MC33219A Datasheet, PDF (23/28 Pages) Motorola, Inc – Voice Switched Speakerphone

Freescale SMeCm33i2c1o9Anductor, Inc.
of speech” to the attenuator control block. Grounding CPT
does the same for the transmit path.
Additionally, the receive background noise monitor is
automatically disabled by the dial tone detector whenever the
receive signal exceeds the detector’s threshold.
Dial Tone Detector Threshold
The threshold for the dial tone detector is internally set at
≈ 20 mV (14 mVrms) below VB (see Figure 29). That
threshold can be adjusted if desired by changing the bias at
RAI. The method used depends on how the input of the
receive attenuator is connected to other circuitry.
a) If the attenuator input (RAI) is DC coupled to the receive
amplifier (Pins 15 to 16 as in Figure 2), or to some other
amplifier in the system, then the threshold is changed by
forcing a small offset on that amplifier. As shown in Figure 32,
connect a resistor (RTO) from the summing node to either
ground or VCC, depending on whether the dial tone detector
threshold is to be increased or decreased. RF and RI are the
resistors normally used to set the gain of that amplifier.
Figure 33. Adjusting Dial Tone Detector
Threshold (AC Coupled)
VCC
56 k
Audio
Signal
Input
Attenuator
RTO
3.0 k
VB
RAI
100 k
VB
DTD
VB
20 mV
To
Control
Circuit
To Increase The Threshold
Figure 32. Adjusting Dial Tone Detector
Threshold (DC Coupled)
VCC or
RTO
GND
VB
3.0 k
Audio
Signal
Input
Attenuator
Audio
Signal
Input
RI
RF
RXI
RXO
RAI
VB
100 k
RTO
56 k
RAI
100 k
VB
DTD
VB
20 mV
To
Control
Circuit
VB Attenuator
To Decrease The Threshold
VB
20 mV
To
Attentuator
Control
Circuit
Adding RTO and connecting it to ground will shift RXO and
RAI upward, thereby increasing the dial tone detector
threshold. In this case, RTO is calculated from:
To increase the threshold, use the first circuit in Figure 33.
The voltage at the top of the 3.0 k resistor is between 90 and
180 mV above VB (depending on VCC). RTO and the 100 k
input impedance form a voltage divider to create the desired
+ ƪ ƫ ^{offset at RAI. RTO is calculated from:}
RTO
((VCC – VB)
DV
0.05)
– 1 (100 k)
+ RTO
VB RF
DV
VB is the voltage at Pin 6, and ∆V is the amount that the
detector’s threshold is increased. For example, if VB = 2.2 V,
and RF = 10 k, and the threshold is to be increased by 20 mV,
RTO calculates to 1.1 MΩ.
Connecting RTO to VCC will shift RXO downward, thereby
decreasing the dial tone detector threshold. In this case, RTO
is calculated from:
+ RTO
(VCC – VB)
DV
RF
For example, if VCC = 5.0 V, VB = 2.2 V, and RF = 10 k and
the threshold is to be decreased by 10 mV, RTO calculates to
2.8 MΩ.
b) If the receive attenuator input is AC coupled to the
receive amplifier or to other circuitry, then the offset is set at
RAI. The circuits in Figure 33 are suggested for changing
the threshold.
For example, if VCC = 5.0 V, and the threshold is to be
increased by 20 mV (∆V), RTO calculates to ≈ 600 kΩ.
If the threshold is to be decreased, use the second circuit
+ ƪ ƫ ^{in Figure 33. RTO is calculated from:}
RTO
(VB 0.05)
DV
–1
(100 k)
RFI Interference
Potential radio frequency interference (RFI) problems
should be addressed early in the electrical and mechanical
design of the speakerphone. RFI may enter the circuit
through Tip and Ring, through the microphone wiring to the
microphone amplifier (which should be short), or through any
of the PC board traces. The most sensitive pins on the
MC33219A are the inputs to the level detectors (RLI, TLI,
XDI) since, when there is no speech present, the inputs are
high impedance and these op amps are in a near open–loop
condition. The board traces to these pins should be kept
MOTOROLA ANALOG IC DEVICE^FD^oA^rT^MA ^{orGeoIntfoo:rwmwatwio.fnreOenscTahlies.cPoromduct,}
23