Datasheet

www.ti.com
f
S
+ Sampling frequency [Hz]
f
C
+ Center frequency [Hz]
a
1
+ *
ǒ
1 ) a
2
Ǔ
cos
ǒ
2pf
C
f
S
Ǔ
a
2
+
1 * tan
ǒ
2pf
b
ńf
S
2
Ǔ
1 ) tanǒ
2pf
b
ńf
S
2
Ǔ
f
b
+ Bandwidth [Hz]
PCM1870
SLAS544A – MAY 2007 – REVISED SEPTEMBER 2007
Calculating Filter Coefficients
The PCM1870 provides a dual-stage notch filter at the digital output of the ADC. The filter characteristics of each
filter stage can be programmed. The characteristics are determined by calculating coefficients for three
parameters, sampling frequency, center frequency and bandwidth, as shown in the following equations. All
coefficients must be written as 2s-complement binary data into registers 97, 98, 99, 100, 101, 102, 103, and 104.
Register 124
B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
Register 124 0 IDX6 IDX5 IDX4 IDX3 IDX2 IDX1 IDX0 RSV RSV RSV RSV RSV RSV G12R G12L
IDX[6:0]: 111 1100b (7Ch) Register 124
G12R: 12-dB Boost for PG2 (Gain Amplifier for AIN1R and AIN2R)
G12L: 12-dB Boost for PG1 (Gain Amplifier for AIN1L and AIN2L)
Default value: 0
These bits are used to boost the microphone signal when the analog input is small. See Register 90 for the
detailed settings.
42 Submit Documentation Feedback Copyright © 2007, Texas Instruments Incorporated
Product Folder Link(s): PCM1870