8
Analog Interface
8 – 17
1
2
π
f
1
R
IN
1
2
π
f
2
C
2
1
2
π
f
2
R
1
VIN
NORM
and VIN
AUX
are biased at the internal voltage reference
(nominally 2.5V) of the ADSP-21msp58/59, which allows the analog
interface to operate from a single supply. The input signal should be
ac-coupled with an external capacitor (C
2
). The value of C
2
is determined
by the input resistance of the analog input (VIN
NORM
, VIN
AUX
), 200 k
Ω
,
and the desired cutoff frequency. The cutoff frequency should be less than
or equal to 30 Hz. The following equations should be used to determine
the values for R
1
, C
1
, and C
2
: R
1
should be less than or equal to 2.2 k
Ω
, C
2
should be greater than or equal to 0.027
µ
F, C
3
should be equal to C
2
.
C
2
=
R
IN
= input resistance of ADSP-21msp58/59 (200 k
Ω
)
f
1
=cutoff frequency
≤
30 Hz
R
1
=
R
1
≤
2.2 k
Ω
20 kHz < f
2
< 40 kHz *
C
1
=
C
3
= C
2
* If minimum (< 0.1 dB) rolloff at 4 kHz is desired, f
2
should be set to 40 kHz.