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Tissue Impedance

IEC 60601

IEC 60601 limits 100 µA maximum dc-leakage current through the body under normal conditions and 500 µA maximum under worst-case, single-fault conditions. An impedance circuit should not exceed that current.

Analog Devices AFE

Maxim AFE

  • Biopotential

    • ECG (waveform)
    • R-R (heart rate)

  • Bioimpedance

    • Resp (respiration)
    • GSR (galvanic skin response, stress level)
    • EDA (electro dermal activity)

  • Max 30001G ($10) for Biopotential and Bioimpedance, evaluation kit available ($113)

  • Maxim 30009 for Bioimpedance, evaluation kit available ($212)

PART Features Resolution Remarks
AD5933 Single channel 3 wire 12bit, 1Msps, 100kHz excitation First Gen, 1024 point FFT, 1kOhm - 1MOhm
AD5940/41 2 channel, 4 wire 16bit, 800ksps, 200kHz Impedance spectroscopy
ADuC355 2 channel, 4 wire 16bit, 800ksps, 200kHz Impedance Spectroscopy, 26MHz ARM Cortex M3 MCU
MAX30002 single channel, BioZ, 2 or 4 wire 17bit
MAX30009 single channel, BioZ, 2 or 4 wire 16 kHz - 806 kHz
MAX30131/132/134 1-,2-,4- channel 12 bit
MAX30001G single channel, 2 or 4 wire Impedance and Potential (ECG)

Skin Equivalent Circuits

There are several impedance models to measure body impedance:

  • Whole Body Impedance Analysis (BIA) or Total Body Composition (TBC)
  • Segmental Body Impedance for smaller regions in the body.
    • Respiratory Rate (RR) model.
    • Lung Composition (LC) model
    • Pulmonary Edema (PE) mopdel.

Usually measurements occur from 1 (or 5) kHz to (100 or) 1MHz.

There are also models to describe the electrode to skin impedance as well as bare tissue impedance.

Paco et al [10] describes a skin electrode model and a tissue model:

Model 1 R r C nF
Skin_Electrode 2k2 150k 150
Tissue 10 56k 47

Ferreira et al [11] describe TBC, RR and LC models:

Model 2 R r C nF
TBC 917.5 665.4 3.42
RR 58.5 25.58 75.7
LC 81.5 22.15 47.7

Model 1 (2014)

Skin

Model 2

Skin

Using AD5933 for Bioimpedance

Circuit

From [1] and [2], made with https://www.circuit-diagram.org/editor

AD5933 Evaluation Board

The evaluation board from Analog Devices includes the reference design describing an analog front end to measure low impedances.

On the excitation side it has a passive high pass filter with $R_{HP}$, $C_{HP}$ and an OpAmp buffer.

On the measurement side it has an amplifier with $V_{DD}/2$ on the positive input and feedback resistor RFB.

The input to the AD5933 has $R_{AD_{IN}}$ of 20K as well as a $R_{AD_{FB}}$ of 20K.

Analog Frontend for Bioimpedance

Munoz et al [2] describe their implementation in a 2022 paper. Several other web publications exist using that design.

For bioimpedance measurements the following analog front end elements should be used: a high-pass filter (HPF), a voltage-to-current converter (VCC), and an instrumentation amplifier (INA).

The HPF is a first order system composed of a 100 kΩ resistor and a 10 nF capacitor to remove the DC components of $V_{out}$. $f_c = 1 / (2 \pi R C)$ resulting in 160Hz.

The HPF output is connected to VCC, which consists of an operational amplifier and two resistors. The current injected into the tissue should not exceed 10 uA.

$R_{VCC current} = 1 kΩ$ (This will exceed 10 µA!)

$R_{VCC_{protect}} = 10 kΩ$

Because the impedance is supposed to vary from tens to hundreds of ohms,

$R_{feedback} = 10 kΩ$

ensures that current flows mainly through $Z_{Load}$, which is connected in parallel to $R_{feedback}$.

The current through VCC is

$R_{VCC_{curent}} = (V_{out} - V_{ref}) / R_{VCC_{current}}$.

Therefore the voltage over $Z_{load}$ is

$V_{Z_{load}} = -(V_{out} - V_{ref}) / R_{VCC_{current}} * Z_{load}$.

The unity gain instrumentation amplifier receives, through its non-inverting input, the voltages on the electrodes connected to $Z_{load}$. Since the ADC converter of the AD5933 is unipolar, a potential ($V_{ref}$) is added as reference voltage to the instrumentation amplifier. The output of the INA is

$V_{INA} = V_{load} + V_{ref}

The current through $R_{AD_{IN}}$ is:

$(V_{INA} - V_{ref})/R_{AD_{IN}}$

The output of the internal OpAmp in the AD5933 is

$I_{RFB} * R_{AD_{FB}}$

Therefore

$V_{AD5933} = (V_{INA} - V_{ref})/R_{AD{IN}} * R_{AD_{FB}}$.

The DSP of the AD5933 calculates the real and imaginary parts of $Z_{load}$, which are read through an inter-integrated circuit (I2C) protocol by a micro controller.

Electrical Components

A list of designs using the AD5933 used the following values for resistors and capacitors:

Design $C_{HP}$ $R_{HP}$ $RFB$ $R_{VCC_{current}}$ $R_{VCC_{protect}}$ $R_{INA_{GAIN}}$ $R_{REFGEN}$ $R_{AD_{IN}}$ $R_{AD_{FB}}$ $VCC +$
Eval [1] 47nF 49.9k 200k N.A. N.A. N.A. 49.9k 20k 20k
Datasheet [3] 47nF N.S. N.S. N.A. N.A. N.A. N.S. 20k 20k
Munoz [2] 10nF 100k N.A. 1k 10k Gain=1 1k 1k 1k GND
Instru Bio [4] 10nF 10k N.A. 1k 1k Gain=10,5.5k VDD/2 1k 1k GND
Instru BIA [5] 10nF 100k N.A. 1M 1M Gain=1.5,100k VDD/2 1k 1k GND
Instru BIA [6] 10nF 100k N.A. 285k 1M VDD/2 VDD/2
Instru Thor[7] 1.2nF 10k N.A. 1M 1M Gain=1.5,100k 1k 1k 1k GND
Instru Thor[8] 100nF 20k N.A. 100k 10k Gain=1.5,100k 3.2/1k 1k 1k GND
UA 2023 10nF 100k N.A. 82k 82k Gain=51,1k 20k 20k 20k VDD/2
UA 2024 10nF 100k 200k 265k 1k Gain=51,1k 10k 20k 20k VDD/2
  • Network Analyzer AD5933
  • Operation Amplifier(s) AD8608 or AD8606 or AD8605
  • Instrumentation Amplifier INA826AID

High Pass

  • $1 / (2 \pi R C)$, should attenuate 50 and 60Hz, need 200Hz..xxkHz
  • 10nF, 100k fc=160Hz

$R_{current}$ and $R_{protect}$

  • Input into VCC is at 1.65V offset with 1V amplitude. With $I_{CC} <10uA$ and $V_{CC} = 2.65V$ and $R_{VCC_{current}}$ will need to be 265kΩ.
  • Setting of Range 1 in AD5933 is 1V if operating at 3.3V., for $10 \mu A$ we need 110kΩ for $R_{VCC_{current}}$.
  • Analog Devices: "Bio-Impedance Circuit Design for Body Worn Systems"

$R_{Gain}$

  • $G = 1 + (49.4k/R_{Gain})$
  • With $R_{INA_{GAIN}}$ = 1kΩ the gain will be $G=51$

Impedance Estimation

Formula to compute impedance from measurements: Description to be completed (check evaluation board and Instructables, also search PubMed).

References

For general reading: Analog Devices: Bio-Impedance Circuit Design for Body Worn Systems"

  1. Analog Devices AD5933 Evaluation Board, User Guide
  2. Munoz et al
  3. AD5933 Datasheet
  4. Instructables Body Composition
  5. Instructables BIA 1
  6. Instructables BIA 2
  7. Instructables Thor 1
  8. Instructables THor 2
  9. IEEE Tutorial
  10. Uwe Pliquett, Andreas Barthel, 2012 J. Phys.: Conf. Ser. 407 012019
  11. Paco Bogonez-Franco et al, 2014, Problems encountered during inappropriate use of commercial bioimpedance devices in novel applications, 7 th International Workshop on Impedance Spectroscopy 2014
  12. J Ferreira et al 2010 J. Phys.: Conf. Ser. 224 012011