SAA7706H Datasheet, PDF (13/52 Pages) NXP Semiconductors – Car radio Digital Signal Processor (DSP)

Philips Semiconductors
Car radio Digital Signal Processor (DSP)
Product specification
SAA7706H
8.1.1
THE REALIZATION OF COMMON MODE INPUT WITH
AIC
A high common mode rejection ratio can be created by the
use of the ground return pin. Pin CD_(L)_GND can be
used in the case that the left and right channel have one
ground return line. CD_(L)_GND and CD_R_GND can be
used for separated left and right ground return lines. The
ground return lines can be connected via the switch
GNDC1/2 and GNDRC1/2 (see Fig.4) to the plus input of
the second operational amplifier in the signal path. The
signal of which a high common mode rejection ratio is
required has a signal and a common signal as input. The
common signal is connected to the CD_(L)_GND and/or
CD_R_GND input. The actual input can be selected with
audio input control AIC1/2(1:0).
In Fig.4 the CD input is selected. In this situation both
signal lines going to S1/2 in front of the ADC will contain
the common mode signal. The ADC itself will suppress this
common mode signal with a high rejection ratio. The inputs
CD_L and CD_R in this example are connected via an
external resistor tap of 82 kΩ and 100 kΩ to be able to
handle larger input signals. The 100 kΩ resistors are
needed to provide a DC biasing of the operational
amplifiers OA1 and OA2. The 1 MΩ resistor provides
DC biasing of OA3 and OA4. If no external resistor tap is
needed the resistors of 100 kΩ and 1 MΩ still have to
provide DC biasing. Only the 82 kΩ resistor can be
removed. The impedance level in combination with
parasitic capacitance at input CD_L or CD_R determines
for a great deal the achievable common rejection ratio.
handbook, full pagewidth
LEFT
from
CD-player
analog
GROUND
LEFT
GROUND
RIGHT
RIGHT
82 kΩ
CD_L 72
100 kΩ
CD_(L)_GND 77
00
01
10 MUX
11
G1
G0
10 kΩ
10 kΩ
OA1
1 MΩ
VREFAD 78
1 MΩ
CD_R_GND 14
MIDREF
82 kΩ
100 kΩ
CD_R 70
00
01
10 MUX
11
10 kΩ
10 kΩ
OA2
off-chip
on-chip
10 kΩ
to MUX S1/2
OA3
AIC1/ 2 (1:0)
GNDC1/ 2
GNDRC1/ 2
10 kΩ
to MUX S1/2
OA4
MGT460
Fig.4 Example of the use of common mode analog input in combination with input resistor tap.
2001 Mar 05
13