8Analog Interface
8 – 17
1
2π f
1
R
IN
1
2π f
2
C
2
1
2π 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 kΩ,
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 kΩ, C
2
should be greater than or equal to 0.027 µF, C
3
should be equal to C
2
.
C
2
=
R
IN
= input resistance of ADSP-21msp58/59 (200 kΩ)
f
1
=cutoff frequency ≤ 30 Hz
R
1
=
R
1
≤ 2.2 kΩ
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.
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