Datasheet

ADA4505-1/ADA4505-2/ADA4505-4
Rev. D | Page 17 of 24
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.
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.
07416-048
8
4
2
1
3
+3.3V
V
OUT
C4
0.1µF
C5
0.047µF
R5
22.6k
R4
22.6k
8
4
6
7
5
R2
22.6k
+3.3V
C2
0.1µF
C3
0.047µF
R3
22.6k
8
4
2
1
3
+3.3V
DUPLICATE OF CIRCUIT ABOVE
CONTROL
WORKING
REFERENCE
R1
5M
C1
1000pF
U2
1/2
ADA4505-2
U1
1/2
ADA4505-2
U1
1/2
ADA4505-2
V+
V–
V+
V–
V+
V–
Figure 58. Four-Pole Butterworth Filter That Can Be Used in a Glucose Meter