AD8506/AD8508

Rev. A | 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 very old technique and widely used, signal-to-noise

ratio and repeatability can be improved using the AD8506 with

its low peak-to-peak voltage noise of 2.8 μV p-p 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, so the I-to-V

converter requires low input bias current. The AD8506 is an

excellent choice because it provides 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- or four-pole Butterworth. Low power

op amps with bandwidths of 50 kHz to 500 kHz should be

adequate. The AD8506 with its 95 kHz GBP and 15 μA typical

of 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 46. With a 3.3 V battery, the

total power consumption of this design is 297 μW typical at

ambient temperature.

8

4

2

1

3

+3.3V

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 46. A Four-Pole Butterworth Filter That Can Be Used in a Glucose Meter