Datasheet
AD8505/AD8506/AD8508
Rev. E | Page 16 of 20
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 well established and widely used technique, signal-to-noise
ratio and repeatability can be improved using the AD8505/
AD8506/AD8508 amplifiers with their low peak-to-peak voltage
noise of 2.8 µV from 0.1 Hz to 10 Hz and voltage noise density
of 45 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
AD8505/AD8506/AD8508 are excellent choices because these
amplifiers provide 1 pA typical and 10 pA maximum of 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 AD8505/AD8506/AD8508 amplifiers with their
95 kHz GBP and 15 µA typical current consumption meet these
requirements. A circuit design of a four-pole Butterworth filter
(preceded by a one-pole, low-pass filter) is shown in Figure 50.
With a 3.3 V battery, the total power consumption of this design
is 297 µW typical at ambient temperature.
06900-044
8
4
2
1
3
+3.3V
V
OUT
C4
0.1µF
C5
0.047µF
R5
22.6kΩ
R4
22.6kΩ
8
4
6
7
5
R2
22.6kΩ
+3.3V
C2
0.1µF
C3
0.047µF
R3
22.6kΩ
8
4
2
1
3
+3.3V
DUPLICATE OF CIRCUIT ABOVE
CONTROL
WORKING
REFERENCE
R1
5MΩ
C1
1000pF
U2
1/2
AD8506
U1
1/2
AD8506
U1
1/2
AD8506
V+
V–
V+
V–
V+
V–
Figure 50. A Four-Pole Butterworth Filter That Can Be Used in a Glucose Meter