ADA4505-1_16 Datasheet, PDF (17/24 Pages) Analog Devices – 10 μA, Rail-to-Rail I/O, Zero Input Crossover Distortion Amplifiers

FOUR-POLE, LOW-PASS BUTTERWORTH FILTER
FOR GLUCOSE MONITOR
There are several methods of glucose monitoring: spectroscopic
absorption of infrared light in the 2 μm to 2.5 μm range, reflec-
tance spectrophotometry, and the amperometric type using
electrochemical strips with glucose oxidase enzymes. The
amperometric type generally uses three electrodes: a reference
electrode, a control electrode, and a working electrode. Although
this is a very old and widely used technique, signal-to-noise
ratio and repeatability can be improved using the ADA4505-x
family, with its low peak-to-peak voltage noise of 2.95 μV from
0.1 Hz to 10 Hz and voltage noise density of 65 nV/√Hz at 1 kHz.
ADA4505-1/ADA4505-2/ADA4505-4
Another consideration is operation from a 3.3 V battery. Glucose
signal currents are usually less than 3 μA full scale; therefore,
the I-to-V converter requires low input bias current. The
ADA4505-x family is an excellent choice because it provides
0.5 pA typical and 2 pA maximum input bias current at ambient
temperature.
A low-pass filter with a cutoff frequency of 80 Hz to 100 Hz is
desirable in a glucose meter device to remove extraneous noise;
this can be a simple two-pole or four-pole Butterworth filter.
Low power op amps with bandwidths of 50 kHz to 500 kHz
should be adequate. The ADA4505-x family, with its 50 kHz GBP
and 7 μA typical current consumption, meets these requirements.
A circuit design of a four-pole Butterworth filter (preceded by a
one-pole low-pass filter) is shown in Figure 58. With a 3.3 V
battery, the total power consumption of this design is 198 μW
typical at ambient temperature.
CONTROL
WORKING
REFERENCE
C1
1000pF
R1
5MΩ
+3.3V
3
8
V+
R2
R3
22.6kΩ 22.6kΩ
1
V–
C3
2
4
U1
0.047µF
1/2
ADA4505-2
C2
0.1µF
+3.3V
U1
1/2
ADA4505-2
5
8
V+
V–
6
4
R4
R5
22.6kΩ 22.6kΩ
7
C5
0.047µF
C4
0.1µF
+3.3V
U2
1/2
ADA4505-2
3
8
V+
1
V–
2
4
V_OUT
DUPLICATE OF CIRCUIT ABOVE
Figure 58. Four-Pole Butterworth Filter That Can Be Used in a Glucose Meter
Rev. D | Page 17 of 24