**2.3 A novel solution measurement of differences**

Certified medical bioimpedance measurement device CircMon BT101, which we have been using so far during the clinical studies, employs a carrier (base value Z0) compensation method [22]. The biggest drawback of this solution is the complexity of both, electronic circuitry and algorithms, for adjusting the compensation signal.

**17**

*Noninvasive Acquisition of the Aortic Blood Pressure Waveform*

*Block diagram of the derivative bioimpedance signal acquisition system.*

Increased energy consumption is a penalty for improvements. The conclusion is that

*The differenced bioimpedance signal dZ(t) measured by 10-bit ADC in numberic value range from −128 to +256. The measured waveform is smoothed by the third-order Savitzky-Golay filter within 10 sidepoints.*

The difference method has been introduced recently for achieving the same result [22, 23]. The main idea behind it lies in digitising of the difference between two consecutive samples instead of direct digitalisation of all the samples. It corresponds to taking a derivative, mathematically. Since the derivative of the constant is zero, the base value Z0 of the impedance Z is eliminated, but the informative

The novel test device in **Figure 6** contains AVR ATXMEGA microcontroller together with BLE 4.0+ module, but instead of high-quality external 32-bit ADC, an

The acquired signal presents now a derivative of the original biological impedance depicted in **Figure 7**. The original signal is restored by digital integration (**Figure 17**), which brings in an additional smoothing effect improving the resulting signal-to-noise ratio (SNR). At the same time, this integration may well be unneces-

Occasionally, one of the signal processing steps after acquiring the impedance signal is differentiation for finding certain peculiar points in the waveform. The first, second and even higher derivatives are used to find and calculate relevant cardiovascular parameters. In that sense, the acquired impedance signal in **Figure 7** is well suited without the integration step. The CAP waveform can also be derived directly from the derivative of the ΔZ(t). The experiments showed the presence of significant noise and

internal rather noisy low-quality 10-bit-embedded ADC was used.

a new, more effective solution should be developed.

variations ΔZ(t) are upraised.

sary in some cases.

**Figure 6.**

**Figure 7.**

*DOI: http://dx.doi.org/10.5772/intechopen.86065*

#### **Figure 6.**

*Wearable Devices - The Big Wave of Innovation*

*Image of the 32-bit impedance acquisition system prototype.*

**16**

**Figure 5.**

**Figure 4.**

*Measured bioimpedance values on top of the radial artery on the Nyquist diagram [23].*

realisation of the synchronous detector.

**2.3 A novel solution measurement of differences**

in Chapter 3.1). Several observations can be drawn from the generalized measurement results in **Figure 5**. The first observation is that the greyish, slightly smeared information carrying signal, ΔZ(t), is tiny compared to the base value Z0 of the acquired bioimpedance. Next, the imaginary part Im Z of the impedance vector Z is nearly 10 times smaller than the real part Re Z. Third, the modulation is roughly in the direction of the vector of the impedance base value Z0. The conclusion is that the role of Im Z is low, and less attention can be paid into the accuracy of the vector measurements when designing the device, especially the synchronous detector of it. A root problem in designing a suitable electronics is whether to use analog or digital

Certified medical bioimpedance measurement device CircMon BT101, which we have been using so far during the clinical studies, employs a carrier (base value Z0) compensation method [22]. The biggest drawback of this solution is the complexity of both, electronic circuitry and algorithms, for adjusting the compensation signal.

*Block diagram of the derivative bioimpedance signal acquisition system.*

#### **Figure 7.**

*The differenced bioimpedance signal dZ(t) measured by 10-bit ADC in numberic value range from −128 to +256. The measured waveform is smoothed by the third-order Savitzky-Golay filter within 10 sidepoints.*

Increased energy consumption is a penalty for improvements. The conclusion is that a new, more effective solution should be developed.

The difference method has been introduced recently for achieving the same result [22, 23]. The main idea behind it lies in digitising of the difference between two consecutive samples instead of direct digitalisation of all the samples. It corresponds to taking a derivative, mathematically. Since the derivative of the constant is zero, the base value Z0 of the impedance Z is eliminated, but the informative variations ΔZ(t) are upraised.

The novel test device in **Figure 6** contains AVR ATXMEGA microcontroller together with BLE 4.0+ module, but instead of high-quality external 32-bit ADC, an internal rather noisy low-quality 10-bit-embedded ADC was used.

The acquired signal presents now a derivative of the original biological impedance depicted in **Figure 7**. The original signal is restored by digital integration (**Figure 17**), which brings in an additional smoothing effect improving the resulting signal-to-noise ratio (SNR). At the same time, this integration may well be unnecessary in some cases.

Occasionally, one of the signal processing steps after acquiring the impedance signal is differentiation for finding certain peculiar points in the waveform. The first, second and even higher derivatives are used to find and calculate relevant cardiovascular parameters. In that sense, the acquired impedance signal in **Figure 7** is well suited without the integration step. The CAP waveform can also be derived directly from the derivative of the ΔZ(t). The experiments showed the presence of significant noise and

disturbance when making provisional experiments with simple stainless steel electrodes [24], and the role of movement artefacts was highly troubling. This implies that the electrode design must be considered more seriously in further research.
