Datasheet

9Maxim Integrated
Digitally-Programmed, Dual
2nd-Order Continuous Lowpass Filter
MAX270/MAX271
Figure 1c. MAX271 Block Diagram—Pin-Strap Mode
Filter Programming
Cutoff Frequency
f
C
is the frequency of 3dB attenuation in the filter
response.
Table 1 shows how data pins D0–D6 allow programming
of 128 cutoff frequencies from 1kHz to 25kHz.
The equations for calculating f
C
from the programmed
code are as follows:
−
−
−
−
=
=
=
=
C
C
C
C
87.5
f x1kHz forcodes 0 63
87.5 CODE
)
(f 1kHz to3.57kHz
262.5
f x1kHz for codes 64 127
137.5 CODE
(f kHz to kHz)
3.57 25
where CODE is the data on pins D0–D6 (0–127). D6 is
the most significant bit (MSB).
Actual cutoff frequencies are subject to some error for
each programmed code. Highest accuracy occurs at
CODE = 0 where filters are trimmed for a 1kHz cut-
off frequency. At higher codes, CODE vs. f
C
errors
increase; the frequency error at CODE = 127 {highest
code) remains typically within Q9.5%. This means that
the actual filter cutoff frequency, when programmed to
CODE = 127, falls between 22.63kHz and 27.38kHz.
DIRECT
CONTROL
D0-D6
17-23
5INA
OUTA
T/H OUT
OUTB
2
1516
A1*
6
V+ V-
10
GND
EN
EN
3
1
9
7
INB
14 12
4
CS* WR*
SHDN*
11
T/H*
A/B
24
TO GND OR V-
T/H
A0*
*PIN HAS NO FUNCTION IN PIN-STRAP MODE.
EN
13
T/H*
EN
PIN-STRAP
MODE
8
MAX270
MODE