Biochemical Sensor Interface Circuits with
Differential Difference Amplifier
Shin-Il Lim, In-Sub Choi,
Han-Ho Lee
Dept. of Electronics Engineering
Seokyeong University
Seoul, Korea
silim@skuniv.ac.kr, retive@naver.com,
Dept. of Information and Communication Engineering
Inha University
Incheon, Korea
hhlee@inha.ac.kr
Abstract—A simple three-electrode CMOS interface system for
electrochemical sensor is described. To maintain a constant
potential between the reference electrode (RE) and working
electrode (WE), only one differential difference amplifier (DDA)
is exploited in this proposed design, while conventional sensor
interface system requires at least 2 operational amplifiers and 2
resistors, or more than 3 operational amplifiers and 4 resistors
for low voltage differential CMOS integrated interface circuits.
The DDA with rail-to-rail design not only enables the full range
operation to supply voltage but also provides simple interface
system with small hardware and low power consumption. This
new interface system was implemented in a 0.35um standard
CMOS technology and experimentally verified.
I.
INTRODUCTION
Electro-biochemical sensors are widely used in glucose
monitoring, pH variation detection, protein classification,
neurotransmitter sensing, oxygen concentration monitoring,
dissolved molecules in liquid and toxic gas detection, etc. [1][9]. Interface circuits for biochemical sensor provide constant
voltage to the sensor and detect the sensor current as the
resistance of sensor varies by its electro-chemical status
(concentration, type, etc) of solution. Amperometric
potentiostat, one of the techniques in electro-biochemical
analysis, is based on the chemical reaction of certain species
that involves a gain of electron (oxidation) and/or a loss of
electrons (reduction) while maintaining a characteristic
potential. Two or three sensor electrodes are commonly used
in amperometric measurement technique. To prevent the effect
of the voltage loss that is caused by the resistance of the
analyte solution, three-electrode potentiostat is usually
preferred for accurate sensor measurement system. Recent invivo biochemical sensor, such as implantable glucose sensor
and neuro-transmitters, needs a miniaturized potentiostat
which can measure the small redox (reduction and oxidation)
current of ~nA range while maintaining a certain redox
potential. Reducing the signal path from sensor to the interface
circuit is very important to reduce the noise and interfering
signals. The best solution for this problem is to place the
interface circuits and sensors on one small package or on one
chip.
A simple and new CMOS interface (potentiostat) circuit
for biochemical sensor is proposed in this design. One
amplifier without any passive components is enough to
maintain the potential between the reference electrode (RE)
and working electrode (WE). The Portable or disposable or
implantable biosensor can be implemented with this small and
robust interface circuits. More details are described in
following sections.
II.
Conventional amperometric potentiostat is shown in Fig.
1(a) [5] and it's fully differential version for wide swing in low
voltage applications is in Fig. 1(b) [6]. If the gain of A2 is big
enough and R1 and R2 have same values in Fig. 1(a), the
source potential VSRC can be equal to the applied cell potential
of VCELL to the sensor that has the Faradaic resistance of RFW,
(VSRC = VCELL). This potential difference of VCELL between the
reference electrode (RE) and working electrode (WE) induces
an sensor reaction current that has the value of VCELL/RFW.
This sensor reaction current can be measured by using transimpedance amplifier (TIA) or by using charge based counting
ADC. The fully differential implementation of conventional
amperometric potentiostat shows larger dynamic range than
single ended mode in low voltage application.
Since the conventional amperometric potentiostat needs
several op-amps and passive resistors for three electrode
biochemical sensor system, it is vulnerable to mismatch, offset
and noise. And it shows larger hardware consumption and
larger power consumption than proposed simple CMOS
interface (potentiostat) circuit for biochemical sensor.
III.
PROPOSED BIOCHEMICAL INTERFACE CIRCUITS
Fig. 2 illustrates the architecture of proposed simple
biochemical interface circuit. The proposed circuits consist of
differential difference amplifier (DDA) and trans-impedance
amplifier (TIA).
This work supported by NRF and ETRI.
978-1-4577-1729-1/12/$26.00 ©2012 IEEE.
CONVENTIONAL BIOCHEMICAL SENSOR DESIGN
176
Figure 2. Proposed biochemical interface circuit
Since the proposed amperometric potentiostat requires one
op-amp without any passive resistors, it does not suffer from
op-amp and resistor mismatches, cumulated offsets and noises.
Moreover, the proposed amprometric potentiostat consumes
less power consumption, requires less hardware area and
hence provides low cost implementation.
(a)
IV.
DESIGN & SIMULATION RESULTS
The chip was implemented and fabricated with a 0.35um
CMOS technology at a single supply voltage of 3.3V. Fig. 3
shows the transistor-level schematic diagram of DDA. For
high gain, a folded cascode structure was used. Two rail-torail input pairs (A and B) were exploited for wide input range
of DDA. And the class AB common-source output stage was
exploited to drive heavy loads. The TIA is also designed with
rail-to-rail folded cascode architecture.
(b)
Figure 1. Conventional amprometric potentiostat
(a) Single ended (b) fully differential
The input-terminal voltages of the DDA are indicated as
VPP, VPN for non-inverting input pairs and VNP, VNN for
inverting input pairs [9]. The basic property of the DDA can
be expressed as equation (1) if the open loop gain of DDA is
sufficiently large.
VPP − VPN = VNP −VNN
(1)
Figure 3. Schematic of the DDA
Since Vcell in Fig. 2 is equal to VS, the sensor output
current Iout is controlled by the sensor resistance RFW with
the relation of equation (2)
Iout =
Vcell (= Vsrc )
RFW
(2)
Output current signal of sensor is converted into voltage
signal by the trans-impedance amplifier (TIA) for further
voltage signal processing
Fig. 4 shows the DC sweep simulation results of the
DDA’s operation range. It works in the full signal range from
0V to 3.3V. Fig. 5 shows the simulation result of transient
simulation when the input source voltage of VS is the 10 KHz
sinusoidal signal of 200 mVPP. The dummy sensor load cell,
RS of 10Ω, and the faradic sensor resistance, RFW of 10MΩ,
were used to characterize the chip. The peak-to-peak sensor
output current (Iout) of 20nA (=200mVPP/10MΩ) is exactly
generated in accordance with the sensor resistance of RFW.
And this Iout is converted to the output voltage of Vout
through the TIA.
177
Figure 4. Simulation results of DC sweep
Figure 6. Layout of proposed circuits
V.
Figure 5. Transient simulation results
Fig. 6 shows the layout of the implemented biochemical
sensor chip. The core area is only 300um×445um without
pads. Table 1 shows the performance summary of the
proposed interface circuits. The DDA has the simulated DC
gain of 135 dB, the CMRR of 175 dB and the phase margin of
65°. The TIA shows the simulated DC gain of 155 dB. Total
power consumption including bias circuits, buffer, DDA and
TIA is 532 uW at the supply voltage of 3.3V
TABLE I.
MEASUREMENT
Fig. 7 shows the cell (solution) kit and biochemical sensor
chip for evaluation. In order to characterize the performance
of the proposed biochemical sensor interface chip,
electrochemical cell with three-electrode (the auxiliary
electrode: AE, the reference electrode: RE, and the working
electrode: WE) was connected to this implemented sensor
interface chip. The triangular waveform as shown in Fig. 8 (a)
is applied to the cell solution of 1 Mol ammonium sulfate
[(NH4)2SO4] through the graphite electrodes to confirm cyclic
voltametry operation. The data was captured at the output of
TIA using a Tektronix MSO4104 mixed signal oscilloscope.
Fig. 8 (b) clearly shows the measured result of oxidation peak
and reduction peak in cyclic voltametry.
PERFORMANCE SUMMARY
Technology
M/H 0.35um CMOS
Operation Voltage
3.3V
Power Consumption
532uW
Core size
300um x 445um(without pads)
DDA gain
135dB
DDA Phase Margin
65˚
DDA CMRR
170dB
TIA gain
125dB
Figure 7. Test set up for evaluation
178
[9]
(a)
(b)
Figure 8. Cyclic voltametry (measured)
(a) Input signal and (b) Output signal.
VI.
CONCLUSION
A simple on-chip interface circuit for biochemical sensor
is proposed and verified. Only one differential difference
amplifier (DDA) maintains a constant potential between the
reference electrode (RE) and the working electrode (WE).
And it also provides current flow through the auxiliary
electrode (AE). One DDA with rail-to-rail design not only
enables the full supply voltage range of operation but also
provides simple integrated bio-electronic system with small
hardware and low power consumption.
ACKNOWLEDGMENT
This work was supported by ETRI SW-SoC R&BD
Center, Human Resource Development project. The CAD tool
and chip implementation were supported by IC Design
Education Center (IDEC).
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