# Analysis, Simulation, and Development of a Low-Cost Fully Active-Electrode Bioimpedance Measurement Module

^{*}

## Abstract

**:**

## 1. Introduction

## 2. The Proposed Module

## 3. Circuit Analysis

## 4. SPICE Simulations

## 5. Implementation and Measurement Results

## 6. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**Discrete component current source configurations. (

**a**) The Enhanced HCP (single-ended). (

**b**) The modified buffered HCP used in this work (single-ended). (

**c**) Current Conveyor based on the AD844 (Bragos CCII). (

**d**) Current Source based on the AD830 differential difference-amplifier (Analog Devices, mirrored).

**Figure 2.**Simplified schematics of three bioimpedance measurement configurations. (

**a**) Passive electrode configuration. (

**b**) Partially-active electrode configuration. (

**c**) Fully-active electrode configuration (this work). The cables’/switches’ parasitic capacitances are indicated with red color.

**Figure 3.**Brief schematic of the proposed active electrode bioimpedance module and its connectivity.

**Figure 4.**The mirrored HCP equivalent circuit for (

**a**) the transconductance calculation, (

**b**) the ${Z}_{out}$ calculation. ${G}_{4,i}^{\prime}$ indicates the parallel combination of $G$ and ${G}_{p,i}=1/{R}_{p,i}$, for $i=\{1,2\}$.

**Figure 5.**Numerical approximations of the open loop gain (

**left**), differential input impedance (

**center**), and output impedance (

**right**) of the ADA4622, AD8034, OPA2210, and AD8672 Opamps (based on Analog Devices and Texas Instruments).

**Figure 6.**Calculated transconductance values of the mirrored-HCP for the ADA4622 Opamp and three resistive load values. The blue line indicates the transconductance when all passive components have their nominal values, while the grey region indicates the effect of their tolerances ($0.1\%$ for the resistors, $5\%$ for the capacitors).

**Figure 7.**Calculated transconductance values of the mirrored-HCP for the AD8034 Opamp and three resistive load values. The blue line indicates the transconductance when all passive components have their nominal values, while the grey region indicates the effect of their tolerances ($0.1\%$ for the resistors, $5\%$ for the capacitors).

**Figure 8.**Calculated transconductance values of the mirrored-HCP for the OPA2210 Opamp and three resistive load values. The blue line indicates the transconductance when all passive components have their nominal values, while the grey region indicates the effect of their tolerances ($0.1\%$ for the resistors, $5\%$ for the capacitors).

**Figure 9.**Calculated transconductance values of the mirrored-HCP for the AD8672 Opamp and three resistive load values. The blue line indicates the transconductance when all passive components have their nominal values, while the grey region indicates the effect of their tolerances ($0.1\%$ for the resistors, $5\%$ for the capacitors).

**Figure 10.**Calculated output impedance values of the mirrored-HCP for the ADA4622, AD8034, OPA2210, and AD8672 Opamps. The read line indicates the transconductance when all passive components have their nominal values, while the grey region indicates the effect of their tolerances ($0.1\%$ for the resistors, $5\%$ for the capacitors).

**Figure 11.**Simulated transconductance magnitude and phase values of the mirrored-HCP for the ADA4622 Opamp and three resistive load values at LT SPICE. The light-colored regions indicate the effect of the passive components’ tolerances ($0.1\%$ for the resistors, $5\%$ for the capacitors).

**Figure 12.**Simulated transconductance magnitude and phase values of the mirrored-HCP for the AD8034 Opamp and three resistive load values at LT SPICE. The light-colored regions indicate the effect of the passive components’ tolerances ($0.1\%$ for the resistors, $5\%$ for the capacitors).

**Figure 13.**Simulated transconductance magnitude and phase values of the mirrored-HCP for the OPA2210 Opamp and three resistive load values at LT SPICE. The light-colored regions indicate the effect of the passive components’ tolerances ($0.1\%$ for the resistors, $5\%$ for the capacitors).

**Figure 14.**Simulated transconductance magnitude and phase values of the mirrored-HCP for the AD8672 Opamp and three resistive load values at LT SPICE. The light-colored regions indicate the effect of the passive components’ tolerances ($0.1\%$ for the resistors, $5\%$ for the capacitors).

**Figure 15.**Simulated output impedance magnitude and phase values of the mirrored-HCP for the ADA4622, AD8034, OPA2210, and AD8672 Opamps. The light-colored regions indicate the effect of the passive components’ tolerances ($0.1\%$ for the resistors, $5\%$ for the capacitors).

**Figure 16.**Transient SPICE analysis on the AD8421 instrumentation amplifier’s output for a 12.3${V}_{p-p}$, 100 kHz sinusoidal ${u}_{id}$. (

**a**) The output voltage for loads of 150 $\mathrm{\Omega}$, 660 $\mathrm{\Omega}$, and 1200 $\mathrm{\Omega}$, in relation to the input. (

**b**) The effect of passive components’ tolerance to the output’s amplitude for ${R}_{L}=660\mathrm{\Omega}$. (

**c**) Phase delay between voltage input ${u}_{id}$ and the output.

**Figure 17.**Transient SPICE analysis on the AD8421 instrumentation amplifier’s output for a 12.3${V}_{p-p}$, 100 kHz sinusoidal ${u}_{id}$. (

**a**) The output voltage for loads of 150 $\mathrm{\Omega}$, 660 $\mathrm{\Omega}$, and 1200 $\mathrm{\Omega}$, in relation to the input. (

**b**) The effect of passive components’ tolerance to the output’s amplitude for ${R}_{L}=660\mathrm{\Omega}$. (

**c**) Phase delay between voltage input ${u}_{id}$ and the output.

**Figure 18.**(

**a**) A fully-active electrode bioimpedance module PCB. An AgCl electrode is placed in the large hole in the center. (

**b**) The bipolar measurement setup (fully differential amplifier connected with two adjacent active electrode modules and nstrumentation).

**Figure 19.**HCP output node voltages for the AD8034 configuration and 150 $\mathrm{\Omega}$ resistive load, when inducing a 10 kHz signal (

**left**), a 100kHz signal (

**center**), and a 300 kHz signal (

**right**).

**Figure 20.**HCP output node voltages for the AD8034 configuration and 660 $\mathrm{\Omega}$ resistive load, when inducing a 10 kHz signal (

**left**), a 100 kHz signal (

**center**), and a 300 kHz signal (

**right**).

**Figure 21.**HCP output node voltages for the AD8034 configuration and 1200 $\mathrm{\Omega}$ resistive load, when inducing a 10 kHz signal (

**left**), a 100 kHz signal (

**center**), and a 300 kHz signal (

**right**).

**Figure 22.**HCP, ADA4622 configuration transconductance results from the numerical analysis, the SPICE simulation (nominal passive component values) and the lab measurements, for ${R}_{L}=$ 150 $\mathrm{\Omega}$, ${R}_{L}=$ 660 $\mathrm{\Omega}$, and ${R}_{L}=$ 1200 $\mathrm{\Omega}$.

**Figure 23.**HCP, AD8034 configuration transconductance results from the numerical analysis, the SPICE simulation (nominal passive component values) and the lab measurements, for ${R}_{L}=$ 150 $\mathrm{\Omega}$, ${R}_{L}=$ 660 $\mathrm{\Omega}$, and ${R}_{L}=$ 1200 $\mathrm{\Omega}$.

**Figure 24.**HCP, OPA2210 configuration transconductance results from the numerical analysis, the SPICE simulation (nominal passive component values) and the lab measurements, for ${R}_{L}=$ 150 $\mathrm{\Omega}$, ${R}_{L}=$ 660 $\mathrm{\Omega}$, and ${R}_{L}=$ 1200 $\mathrm{\Omega}$.

**Figure 25.**HCP, AD8672 configuration transconductance results from the numerical analysis, the SPICE simulation (nominal passive component values) and the lab measurements, for ${R}_{L}=$ 150 $\mathrm{\Omega}$, ${R}_{L}=$ 660 $\mathrm{\Omega}$, and ${R}_{L}=$ 1200 $\mathrm{\Omega}$.

**Figure 26.**HCP output node voltages for 300 kHz input signal for the ADA4622 configuration (

**left**), the AD8034 configuration (

**center**), and the OPA2210 (

**right**). The resistive load is ${R}_{L}=$ 660 $\mathrm{\Omega}$. Note the significant distortion for the OPA2210 configuration.

**Figure 27.**AD8421 instrumentation amplifier output for a 150 $\mathrm{\Omega}$ resistive load and $12.3{V}_{p-p}$ sinusoidal ${u}_{id}$ at 10 kHz (

**left**), 100 kHz (

**center**), and 300 kHz (

**right**). The AD8034 Opamp configuration has been used in the HCP.

**Figure 28.**AD8421 instrumentation amplifier output for a 660 $\mathrm{\Omega}$ resistive load and $12.3{V}_{p-p}$ sinusoidal ${u}_{id}$ at 10 kHz (

**left**), 100 kHz (

**center**), and 300 kHz (

**right**). The AD8034 Opamp configuration has been used in the HCP.

**Figure 29.**AD8421 instrumentation amplifier output for a 1200 $\mathrm{\Omega}$ resistive load and $12.3{V}_{p-p}$ sinusoidal ${u}_{id}$ at 10 kHz (

**left**), 100 kHz (

**center**), and 300 kHz (

**right**). The AD8034 Opamp configuration has been used in the HCP.

${\mathit{R}}_{1,\mathit{i}}$ | ${\mathit{R}}_{2,\mathit{i}}$ | ${\mathit{R}}_{3,\mathit{i}}$ | ${\mathit{R}}_{4,\mathit{i}}$ | ${\mathit{R}}_{\mathit{p},\mathit{i}}$ | ${\mathit{R}}_{5,\mathit{i}}$ | ${\mathit{R}}_{6,\mathit{i}}$ | ${\mathit{R}}_{\mathit{L}}$ | ${\mathit{C}}_{1,\mathit{i}}$ |
---|---|---|---|---|---|---|---|---|

1 k$\mathrm{\Omega}$ | 1 k$\mathrm{\Omega}$ | 1 k$\mathrm{\Omega}$ | 1 k$\mathrm{\Omega}$ | 100 k$\mathrm{\Omega}$ | 10 k$\mathrm{\Omega}$ | $\{150,660,1200\}\phantom{\rule{0.166667em}{0ex}}\mathrm{\Omega}$ | 10 M$\mathrm{\Omega}$ | 1 nF |

**Table 2.**Comparison of implemented discrete-component VCCS output impedances. Ranges denote possible differences between simulated and measured values, and/or variations, due to the resistors’ tolerances.

Resource | Topology | Opamp | Tolerance | $|{\mathit{Z}}_{\mathit{out}}|$, 10 kHz | $|{\mathit{Z}}_{\mathit{out}}|$, 100 kHz | $|{\mathit{Z}}_{\mathit{out}}|$, 1 MHz |
---|---|---|---|---|---|---|

[15] | Enhanced HCP | Not Mentioned | $1\%$ | 750 k$\mathrm{\Omega}$–4.5 M$\mathrm{\Omega}$ | 670 k$\mathrm{\Omega}$ | 70 k$\mathrm{\Omega}$ |

[15] | CCII with AD844 | No Opamp | $1\%$ | 2 M$\mathrm{\Omega}$ | 288 k$\mathrm{\Omega}$–700 k$\mathrm{\Omega}$ | 70 k$\mathrm{\Omega}$ |

[18] | Enhanced HCP | OPA655 | $1\%$ | 80 k$\mathrm{\Omega}$–10 M$\mathrm{\Omega}$ | 80 k$\mathrm{\Omega}$–2 M$\mathrm{\Omega}$ | 80 k$\mathrm{\Omega}$–300 k$\mathrm{\Omega}$ |

[32] | Enhanced HCP | LM741 | $0.1\%$ | 200 k$\mathrm{\Omega}$ | 10 k$\mathrm{\Omega}$ | 5 k$\mathrm{\Omega}$ |

[32] | Buffered HCP | LM741 | $0.1\%$ | 300 k$\mathrm{\Omega}$ | 20 k$\mathrm{\Omega}$ | 7 k$\mathrm{\Omega}$ |

[32] | Bridge HCP | LM741 | $0.1\%$ | 600 k$\mathrm{\Omega}$ | 35 k$\mathrm{\Omega}$ | 7 k$\mathrm{\Omega}$ |

[32] | Enhanced HCP ($3.5$pF ${C}_{stray}$) | AD818 | $0.1\%$ | 3 M$\mathrm{\Omega}$ | 200 k$\mathrm{\Omega}$ | 60 k$\mathrm{\Omega}$ |

[34] | Enhanced-Optimized Difference Evolution | AD825 | $0.05\%$ | 5 M$\mathrm{\Omega}$ | 3 M$\mathrm{\Omega}$ | 100 k$\mathrm{\Omega}$ |

This Work | Mirrored-Buffered HCP- Active Electrode | AD8034 | $0.1\%$ | 2.5 M$\mathrm{\Omega}$–4 M$\mathrm{\Omega}$ | 0.9 M$\mathrm{\Omega}$–2 M$\mathrm{\Omega}$ | 100 k$\mathrm{\Omega}$–250 k$\mathrm{\Omega}$ |

**Table 3.**Comparison between expected and measured voltage amplitudes at the AD8421 output (bipolar measurement).

Test Case | Expected ${\mathit{mV}}_{\mathit{p}-\mathit{p}}$ | Measured ${\mathit{mV}}_{\mathit{p}-\mathit{p}}$ | Error (%) |
---|---|---|---|

${R}_{L}=$ 150 $\mathrm{\Omega}$, $f=10$ kHz | 1014 | 990 | 2.36 |

${R}_{L}=$ 150 $\mathrm{\Omega}$, $f=100$ kHz | 1014 | 960 | 5.36 |

${R}_{L}=$ 150 $\mathrm{\Omega}$, $f=300$ kHz | 1014 | 900 | 11.2 |

${R}_{L}=$ 660 $\mathrm{\Omega}$, $f=10$ kHz | 4465 | 4460 | 0.11 |

${R}_{L}=$ 660 $\mathrm{\Omega}$, $f=100$ kHz | 4465 | 4300 | 3.70 |

${R}_{L}=$ 660 $\mathrm{\Omega}$, $f=300$ kHz | 4465 | 4100 | 8.17 |

${R}_{L}=$ 1200 $\mathrm{\Omega}$, $f=10$ kHz | 8118 | 7960 | 1.94 |

${R}_{L}=$ 1200 $\mathrm{\Omega}$, $f=100$ kHz | 8118 | 7760 | 4.41 |

${R}_{L}=$ 1200 $\mathrm{\Omega}$, $f=300$ kHz | 8118 | 7320 | 9.83 |

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Dimas, C.; Alimisis, V.; Georgakopoulos, I.; Voudoukis, N.; Uzunoglu, N.; Sotiriadis, P.P.
Analysis, Simulation, and Development of a Low-Cost Fully Active-Electrode Bioimpedance Measurement Module. *Technologies* **2021**, *9*, 59.
https://doi.org/10.3390/technologies9030059

**AMA Style**

Dimas C, Alimisis V, Georgakopoulos I, Voudoukis N, Uzunoglu N, Sotiriadis PP.
Analysis, Simulation, and Development of a Low-Cost Fully Active-Electrode Bioimpedance Measurement Module. *Technologies*. 2021; 9(3):59.
https://doi.org/10.3390/technologies9030059

**Chicago/Turabian Style**

Dimas, Christos, Vassilis Alimisis, Ioannis Georgakopoulos, Nikolaos Voudoukis, Nikolaos Uzunoglu, and Paul P. Sotiriadis.
2021. "Analysis, Simulation, and Development of a Low-Cost Fully Active-Electrode Bioimpedance Measurement Module" *Technologies* 9, no. 3: 59.
https://doi.org/10.3390/technologies9030059