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.

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

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