serial PCM word, clocked by BCLK , out of D . If the FS
X
VPO+
X
X
pulse is high for more than eight BCLK periods, the D and
Voltage Power Output (Non–Inverted)
(MC145554/67 Only)
X
X
TS outputs will remain in a low–impedance state until FS
is brought low. The length of the FS pulse is used to deter-
X
X
X
This non–inverted output of the receive push–pull power
amplifier pair can drive 300 Ω to 3.3 V peak.
mine whether the transmit and receive digital I/O conforms to
the Short Frame Sync or to the Long Frame Sync conven-
tion.
POWER SUPPLY
GNDA
Analog Ground
TS
X
Transmit Time Slot Indicator
This terminal is the reference level for all signals, both ana-
log and digital. It is 0 V.
This is an open–drain output that goes low whenever the
D
output is in a low–impedance state (i.e., during the trans-
X
V
mit time slot when the PCM word is being output) for en-
abling a PCM bus driver.
CC
Positive Power Supply
V
is typically 5 V.
CC
ANLB
V
BB
Negative Power Supply
Analog Loopback Control Input (MC145564/67 Only)
When held high, this pin causes the input of the transmit
V
is typically – 5 V.
BB
RC active filter to be disconnected from GS and connected
X
to VPO+ for analog loopback testing. This pin is held low in
normal operation.
FUNCTIONAL DESCRIPTION
ANALOG INTERFACE AND SIGNAL PATH
The transmit portion of these codec–filters includes a low–
noise gain setting amplifier capable of driving a 600 Ω load.
Its output is fed to a three–pole anti–aliasing pre–filter. This
pre–filter incorporates a two–pole Butterworth active low–
pass filter, and a single passive pole. This pre–filter is fol-
lowed by a single ended–to–differential converter that is
clocked at 256 kHz. All subsequent analog processing uti-
lizes fully differential circuitry. The next section is a fully–dif-
ferential, five–pole switched capacitor low–pass filter with a
3.4 kHz passband. After this filter is a 3–pole switched–ca-
pacitor high–pass filter having a cutoff frequency of about
200 Hz. This high–pass stage has a transmission zero at dc
that eliminates any dc coming from the analog input or from
accumulated operational amplifier offsets in the preceding fil-
ter stages. The last stage of the high–pass filter is an auto-
zeroed sample and hold amplifier.
ANALOG
GS
X
Gain–Setting Transmit
This output of the transmit gain–adjust operational amplifi-
er is internally connected to the encoder section of the
device. It must be used in conjunction with VF I– and VF I+
to set the transmit gain for a maximum signal amplitude of
2.5 V peak. This output can drive a 600 Ω load to 2.5 V peak.
X
X
VF I–
X
Voice–Frequency Transmit Input (Inverting)
This is the inverting input of the transmit gain–adjust
operational amplifier.
One bandgap voltage reference generator and digital–to–
analog converter (DAC) are shared by the transmit and
receive sections. The autozeroed, switched–capacitor band-
gap reference generates precise positive and negative refer-
ence voltages that are independent of temperature and
power supply voltage. A binary–weighted capacitor array
(CDAC) forms the chords of the companding structure, while
a resistor string (RDAC) implements the linear steps within
each chord. The encode process uses the DAC, the voltage
reference, and a frame–by–frame autozeroed comparator to
implement a successive–approximation conversion algo-
rithm. All of the analog circuitry involved in the data con-
version — the voltage reference, RDAC, CDAC, and
comparator — are implemented with a differential architec-
ture.
VF I+
X
Voice–Frequency Transmit Input
(Non–Inverting)
This is the non–inverting input of the transmit gain–adjust
operational amplifier.
VF O
R
Voice–Frequency Receive Output
This receive analog output is capable of driving a 600 Ω
load to 2.5 V peak.
VPI
Voltage Power Input (MC145564/67 Only)
The receive section includes the DAC described above, a
sample and hold amplifier, a five–pole 3400 Hz switched
capacitor low–pass filter with sinX/X correction, and a two–
pole active smoothing filter to reduce the spectral com-
ponents of the switched capacitor filter. The output of the
smoothing filter is a power amplifier that is capable of driving
a 600 Ω load. The MC145564 and MC145567 add a pair of
power amplifiers that are connected in a push–pull configu-
ration; two external resistors set the gain of both of the
This is the inverting input to the first receive power ampli-
fier. Both of the receive power amplifiers can be powered
down by connecting this input to V
.
BB
VPO–
Voltage Power Output (Inverted) (MC145564/67 Only)
This inverted output of the receive push–pull power ampli-
fiers can drive 300 Ω to 3.3 V peak.
MC145554•MC145557•MC145564•MC145567
MOTOROLA
4
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