Passive Component Selection; Input Coupling Capacitor Selection (C22 To C25) - Analog Devices SSM2380 User Manual

UG-248

PASSIVE COMPONENT SELECTION

Although the evaluation board is preloaded with the passive
components required for a basic configuration, the user may
wish to evaluate the same circuit with different component
values or filter designs. Selecting the proper components is the
key to achieving the performance required at the budgeted cost.
INPUT COUPLING CAPACITOR SELECTION (C22
TO C25)
The input coupling capacitors, C22 to C25, should be large
enough to couple the low frequency signal components in the
incoming signal and small enough to filter out unnecessary
low frequency signals. For music signals, the cutoff frequency
chosen is often between 20 Hz and 30 Hz to preserve the low
frequency components of the signal; for applications with small
speakers, a higher cutoff frequency is often chosen to reduce
the power wasted on audio that cannot be reproduced by the
speaker.
The value of each input capacitor is calculated by
C = 1/(2R f )
IN C
where:
R is the amplifier's input resistance.
IN
f
is the cutoff frequency.
C
Because R varies with amplifier gain value from 7.3 kΩ
IN
to 55 kΩ over the entire gain range of the SSM2380, this
calculation must be performed carefully to ensure that the
low frequency performance is acceptable at all gain levels.
The relationship between gain and input resistance is shown
in Table 3.
As an example calculation, suppose that the low frequency
cutoff is to be no higher than 200 Hz and that the amplifier
gain varies between 6 dB and 18 dB. In this case, the input
resistance is between 39.6 kΩ and 13.6 kΩ. Because the cutoff
frequency is highest when the input resistance is small, the
calculation should be done using the minimum value—in
this case, giving a minimum capacitance of approximately
59 nF. A larger standard value (perhaps 100 nF) should be
used to account for the ordinary variation due to tolerance,
temperature coefficient, and so on.
Table 3. Input Impedance for I
LGAIN[5:0], RGAIN[5:0]
101110
101101
101100
101011
101010
101001
101000
100111
100110
100101
100100
100011
100010
100001
100000
011111
011110
011101
011100
011011
011010
011001
011000
010111
010110
010101
010100
010011
010010
010001
010000
001111
001110
001101
001100
001011
001010
001001
001000
000111
000110
000101
000100
Rev. 0 | Page 12 of 20
Evaluation Board User Guide
2
C Control Mode
Gain (dB)
24.0
23.5
23.0
22.5
22.0
21.5
21.0
20.5
20.0
19.5
19.0
18.5
18.0
17.5
17.0
16.5
16.0
15.5
15.0
14.5
14.0
13.5
13.0
12.5
12.0
11.5
11.0
10.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
R (kΩ)
IN
7.3
7.7
8.1
8.5
9.0
9.5
10.0
10.5
11.1
11.7
12.3
12.9
13.6
14.3
15.0
15.8
16.6
17.4
18.3
19.2
20.1
21.1
22.1
23.1
24.2
25.3
26.4
27.6
28.8
30.0
31.3
32.6
34.0
35.3
36.7
38.1
39.6
41.1
42.6
44.1
45.6
47.1
48.7