ADA4505-2

Rev. 0 | Page 17 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 ADA4505-2

with its low peak-to-peak voltage noise of 2.95 μV p-p from

0.1 Hz to 10 Hz and voltage noise density of 55 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 ADA4505-2 is an

excellent choice because it provides 0.5 pA typical and 2 pA

maximum of input bias current at ambient temperature.

A low-pass filter with a cutoff frequency of 80 Hz to100 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 ADA4505-2 with its 50 kHz GBP and 10 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 53. 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 53. A Four-Pole Butterworth Filter That Can Be Used in a Glucose Meter