An active electrode for biopotential recording from small localized biosources
 Emil S Valchinov^{1} and
 Nicolas E Pallikarakis^{1}Email author
DOI: 10.1186/1475925X325
© Valchinov and Pallikarakis; licensee BioMed Central Ltd. 2004
Received: 02 June 2004
Accepted: 22 July 2004
Published: 22 July 2004
Abstract
Background
Laser biostimulation is a wellestablished procedure in Medical Acupuncture. Nevertheless there is still a confusion as to whether it works or the effect is just placebo. Although a plethora of scientific papers published, showing positive clinical results, there is still a lack of objective scientific proofs about the biostimulation effect of lasers used in Acupuncture. The objective of this work was to design and build a body surface electrode and an amplifier for biopotential recording from acupuncture points, considered here as small localized biosources (SLB). The design is aimed for studying SLB potentials provoked by laser stimulus, in search for objective proofs of the biostimulation effect of lasers used in Medical Acupuncture.
Methods
The active electrode presented features a new adjustable anchoring system and fractionation of the biopotential amplifier between the electrode and the cabinet's location. The new adjustable electrode anchoring system is designed to reduce the electrodeskin contact impedance, its variation and motion artifacts. That is achieved by increasing the electrodeskin tension and decreasing its relative movement. Additionally the sensing element provides local constant skin stretching thus eliminating the contribution of the skin potential artifact. The electrode is attached to the skin by a doublesided adhesive pad, where the sensing element is a stainless steel, 4 mm in diameter. The fractionation of the biopotential amplifier is done by incorporating the amplifier's frontend opamps at the electrodes, thus avoiding the use of extra buffers. The biopotential amplifier features two selectable modes of operation: semiACmode with a 3 dB bandwidth of 0.32–1000 Hz and ACmode with a bandwidth of 0.16–1000 Hz.
Results
The average measured DC electrodeskin contact impedance of the proposed electrode was 450 kΩ, with electrode tension of 0.3 kg/cm^{2} on an unprepared skin of the inner forearm. The peaktopeak noise voltage measured at the amplifier output, with input terminals connected to common, was 10 mV_{pp}, or 2 μV_{pp} referred to the input. The commonmode rejection ratio of the amplifier was 96 dB at 50 Hz, measured with imbalanced electrodes' impedances. The prototype was also tested practically and sample records were obtained after a low intensity SLB laser stimulation. All measurements showed almost a complete absence of 50 Hz interference, although no electrolyte gel or skin preparation was applied.
Conclusion
The results showed that the new active electrode presented significantly reduced the electrodeskin impedance, its variation and motion artifact influences. This allowed SLB signals with relatively high quality to be recorded without skin preparation. The design offers low noise and major reduction in parts, size and power consumption. The active electrode specifications were found to be better or at least comparable to those of other existing designs.
Background
The noninvasive nature of laser biostimulation have made lasers an attractive alternative in Medical Acupuncture at the last 25 years. Unfortunately there is still a confusion as to whether they work or their effect is just placebo. Although a plethora of scientific papers published, showing positive clinical results, there is still a lack of objective scientific proofs about the biostimulation effect of lasers used in Acupuncture. The objective of this work was to design and built a body surface electrode and an amplifier for biopotential recording from acupuncture points. The latter are considered here as small localized biosources (SLB). As discussed by other authors, SLB are small area body regions with specific electrical, physiological and anatomical properties (e.g. high density of gap junctions, relatively low impedance etc.) [1–4]. They appear to be highly sensitive to mechanical, thermal, electrical or electromagnetic stimulation and are found to take place from the epidermis (stratum granulosum) to a maximum depth of 2 cm [5–8].
The active electrode is aimed for studying SLB potentials provoked by laser stimulus, in search for objective proofs of the biostimulation effect of lasers used in Medical Acupuncture.
Methods
Electrode design
The attempt to define the optimal parameters of the active electrode was based on a set of preliminary measurements performed in our laboratory. Anatomical, physiological and electrical characteristics of the signal source were considered. The SLB AC signal level, after stimulation, varied from subject to subject, but did not exceed 1 mV peaktopeak (pp). Additionally SLB occasionally manifested a high DC potential up to 200 mV, implying the use of differential amplifier with optional DC coupling and wide DC input range. The frequency band of the signal of interest was found to be in the range 0–200 Hz. The preliminary experiments showed that SLB potentials were best recorded with small pasteless electrodes although their contact impedance depends strongly on sweat gland secretion. The application of electrolytic gel resulted in significant reduction of the SLB signal amplitude, probably due to smoothing of the potential caused by saturation of the epidermis with electrolyte. Moreover, potentials between closely spaced SLB might be shortened by the application of excessive gel or large surface electrodes. An additional difficulty is that the SLB are often situated at convex or concave body surface areas where large flat electrodes could not be easily affixed. Skin abrasion with sandpaper is also not recommended since it can cause skin irritation and SLB potential changes. However, the use of small passive dry electrodes on an unprepared skin results in high electrodeskin impedance, motion artifacts, high powerline cable interference and noise. When the electrodes are DC coupled to the amplifier, a motion induced interfering signal appears at the amplifier input, mainly due to:

Electrokinetic effect – the disturbance of the double layer of charge at the electrodeskin interface causes variations of the DC polarization potential [9].

Skin potential or skin stretch artifact – stretching of the skin causes a change of the potential of the barrier layer between the epidermis and the dermis [10].

Variation in the electrodeskin contact resistance – caused by the amplifier input bias current and the current flowing due to the polarization potential.
V _{ mot }= ΔV _{ pol }+ ΔV _{ skin }+ (ΔR _{ d }+ ΔR _{ s }) (V _{ POL }/ R _{ i }+ i _{ b }) (1)
where ΔV_{skin} is the skin stretch artifact and i_{b} is the amplifier input bias current. It was deduced that in order to keep the resistive interfering component less than 10 μV when DC coupling is employed and with both currents contributing equally to it, then i_{b}<50 pA and R_{i}>1 GΩ [11]. If an AC coupling is used then the resistive component of the motion artifact is eliminated.
Basic amplifier circuit
where A_{d1}(s) is the differentialmode (DM) gain and A_{c1}(s) is the commonmode (CM) gain of opamp A_{1}. If we take the usual definitions for the DM input signal, V_{d}=(E_{1}E_{0}), and for the CM input signal, V_{c}=(E_{1} + E_{0})/2, then the output voltage can be also written as
U _{1}(s) = A _{ d }(s)V _{ d }+ A _{ c }(s)V _{ c } (3)
It can be shown that the respective expressions for the DM gain A_{d}(s), and the CM gain A_{c}(s), are given by
Considering (4) and (5), then the commonmode rejection ratio CMRR(s) is
Assuming opamps A_{0} and A_{1} are ideal then the only factor contributing to the CMRR is the mismatching of the resistors. Thus we can define a commonmode rejection ratio for the resistors, CMRR_{R}. By taking 1/A_{d0}(s) = A_{c}(s) = 0 in (6) we obtain
Therefore CMRR_{R}(s) approaches infinity if the relevant impedances are chosen according to
If the condition in (8) is fulfilled and opamps A_{0} and A_{1} are ideal, then (4) simplifies to
Considering (7), equation (6) can be written as
where CMRR_{A1}(s) is the CMRR of opamp A_{1}. Further if we assume that Z_{1}, R_{2}, R_{3} and Z_{E} have tolerance t, then from (7) and (8) we can deduce that the worst case condition will be when Z_{1} = Z_{10}(1t), R_{2} = R_{20}(1+t), R_{3} = R_{30}(1t) and Z_{E} = Z_{E0}(1+t) where Z_{10}, R_{20}, R_{30}, and Z_{E0}, are the respective nominal values. Equation (7) then can be written as
The CMRR_{A1} has the form
where ω_{r} is the frequency where CMRR_{A1} has decreased by 3 dB and is usually between 100 Hz and 1 kHz. The open loop gain A_{d0}(s), also decreases at higher frequencies with a corner frequency ω_{0} = 1/τ_{0} which is usually lower than ω_{r}, if A_{0} and A_{1} are of the same type. Therefore the CMRR is mainly determent at low frequency by the matching of the resistors and the DM gain, and at high frequencies by the open loop response of opamp A_{0}, rather than its CMRR. If we take the advantage of the fact implicit in (10), and achieve
then theoretically the CMRR becomes infinite. In part this can be achieved by the use of a capacitor and resistor in parallel for the impedance Z_{1} (Fig. 3), and then trimming R_{2}. Thus the need of lowtolerance components is eliminated. Therefore Z_{1}(s) will have the form
It has been shown [20] that a good approximation for the optimal value of the capacitor C_{1} is
where GBP_{A0} is the gain bandwidth product of opam A_{0}. Trimming R_{2} is a good solution for achieving an ultra high CMRR for demanding application, however it is not practical since the trimmer has to be incorporated in the electrode. Alternatively, Z_{E} or R_{3} can be trimmed, which however will alter also the amplifier DM gain. Considering equation (12) it can be shown that for application with relatively high DM gain and proper opamp selection, both trimming and compensation (C_{1}) can be omitted, without significantly degrading the CMRR. For example, if the usual 1% tolerance resistors are used and opamps with CMRR of 100 dB and DM open loop gain of 120 dB at 50 Hz, then for an amplifier with DM gain of 5000, a CMRR of 96 dB can be achieved without trimming.
In the amplifier circuit shown in Fig. 3, Z_{E} is replaced with an active DC rejection/suppression circuit [20]. It includes an integrator (A_{2}, R_{i}, C_{i}) and two potential dividers (R_{6}, R_{5} and R_{4}, R_{3}). The amplifier can operate in ACmode or in semiACmode. The two modes are selectable by the switch S_{1}: ACmode with S_{1} open and semiACmode with S_{1} closed. In ACmode the DC signals are rejected, where in semiACmode they are suppressed. If R_{6} = R_{6} *, R_{5} = R_{5} * and R_{i} = R_{i} *, then the respective expressions for the equivalent impedance Z_{E}(s) for the two modes are given by
where τ_{i} = R_{i}C_{i} is the time constant of the integrator, and τ_{2} are the respectively the DM open loop gain and the time constant of the first pole of opamp A_{2}. Whenever R_{i}>>R_{5} then k ≈ (R_{6}/R_{5}+1), which is true with the time constants and voltage gains, typical in biopotential recordings. For signals bellow the amplifier highpass cutoff frequency, Z_{E}(s) decreases due to the active DC rejection/suppression circuit. For DC signals equation (8) is maximally imbalanced and thus CMRR_{R}(0) ≈ A_{d}(0). Since for biopotential amplifiers A_{d}(0) is much lower than CMRR_{A1}(0) and A_{d0}(0), therefore CMRR(0) ≈ A_{d}(0), which represents the worst case.
If we consider only the 3 dB bandwidth and assume that opamp A_{2} is ideal, then (17) and (18) simplify to
Therefore, in this case (9) can be written as
which represents the mid band DM gain for both modes. After substituting (17) and (18) in (9), it can be shown that the respective DC differential gains for the two modes are given by
where 2k is approximately the DC gain of the stopped integrator (A_{2}, R_{i}, C_{i}, R_{i} *, R_{5} *, R_{6} *) in semiACmode. Thus DC signals meet lower gain, in order to prevent saturation from large electrode offsets or other high DC potentials.
The active electrodes' input resistances R_{iE0} and R_{iE1}, are not equal due to the different closed loop gains of opamps A_{0} and A_{1}, and can be expressed as
where R_{iA0} and R_{iA1} are the input resistances of opamps A_{0} and A_{1}. However, at higher frequencies, the electrodes' input impedances are much lower and about the same (assumed that A_{0} and A_{1} are of the same type), due to the opamps' input and additional stray capacitance, being in parallel to the high opamps' input resistance.
The output noise spectral density for the 3 dB bandwidth is approximately the same for both modes and can be written as
where e_{n0}, e_{n1} and e_{n2} are the respective voltage noises of opamps A_{0}, A_{1} and A_{2}.
Assuming E_{0} is connected to common (Fig. 3), then the amplifier transfer function H(s) is given by
After substituting Z_{E}(s) and A_{d1}(s) in (24), it can be shown that H(s) has three poles and two zeros for both modes. However, with the time constants and voltage gain used in the current application, one pole almost coincides with one zero. Therefore, H(s) can be approximated very well by a transfer function with two poles and one zero. The respective approximations for ACmode and semiACmode are given by
The circuits described by the transfer functions H_{AC}(s) and H_{sAC}(s) are stable because all the poles are situated in the left half of the complex splane and there are no resonance effects as the poles are on the real saxis.
Practical amplifier circuit
Because of the large integrator's time constant, the amplifier has a very slow response after overloads (≤ 10τ_{i}), caused by large signal disturbances. Thus a deblocking circuit was added at the cabinet's location, for temporary reduction of the time constant during overload [24]. It is controlled by the output voltage U1 through the low pass filter (R_{13}, C_{2}). The filter output controls two threshold triggers (A_{3}, A_{4}), which through D_{1},D_{2} control the MOS transistor T_{1}, acting like a switch. When the output signal reaches its range limits (defined by R_{14}, R_{15}, R_{16}), T_{1} opens and the new reduced time constant τ_{i}* = (R_{7}//R_{11})C_{2}, pulls the output signal to the zero level. This state is maintained for additional hundred milliseconds (R_{13}C_{2}) and then is switched back to its original value.
The connection between the amplifier common and the signal source is implemented by a driven rightleg (DRL) circuit. The CM voltage at the output of A_{0} is reduced by a factor equal to the DRL circuit gain (A_{DRL} = 314 at 50 Hz), which theoretically should give a 50 dB extra CMRR at 50 Hz. In addition, in case of a faulty opamp, the DRL circuit will limit the maximum patient current to a safe level of 50 μA.
Results
Active electrode specifications
Parameter  semiACmode  ACmode 

Bandwidth (3 dB)  0.32–1000 Hz  0.16–1000 Hz 
DC gain  3.22  ≈ 0 
AC mid band gain  74 dB  
Differential mode AC input range  0.005–1 mV_{pp}  
Differential mode DC input range  ± 370 mV  
Common mode input range  ± 2 V  
Input noise current  1 pA_{rms} @ 0.1–200 Hz  
Input bias current  1.5 pA  
Input impedance, Active Electrode  320 MΩ @ 50 Hz (1000 GΩ //10 pF)  
CMRR  96 dB @ 50 Hz  
Output offset  0.7 mV  
Input noise voltage  2 μV_{pp} (0.33 μV_{rms}) @ 0.1–200 Hz  
Power consumption  11 mW @ one channel 
Discussion
The best solution for an active electrode would be to perform the entire analog signal processing at the electrode site. This could be achieved with a custom made integrated circuit, but the cost would be much higher. We found a good alternative in using SMD technology and integrating only the frontend of the amplifier into the electrode.
The ultra high input resistance of the electrode is degraded at higher frequencies by the opamp's input capacitance in parallel with the stray capacitance due to the electrode Printed Circuit Board (PCB). Nevertheless, combining an opamp with low input capacitance and a proper PCB design, allowed a relatively high input impedance to be achieved at 50 Hz. That decreased the amplifier sensitivity to high electrodeskin impedance imbalances, by reducing the transformation of the CM interference signal into unwanted DM signal. Unfortunately, most data sheets do not properly specify opamp's input capacitance, neither DM nor CM.
The active electrode presented is not suitable for applications requiring a low differential gain and large signal bandwidth due to the decreasing CMRR at higher frequencies, if not properly compensated. On the other hand, below the highpass cutoff frequency, the CMRR is degraded by the active feedback circuit, and reaches its minimum value for DC signals, equal to the DM gain. The circuit can accept high value input filter resistances, which will also limit the patient auxiliary current in case of fault condition of opamps A_{0} and A_{1}. Because of the limited electrode space, it is preferable that the frontend opamps feature internal electrostatic discharge protection circuitry, rather than building an external one.
Conclusions
The new electrode anchoring system significantly reduced the electrodeskin impedance, its variation and motion artifact influences. The proposed amplifier fractionation resulted in lower noise and less parts. Moreover splitting the amplifier between the electrodes and the cabinet's location allowed the use of an automatic DC deblocking system and mode switching. The prototype tests showed that with the active electrode presented, SLB signals with relatively high quality could be recorded without skin preparation. The 50 Hz interference pickup by the electrode leads was practically eliminated. Because high electrodeskin impedances are tolerated, no electrolytic gel is needed. This allows fast application of the electrodes, minimizes patient discomfort and eliminates the risk of infection.
With proper opamps selection, the active electrode specifications were found to be better or at least comparable to those of other existing designs. The design offers low noise and major reduction in parts, size and power consumption. It is currently used in studying laser provoked SLB potentials and their propagation, aiming to gain a better insight into the biostimulation effect of lasers in Medical Acupuncture.
Declarations
Authors’ Affiliations
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