Datasheet

AD706
REV. D
–8–
A 1 Hz, 4-Pole, Active Filter
Figure 25 shows the AD706 in an active filter application. An
important characteristic of the AD706 is that both the input bias
current, input offset current and their drift remain low over
most of the op amp’s rated temperature range. Therefore, for
most applications, there is no need to use the normal balancing
resistor. Adding the balancing resistor enhances performance at
high temperatures, as shown by Figure 26.
Table II. 1 Hz, 4-Pole, Low Pass Filter Recommended Component Values
Section 1 Section 2
Desired Low Frequency Frequency C1 C2 C3 C4
Pass Response (Hz) Q (Hz) Q (F) (F) (F) (F)
Bessel 1.43 0.522 1.60 0.806 0.116 0.107 0.160 0.0616
Butterworth 1.00 0.541 1.00 1.31 0.172 0.147 0.416 0.0609
0.1 dB Chebychev 0.648 0.619 0.948 2.18 0.304 0.198 0.733 0.0385
0.2 dB Chebychev 0.603 0.646 0.941 2.44 0.341 0.204 0.823 0.0347
0.5 dB Chebychev 0.540 0.705 0.932 2.94 0.416 0.209 1.00 0.0290
1.0 dB Chebychev 0.492 0.785 0.925 3.56 0.508 0.206 1.23 0.0242
NOTE
Specified Values are for a –3 dB point of 1.0 Hz. For other frequencies simply scale capacitors C1 through C4 directly, i.e.: for 3 Hz
Bessel response, C1 = 0.0387 mF, C2 = 0.0357 mF, C3 = 0.0533 mF, C4 = 0.0205 mF.
OUTPUT
*WITHOUT THE NETWORK,
PINS 1 & 2, AND 6 & 7 OF THE
AD706 ARE TIED TOGETHER.
CAPACITORS C1 & C2
ARE SOUTHERN ELECTRONICS
MPCC, POLYCARB 5%, 50 VOLT
+
–
C4
C3
0.1F
+V
S
OPTIONAL BALANCE
RESISTOR NETWORKS*
1/2
AD706
1/2
AD706
INPUT
C1
C2
+
–
R1
1M
0.1F
R2
1M
R3
1M
R4
1M
–V
S
R5
2M
C5
0.01F
3
2
4
1
5
6
7
8
R6
2M
C6
0.01F
Figure 25. A 1 Hz, 4-Pole Active Filter
TEMPERATURE – C
180
–40 0 +40
60
120
0
–60
–120
–180
OFFSET VOLTAGE OF FILTER CIRCUIT (RTI) – V
+80 +120
WITHOUT OPTIONAL
BALANCE RESISTOR, R3
WITH OPTIONAL BALANCE
RESISTOR, R3
Figure 26. V
OS
vs. Temperature Performance
of the 1 Hz Filter