capacitance-to-digital: the upgrades of single chip detector

Electrophoresis 2014, 00, 1–6 1

Tomas Drevinskas1

Audrius Maruska1

Vitalis Briedis2

1Faculty of Natural Sciences,Department of Biology,Vytautas Magnus University,Kaunas, Lithuania

2Faculty of Pharmacy, LithuanianUniversity of Health Sciences,Kaunas, Lithuania

Received July 4, 2014Revised September 11, 2014Accepted September 30, 2014

Research Article

Capacitance-to-digital: The upgrades ofsingle chip detector

The capacitance-to-digital single chip detector was upgraded. The paper discusses hard-ware issues and benefits of the designed/upgraded detector. The device can be operatedfrom rechargeable lithium-ion battery as stand-alone, portable system and is capable oftransmitting real-time data wirelessly. The detector and additional modules (battery, batteryholder, microcontroller board, wireless module) weight is less than 85 g. Electrophoreticseparation in low conductivity 20 mM MES/L-His buffer, pH 6.1, was performed in orderto evaluate detection parameters. The system is capable of quantification of potassium ionsdown to 0.31 �M. Investigation of differential signal acquisition configuration showed im-proved performance regarding external noise and temperature fluctuations. The systemcan be a solution for stand-alone, field-portable capillary format separation detector.

Keywords:

Capacitance-to-digital converter / Capillary electrophoresis / MiniaturizationDOI 10.1002/elps.201400320

� Additional supporting information may be found in the online version of thisarticle at the publisher’s web-site

1 Introduction

Since the introduction of chromatography in 1903 by Tsvett [1]and electrophoresis in 1930s by Tiselius [2], separation meth-ods were upgraded/developed into sophisticated analyticaltools for complex mixtures analysis [3]. Separation systemsare coupled to a detector that detects migrating analytesthrough the detection window. Such systems are commonlypurchased as benchtop devices. Numerous suppliers can befound providing different bench-top size analytical systemsworldwide. Contrary, field-portable, stand-alone, miniatur-ized separation/detection systems, or modules are describedmore often in scientific literature [4]. Demanding industrialguidelines [5], ambitiously growing scientific community [6]require automated, miniaturized, wireless, smart systemsfor chemical analysis, environmental monitoring, processcontrol etc.

Numerous scientific articles describe miniaturizedanalytical tools and detectors integrated into separation

Correspondence: Professor Audrius S. Maruska, Faculty of Natu-ral Sciences, Department of Biology, Vytautas Magnus University,Vileikos 8–206, LT44404 Kaunas, LithuaniaE-mail: [email protected]: +37-037-327908

Abbreviations: CDC, capacitance-to-digital converter; GND,ground; IC, integrated circuit; LDO, low dropout; PCB, printedcircuit board

systems [7, 8]. Not only separation devices require moderndetection systems, but miniaturized mass analyzers [9], fluo-rescence detectors [10], spectrophotometers [11], impedanceanalyzers [12], capacitive sensors [13], gas sensors [14],Fourier transform infrared spectrometers [15] as well. Thewireless data transmission implementation into moderndetectors is presented in the scientific literature [16].

Integration of modern detection systems or couplingthem with separation systems is a challenge. Special elec-trode optimization [17], additional coupling interface [18] etc.are needed. Frequently, after coupling the device or systemhas bigger dimensions [17], or such systems need additionalunits for operation (laboratory power supply, alternatingcurrent source, connectors etc.) [19].

Single chip techniques provide numerous advantages:low noise, low power consumption, small-size of the de-vice [20]. Therefore capacitive sensor AD7745 integrated cir-cuit (IC) was selected as a candidate designing the single chipdetector [21]. Manufacturers state, that this IC can be poweredfrom a low voltage source of 3.3 V and consumes 700 �A cur-rent. Such characteristics provide a possibility for low powerconsumption applications.

Often designers/engineers face additional problems,when designing miniaturized systems. Printed circuit boardtraces routing can be a challenge due to cross-talk and otherparasitic effects [22].

Capacitance-to-digital (CDC) technology allows measure-ment of capacitance and conversion of it to digital code [23].There are several publications with this technology applied

C© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com

2 T. Drevinskas et al. Electrophoresis 2014, 00, 1–6

for liquid holdup measurement [24], detection of volatile com-pounds [14], structural health monitoring [16] etc. This tech-nology has already been applied in separation science [25,26].

In our previous work the single chip (AD7745) was usedas whole detector. The device operation required laboratorypower supply, additional electronics prototyping board, andUSB cable for data transmission [27]. Only one single-endedelectrode configuration was investigated in the former set-up. Differential signal acquisition concept can be applied fordetection. Such technique provides higher resistance to ex-ternal noise [28]. In addition, Wheatstone bridge is a widelyused measurement technique in the modern devices (digi-tal scales, pressure sensors etc.). This measurement imple-ments four wires and four resistances (impedances). Usuallythree resistances are equal and known and one is used as asensor [29]. This method allows a simple proportional calcu-lation of unknown resistive value of the sensor. Inequalitiesdue to the presence of different length wires are neglected,therefore only the change of sensor resistivity is recorded[29, 30].

The aim of this work was to design a stand-alone,battery-powered, wireless, capacitance-to-digital single chipdetector for capillary electrophoresis and the improvementof detection parameters using different signal acquisitionconfigurations.

2 Materials and methods

2.1 Chemicals

L-Histidine (L-His) (99.0%), monohydrate MES (98.0%) werepurchased from Alfa Aesar (Germany), sodium hydrox-ide (NaOH) (99.0%) was purchased from Reachem (Slo-vakia), potassium dihydro phosphate (NaH2PO4) (99.3%) wasfrom Fisher Scientific (USA). 2-Amino-2-(hydroxymethyl)-1,3-propanediol (TRIS) (99.8%) was purchased from Merck(Germany), sodium chloride (NaCl) was purchased fromLachema (Czech Republic), acetic acid (99.8%) was purchasedfrom Lach-ner (Czech Republic). Histamine (�97.0%) andmelittin (91.8%) were from Sigma-Aldrich (Germany). Beevenom was kindly provided by the local beekeeper. Bidistilledwater (produced in a laboratory) was used for preparation ofthe solutions.

2.2 Instrumentation

AD7745 chip was purchased from Analog Devices (USA),LT1761 3.3 V low dropout (LDO) voltage regulator was pur-chased from Linear Technology (USA). Other electronic partsand wires were purchased from the local store, APC220 wire-less data transmission module was from DFROBOT (China).Sparkfun pro micro prototype development board was pur-chased from SparkFun (USA). Li-ion (3.7 V, 1800 mAh) cellphone battery from Kruger & Matz (Poland) was used topower the device. Analyses were performed using HP 3D CE

capillary electrophoresis system (Agilent Technologies, Wald-bron, Germany). The data acquisition and peak integrationsoftware was programmed in house using open-source soft-ware Processing 2 (USA). Other calculations were performedusing Microsoft Excel (USA).

2.3 Sample preparation and electrophoretic

separation

Stock solutions of analytes (NaCl, KH2PO4, TRIS, histamine,melittin, and bee venom) and 20 mM MES/L-His buffer/BGE,pH 6.1 were prepared by dissolving adequate mass of sub-stance in bidistilled water and frozen at −20°C to pre-vent microbial growth. BGE of 1 M acetic acid (pH 2.4)was prepared by diluting adequate volume of acetic acid withbidistilled water. Before use, the stock solutions were de-frosted and diluted to required concentration using bidistilledwater. Buffer solution was degassed in vacuum for 20 s beforeanalysis.

Electrophoretic separations were performed using 50 �mid, 365 �m od fused silica capillary. Analysis temperature wasset at 25°C. A 14 and 17 kV voltages were used for detectionevaluation and real-sample analysis correspondingly. Sam-ples were injected hydrodynamically at 50 mbar for 5 s. Sepa-ration in MES/L-His buffer was performed in less than 5 minand separation in 1 M acetic acid was performed in 12 min.For evaluation of detection parameters 20 mM MES/L-Hisbuffer was selected as BGE. Metal cations were separatedusing TRIS buffer [31]. Capillary was flushed for 2 min with0.1 M NaOH, bidistilled water, and BGE prior to each analysis.Each analysis was repeated three times. LOQ were calculatedin accordance with valid pharmaceutical analytical methodvalidation guidelines [32].

3 Results and discussion

3.1 Design of battery powered detector

Battery holder, holder clips, and detection cell case were3D-printed using a Printrbot 3D printer (Printrbot, USA).Microcontroller development board and wireless data trans-mission module was glued on top of the battery holder us-ing hot glue. AD7745 chip was soldered on 2-sided printedcircuit board (PCB) that was carefully routed in accordancewith technical documentation [23] and mixed signal lay-out guidelines [33]. The AD7745 contains digital and ana-log signal conversion parts, therefore IC placement affectsthe designed device performance. The device was designedof several modules (prototyping board, wireless transmis-sion module, and AD7745 capacitance detector), thereforesolid/unsplit grounding cannot be applied [33].

In our previous applications the single chip detectorwas powered from laboratory power supply [27]. All ground(GND) wires were connected to the single point of powerplug grounded wire. The battery powered, stand-alone devices


Electrophoresis 2014, 00, 1–6 Microfluidics and Miniaturization 3

Figure 1. Schematic diagram of thedetector. (A) Single-ended configu-ration. (B) Differential configuration.(C) Wheatstone bridge configuration.Drawn using Cadsoft Eagle (USA).

Figure 2. Photographs of different con-figuration detectors. (A) Single-endedconfiguration, (B) differential configura-tion, (C) shielded device with copper foil.

cannot be connected to a common grounded wire, thereforeother solutions of this problem should be made.

High resolution analog-to-digital converters require avery low noise power supply [34]. Consequently, low noise,micro-power, LDO voltage regulator LT1761 was used for on-board voltage stabilization. Stabilized voltage of 3.3 V pow-ered the AD7745 chip. Decoupling capacitors were selectedas suggested in the datasheet [35]. Power supply of AD7745was bypassed using 100 nF ceramic, 1 and 10 �F tantalumcapacitors (Fig. 1).

Electrode geometry of the device was similar to geom-etry used in our previous work [27]. Electrodes were madefrom stainless steel sleeves (syringe needles) (0.4 mm id,0.6 mm od, 19 mm length) and soldered onto the PCB leav-ing a gap of 0.2 mm between actuator and sensor electrodes(Supporting Information Fig. 1). Shielded wires were usedfor power supply from the battery and for digital data con-nections. Data were collected at 9 Hz rate via inter-integratedcircuit interface and instantly sent to the computer via a wire-less system. The total mass of the designed detector was 82.5 g(Supporting Information Fig. 2).

Three different electrode configurations were tested: (i)one electrode couple in single-ended mode (Fig. 1A); (ii) twoelectrode couples in differential mode (Fig. 1B); (iii) fourelectrode couples—Wheatstone bridge (Fig. 1C) (differentialmode). In differential operation mode offset calibration wasperformed. This procedure was necessary for compensation

of wire/trace inequality and different parasitic values for eachinput pin of AD7745. Software code was adjusted so that thecapacitive offset calibration register [22] would have 15 fFoffset at start (Supporting Information Vid. 1).

3.2 Performance evaluation

First of all the previously reported version of single-endedsingle chip detector [27] was reproduced (Fig. 2A). Severalchanges were implemented, namely: (i) PCB was etched withGND layer under all electrode area; (ii) AD7745 chip andpassive electronic parts were completely shielded. Unshieldeddetector with single-ended configuration resulted in highernoise floor than unshielded detector with differential signalacquisition configuration (Fig. 2B). Resolution parameterswere calculated (Table 1).

Table 1. Comparison of performance of different configurationdetectors

No. Shield Effective resolution(peak-to-peak bits)

Wheatstone bridge(Differential)

Single-ended Differential

1 Shielded 16.6 17.3 16.72 Unshielded 13.4 16.7 16.3



Table 2. Detector performance regarding capillary washprocedure

Effective resolution (peak-to-peak bits)

No. Flushing solution Active electrodecouple washed

Active and referenceelectrode coupleswashed

1 Bidistilled water 17.3 17.32 MES/L-His buffer 17.3 17.33 0.1 M NaOH 12 17

The results obtained show, that single-ended electrodeconfiguration is highly susceptible to noise, therefore the de-vice must be completely shielded (Fig. 2C). The unshieldeddevice differential electrode configuration offered similar per-formance to the shielded device.

During detector performance evaluation it was noticed,that the noise floor of a detector differs when different so-lutions are flushed through the capillary. The sensor elec-trode connected to Cin(±) and the high conductivity liquid(0.1 M NaOH) inside the capillary form a capacitively cou-pled antenna which degrades signal-to-noise ratio/effectiveresolution. However, undesirable background noise can bereduced by injecting the same liquid into reference electrodecouple. Consequently, two antennas are formed providingsame pattern of noise into Cin+ and Cin– of AD7745 chipinputs resulting in subtracted noise and higher performance(Table 2). Similar tendencies showing reduced performanceof capacitively coupled contactless conductivity detector (C4D)detectors and higher background noise are presented by otherresearchers [36]. The latter issue is the main cause of degradedperformance for single-ended C4D detectors when high con-ductivity buffer is used for separation. In another work [30]slight degradation of effective resolution from 16.8 bits (lowconductivity BGE) to 16.0 bits (high conductivity BGE) bitswas reported.

Another valuable feature of the detector was noticed dur-ing the capillary cassette temperature stabilization period.Low baseline shifting was observed when the temperature ofcapillary cassette is increased or decreased. The effect of tem-perature fluctuations was evaluated and compared betweendifferential and single-ended electrode configurations. Dur-

Table 3. Recorded values of capacitance for different electrodeconfigurations and different flush solutions

No. Solution Configuration Value at15°C (fF)

Value at30°C (fF)

Difference(fF)

1 Bidistilled water Single-ended 145.2 155.7 10.52 Bidistilled water Differential 19.2 20.0 0.83 20 mM MES/L-His

bufferSingle-ended 287.1 315.5 28.4

4 20 mM MES/L-Hisbuffer

Differential 9.6 13.5 3.9

ing temperature fluctuations test the capillaries inside mea-surement electrode couple (for single-ended configuration)and measurement/reference electrode couple (for differen-tial electrode configuration) were injected with the test liquid(water and 20 mM MES/L-His buffer). Temperature shiftsfrom 15 to 30°C and vice versa were set. During temperaturechange period in capillary cassette the values of capacitancewere recorded. Lower baseline change during the tempera-ture shifting was observed for differential electrode configura-tion than for single-ended electrode configuration (Table 3).The differential signal acquisition mode showed increasedperformance during temperature fluctuations.

One of the main problems of contactless conductivity de-tection (or similar complex impedance measurement tech-niques) is caused by fluctuating temperature [36, 37]. Inthis case, temperature fluctuations affect both: active andreference electrode couples, therefore, undesirable baselineshifting is negligible or reduced (Fig. 3).

3.3 Separation and detection evaluation

In our previous work [27] it was shown that CDC detectoris capable of detecting of inorganic and organic cations andanions in high and low conductivity buffers. Detection capa-bilities in different BGE solutions were investigated. In thispaper, a low conductivity buffer was tested using model ana-lytes in order to evaluate performance of the designed device.Separation of three cationic analytes was performed in 20 mMMES/L-His pH 6.1 buffer solution. Baseline separation wasachieved in less than 5 min (Fig. 4).

Figure 3. Temperature fluctuationsinfluence on CDC detector reading.Temperature shifted down from 30°Cto 15°C and vice versa. Gray line—single-ended electrode configurationreading, black line—differential elec-trode configuration reading. Condi-tions: 50 �m id, 365 �m od fused silicacapillary (A) 20 mM MES/L-His bufferflushed, (B) bidistilled water flushed.Detection: 3.3 V, 32 kHz.


Electrophoresis 2014, 00, 1–6 Microfluidics and Miniaturization 5

Figure 4. Separation of 1–K+ (44 �M), 2–Na+ (52 �M) ions and3–TRIS (28 �M). Conditions: 50 �m id fused silica capillary,effective length (Leff)–37.5 cm, total length (Ltot)–50.0 cm, 14 kVvoltage potential, 25°C separation temperature, injection at50 mbar * 5 s. BGE–20 mM MES/L-His buffer, pH 6.1. Detection:3.3 V, 32 kHz.

Table 4. Calculated LOQ for different electrode configurationdetectors

No. Range (�M) LOQ (�M)

Single-ended Differentialelectrodes

Wheatstonebridge

K+ 11–27500 0.54 0.31 0.56Na+ 14–3500 0.49 0.25 0.45TRIS 7–1875 0.83 0.55 0.83

Obtained results showed excellent linearity (R2 � 0.99) ofthe calibration curves for the selected analytes and electrodeconfigurations tested. High migration time repeatability(RSD � 1.2%) and good peak area repeatability (RSD � 2.2%)were achieved. LOQs were calculated for all the analytes anddifferent electrode configurations used. LOQ for K+ were0.54 using single-ended setup and 0.31 �M using differen-tial electrode setup (Table 4). In the previous work, LOD(LOD is assumed three times lower than LOQ) of K+ was1.6 �M using the same conditions, which shows an improve-ment of more than eight times for similar electrode con-figurations. The improvement of sensitivity is caused by animplementation of on-board voltage regulator. The total im-provement of sensitivity of upgraded detector was more than15 times.

Better performance was expected from Wheatstonebridge electrode configuration, however the results were verysimilar to that obtained using single-ended configuration.This can be explained by the fact that in Wheatstone bridgeelectrode configuration the detection cell was separated fromother electronic parts. Although shielded wires were usedfor signal transmittance, separated modules usually decreaseperformance of the device [33].

What is most important, the differential electrode config-uration provides a subtraction of parasitic effects, therefore it

can be stated, that the measurement is performed only in theliquid present inside the capillary.

The detector was designed manually (all electronic partsand electrodes were soldered by hand). The use of sophis-ticated pick-and-place machines for electrode soldering isexpected to improve the symmetry that should increasethe performance of the detector. Such techniques are ap-plied in the difference or instrumentation amplifiers, whensymmetry plays an important role on device performance[28].

3.4 Real-sample separation

The results obtained by other authors show that bee venomcan be successfully separated in acidic BGE [38, 39]. In thepresent work, separation of bee venom was performed in theBGE of 1 M acetic acid, pH 2.4. Conditions were not optimizedfor separation, since the intention was only to provide datashowing, that separation and detection of complex mixture ispossible using the detection setup of designed configuration.Separation was performed in 12 min (Fig. 5). Analytes weredetected using a CDC detector with differential electrode con-figuration. The reference electrode couple contained emptyfused silica capillary (50 �m id, 365 �m od). The results ob-tained showed excellent linearity (R2 � 0.99) of the histaminecalibration curve. LOQ for histamine was 0.4 �M. Perfectmigration time repeatability (RSD � 1.0%) and good peakarea repeatability (RSD � 3.6%) were achieved. Histamineand melittin peaks were identified in the electropherogramof separated bee venom using external standard method.Determined concentration of histamine in the sample was40 �M and histamine part in bee venom was found to be1.0%.

Figure 5. Separation of bee venom sample (0.4 mg/mL) dissolvedin water. Conditions: 50 �m id, 365 �m od fused silica capillary,Leff—37.5 cm, Ltot—50.0 cm, 17 kV voltage potential, 25°C sepa-ration temperature, injection at 50 mbar * 5 s. BGE—1 M aceticacid, pH 2.4. Detection: 3.3 V, 32 kHz. Peaks: 1–40 �M histamine,2–melittin.



4 Concluding remarks

Stand-alone battery-powered capacitance-to-digital detectorfor capillary electrophoresis, capable of transmitting real-timedata wirelessly, was designed. Different signal acquisitionelectrode configurations were evaluated, showing approxi-mately 15 times higher LOQs comparing it to the previouswork. The effect of external noise and thermal fluctuationson detector performance was reduced. Developed system wasapplied for the real-sample analysis.

This research was financed by the Research Fund of VytautasMagnus University, grant No. P-BF-13-07. Instrumental sup-port from Agilet Technologies (donation of HP 3D CE capillaryelectrophoresis system) is highly acknowledged.

The authors have declared no conflicts of interest.

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capacitance-to-digital: the upgrades of single chip detector

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