM channels enable segmental body impedance analysis (BIA) between body extremities, which allows for a full picture of the subject’s body composition.

The AFE4300 is a low-cost analog front-end incorporating two separate signal chains: one chain for weight-scale (WS) measurement and the other for body composition measurement (BCM) analysis. A 16-bit, 860-SPS analog-to -digital converter (ADC) is multiplexed between both chains. The weight measurement chain includes an instrumentation amplifier (INA) with the gain set by an external resistor, followed by a 6-bit digital-to-analog converter (DAC) for offset correction , and a circuit to drive the external bridge/load cell with a fixed 1.7 V for ratiometric measurements.

The AFE4300 can also measure body composition by applying a sinusoidal current into the body. The sinusoidal current is generated with an internal pattern generator and a 6-bit, 1-MSPS DAC. A voltage-to-current converter applies this sinusoidal current into the body, between two terminals. The voltage created across these two terminals as a result of the impedance of the body is measured back with a differential amplifier, rectified, and its amplitude is extracted and measured by the 16-bit ADC.

The AFE4300 operates from 2 V to 3.6 V, is specified from 0℃ to +70℃, and is available in a TQFP-80 package.

AFE4300 main features:

• Weight-Scale Front-End:

– Supports up to Four Load Cell Inputs

– On-Chip Load Cell 1.7-V Excitation Voltage for Ratiometric Measurement

– 68-nVrms Input-Referred Noise (0.1 Hz to 2 Hz)

– Best-Fit Linearity: 0.01% of Full-Scale

– Weight-Scale Measurement : 540 μA

• Body Composition Front-End:.

– Supports Up To Three Tetra-Polar Complex Impedance Measurements

– 6-Bit, 1-MSPS Sine-Wave Generation Digital-to-Analog Converter (DAC)

– 375-μArms, ±20% Excitation Source

– Dynamic Range : 0 Ω to 2.8 kΩ

– 0.1-Ω Measurement RMS Noise in 2-Hz BW

– Body Composition measurement : 970 μA

• Analog-to_Digital Converter (ADC):

– 16 Bits, 860 SPS

– Supply current: 110 μA

AFE4300 application:

• Weight Scales with Body Composition Measurements

Figure 1. AFE4300 functional block diagram

Figure 2. AFE4300 block diagram

Figure 3. AFE4300 Weighing Instrument Front-End Block Diagram

Figure 4. AFE4300 AC Rectifier Mode Body Composition Meter (BCM) Front-End Block Diagram

Evaluation Board AFE4300EVM-PDK

AFE4300EVM-PDK is a platform for evaluating the AFE4300 device. AFE4300 is a low cost Analog-Front-End incorporating two separate signal chains, one targeting weight measurement and the other targeting body composition analysis. The EVM provides access for load cell terminals to support up to four load cell inputs for weigh scale measurements, BCM terminals to support up to three tetra-polar complex impedance measurements. Users can also feed any type of single ended or differential signals directly to the AFE4300 through the auxiliary (AUX1 and AUX2) inputs . External support circuits such as anti-aliasing network for load cell terminals, trim-pot for setting the gain of the Instrumentation Amplifier at the Weigh Scale Front-end, Instrumentation amplifiers for the BCM differential terminals, load cell simulation block, impedance simulation block , clocks and external references are provided for testing purposes.

The evaluation module comes with a LabVIEW-based graphical user interface that allows the user to easily configure the device to capture data and perform analysis on the data. Note that the AFE4300EVM-PDK is intended for prototyping and evaluation purposes only and is not intended to be used with real subjects or patients.

The main features of the evaluation board AFE4300EVM-PDK:

Support for up to four load cell inputs

Support for three tetra-polar impedance measurements

On-board load cell simulation block

Acquire data at up to 860 samples/second in data capture mode

USB based power and PC application connectivity

Figure 5. Outline Drawing of Evaluation Board AFE4300EVM-PDK

Figure 6. Evaluation Board AFE4300EVM-PDK Block Diagram

Figure 7. Evaluation Board AFE4300EVM-PDK Circuit Diagram (1)

Figure 8. Evaluation Board AFE4300EVM-PDK Circuit Diagram (2)

Figure 9. Evaluation Board AFE4300EVM-PDK Circuit Diagram (3)

Figure 10. Evaluation Board AFE4300EVM-PDK Circuit Diagram (4)

Figure 11. Evaluation Board AFE4300EVM-PDK Circuit Diagram (5)

Figure 12. Evaluation Board AFE4300EVM-PDK Circuit Diagram (6)

Evaluation Board AFE4300EVM-PDK Bill of Materials (BOM):

Figure 13. Evaluation Board AFE4300EVM-PDK Component Layout (Top Layer)

Figure 14. Evaluation Board AFE4300EVM-PDK Component Layout (Bottom Layer)

For details, see:

http://www.ti.com/lit/ds/sbas586a/sbas586a.pdf

and

http://www.ti.com/lit/ug/sbau201/sbau201.pdf

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