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.

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