Optical DAA2000 Theory of Operation
Appnote 71
by Alan Maenchen
Introduction
The DAA circuit, known formally as the “Digital Access
Arrangement“ is the physical connection to the telephone line
,
known as the “local loop”The DAA performs four critical func-
.
tions:
1. Line termination
2. Isolation
3. Hybrid
4. Ring Detection
The following is a description of how the Infineon optical DAA
circuit fulfills these requirements, as well as providing other
useful features. The DAA2000 can fulfill a wide variety of tele-
phone interface applications including voice, FAX, and data. This
application note assumes the DAA is used as the interface to a
data modem.
General Architecture
The DAA block diagram is shown in Figure 1. This is a simplified
schematic showing how the circuit operates.
The circuit is broken into two parts; the Line side circuit and the
Modem side circuit. Telephone regulations require a high volt-
age barrier between these two sides which is provided by the
analog opto-isolators.
The Line side circuit derives its operating power from the tele-
phone line directly. A bridge rectifier allows operation from
either battery polarity. A fuse and a high voltage transient pro-
tection device are used to preserve telephone line integrity in
the event of a lightning, or other high energy pulse coming in
from the loop.
Figure 1. Simplified DAA Block Diagram
Loop current
Fuse
TIP
Current
sensor
The DL207 Line side chip includes a HYBRID circuit that sepa-
rates the TX and RX signals by doing a 2 wire to 4 wire conver-
sion. The success of this circuit (trans-hybrid balance) depends
on the line termination impedance shown in Figure 1 as a sim-
ple resistor. In fact, the termination impedance is a combination
of an external resistor and an external capacitor. The value of
these components depends on the telephone regulations of the
country concerned.
The RX audio signal is transmitted across the high voltage bar-
rier with an analog opto-isolator. This circuit is capable of very
good linearity and broad bandwidth by using a novel feedback
technique. In a similar circuit, the TX audio is passed across the
barrier with a second analog opto-isolator.
On-hook and off-hook functions are taken care of by turning on/
off the series switch. In the DAA circuit, this is a DMOS transis-
tor capable of withstanding at least 350 V. When in the “on-
hook” condition, the switch is open and the DAA draws no cur-
rent from the loop. The DAA can be commanded to go “off-
hook” by asserting the “hook command” input to the
DM207 This command is communicated to the DL207 via the
.
IL388DAA analog opto-isolator. An internal multiplexing opera-
tion takes place which combines this command, the Caller ID
command, and the actual TX audio signals. When the off-hook
command is given, the DL207 switches on the hook switch by
fully saturating the DMOS transistor.
The Caller-ID function is sometimes referred to as the “snoop”
mode. When this command is given, the DL207 turns on the
hook switch just a little. In this mode, the loop current is kept
IL388DAA
Line status
RING detector
RX audio
Transient
protection
RING
detector
RING
Hook switch
Hybrid
network
IL388DAA
Bias
supply
1.25 V
TX audio
HOOK command
CALLER ID
DL207
2001 Infineon Technologies Corp. • Optoelectronics Division • San Jose, CA
www.infineon.com/opto • 1-888-Infineon (1-888-463-4636)
6–96
DM207
March 07, 2001-12
low enough to keep the central office (CO) from seeing an off-
hook situation, yet it allows the DL207 to transmit the RX audio
signal over to the DM207
.
When in the “off-hook” mode, the DL207 passes the full loop
current through itself. Power for the chip is derived by an inter-
nal circuit which acts like a Zener diode. The voltage across the
chip from V
DD
to V
SS
is maintained at about 5 V. With the hook
switch fully “on” loop current is determined by the external loop
,
resistance. In some countries, the current must be limited to 60
mA maximum by the DAA. In those cases, an external current
limiting circuit must be added.
To prevent damage when the DAA is accidently connected to a
digital PBX line with unlimited current capability, the DAA has an
internal shut-down circuit. This circuit is sensitive to total power
dissipated by both hook switch and the DL207 and internal tem-
,
perature. When power from the line exceeds about 3 to 4 watts
at room temperatures, the DL207 turns off the hook switch. In
this situation the hook switch itself can generate significant
heat. Therefore the switch should be thermally connected to the
DL207 with ground plane techniques. This shutdown function is
linearly de-rated to 50 mA at +70
°
C.
A ring detector is built into the DL207 which can be configured
,
as either a “full wave” detector or a “half wave” detector. In the
full wave mode, a pulse is generated at the DM207 “ring
detect” output pin for each half cycle of the ring signal. In the
half wave mode, one pulse is generated for each full cycle of
the ring signal. See Figure 2.
Figure 2. Ring Detection
Figure 3. Matched Detectors
V
DD
Audio
input
+
–
IL388DAA
–
+
Audio
output
V
SS
Figure 3 shows the method of using matched detectors. In this
circuit, the audio input signal is dropped through the input resis-
tor to virtual ground. The op-amp output drives a transistor,
which draws current through the LED. The source side photo-
diode generates a current which goes back to the input source
and closes the loop. The closed loop linearizes the LED. The
secondary side photo-diode is an exact duplicate of the first and
is illuminated with the same light from the LED. In the output
op-amp, the photo-current generated is forced to flow through
the feedback resistor. That generates a voltage equal to the
original input signal.
The DAA circuit uses this same principle of matched detectors.
Figure 4 is the DAA version, which also includes the hybrid func-
tion. The complete signal details are not shown for simplicity.
The Hybrid Circuit
The hybrid circuit operates by modulating the loop current with
the TX audio signal. This can be seen in Figure 4. The TX audio
current is generated in the lower opto-isolator in the secondary
photo-diode. That current is impressed across a 600
Ω
and a
1200 ohm series pair of resistors. The voltage generated across
the 1200 ohm resistor modulates the current flowing through
R1. This external resistor is normally 16.5
Ω
, but the value can
change slightly as needed for other than 600 ohm termination
impedances. R1 also affects transmit gain and trans-hybrid bal-
ance.
The receive audio signal is picked up with the induced voltage
across the 3600 ohm resistor. This, of course, also has the TX
signal as well. The TX signal is cancelled by feeding an equal but
opposite amount of the original TX signal into the summing
amplifier at the top. The balance of this cancellation is depen-
dent on the matching accuracy of the resistors shown. All resis-
tors except for R1 are internal to the DL207 and they are
,
matched in ratio to a very high degree and will accurately track
over temperature.
The balance is also referred to as “trans-hybrid loss” Effectively,
.
this is the amount of RX signal which gets inadvertently fed into
the TX signal path. If the match is perfect, then the trans-hybrid
loss is infinite. Typical DAA2000 applications have a trans-hybrid
loss of around 20 to 30 dB, which is far better than traditional
transformer circuits.
Audio AGC Compensation
Optical isolators can have gain variations that change with tem-
perature and age. To compensate for these variations, the Infin-
eon DAA2000 has an internal AGC circuit, which adjusts the
audio gain based on measured DC gain characteristics
Ring Signal
Full wave
detector
Half wave
detector
Ring detector, (Not to scale)
Using Optical Isolators for Crossing the Barrier
For many years, the only practical solution to providing high
voltage isolation between the CO loop and the modem (or any
telephone device) has been the use of a transformer. In modern
equipment design, space, weight, and cost are all prime con-
cerns. The venerable transformer does not fit into today’s solu-
tions. Optical isolators can provide excellent high voltage
isolation in a very compact size. The IL388DAA devices have a
2500 V rating.
The use of linear opto-isolators is a well-established technique
in instrumentation designs. Infineon uses this same basic idea
in the DAA2000 by using standard, off-the-shelf linear opto-iso-
lators with their custom linear chips, the DL207 and DM207
.
The way this circuit achieves such good linearity is by using
matched detectors in the IL388DAA with one used in the feed-
back circuit, and the other used to cross the barrier.
2001 Infineon Technologies Corp. • Optoelectronics Division • San Jose, CA
www.infineon.com/opto • 1-888-Infineon (1-888-463-4636)
Appnote 71
6–97
March 07, 2001-12
Figure 4. Trans-hybrid Circuit and Optical Coupling
22
k
V
DD
+
–
3600
1200
+
–
1V
V
DD
35 uA
IL388DAA
68 nF
V
SS
3600
+
–
V
DD
IL388DAA
–
+
RX
Audio
ACREF
.7V
–
+
600
60
k
10–120 mA
Loop Current
TX
Audio
R1
V
SS
Figure 5. AGC Compensation
DM207 or DL207
IL388DAA
C1A
C1
C1B
Bandgap
reference
C2
V
SS
AGC
Gain of the converter is based on two external components
connected to pin 19 of the DL207 Given the reference values
.
of 20 M
Ω
and 68 nF the gain of this circuit is about 0.5 Hz/volt.
,
The resistor value must not be lower than 10 M
Ω
otherwise
the circuit will not meet FCC part 68 on-hook DC resistance
specifications. The capacitor may be varied between 10 and
100 nF with resulting change in pulse width and V/F gain.
Figure 6. Line Monitoring with V/F
TIP
V
DD
24
V/F 19
DL207
V
SS
23
RING
Hook
switch open
The AGC circuit works by adjusting the gain of a low distortion,
variable gain current amplifier so that the average photo-diode
output DC current is constant. This has the effect of normalizing
the AC gain at the output since the AC gain strictly tracks the
DC gain of the linear optical isolator. DC averaging is provided
by a low pass filter whose time constants are set by C1 and C2.
With the values recommended in the reference circuit, the time
constant is about 400 mS. Increasing the values of the capaci-
tors will improve low frequency distortion, but will increase set-
tling time. The ration of C2 to C1 should be kept at 4.5 to 1 to
provide optimum damped settling.
Note that there are similar AGC circuits in both the DL207 and
the DM207 In both chips, the AGC functions on the “received”
.
audio from the opto-isolator. In the DL207 this is actually the TX
,
signal to be transmitted into the loop. In the DM207 it is the RX
,
signal received from the loop.
On Hook Operation
In the “On-Hook” condition, the DAA appears as a very high
impedance and draws only a few microamperes of line current.
A V/F (voltage to frequency) circuit monitors the open line
voltage. This voltage measurement appears at the DM207 at
pin 23, LSTAT in the form of a series of pulses. Each pulse is
about 4 mS wide, and the frequency of the pulses is propor-
tional to the line voltage. The pulses are sent across the barrier
by pulsing the RX opto-isolator during the discharge time of the
RC circuit.
2001 Infineon Technologies Corp. • Optoelectronics Division • San Jose, CA
www.infineon.com/opto • 1-888-Infineon (1-888-463-4636)
This circuit is provided to allow detection of line status without
having to go off-hook. It can be used to detect when an exten-
sion phone goes off-hook, or if the modem is connected to a
dead line. Most modem problems are improper connection to
the line.
Due to the wide variety of other telephone equipment available,
it is not possible to reliably determine line condition by monitor-
ing line voltage alone. Generally, line voltages below 12 to 14 V
indicate the line is in use. Voltages above 18 to 20 V indicate the
line is available. Voltages in the 12 to 20 V range are ambiguous.
This is because telephone standards have no maximum off-
hook voltage drop recommendations for line currents over 26
mA. A common situation where a high line voltage is encoun-
tered while the line is in use, is when a user has inserted a
zener adapter in series with their answering telephone jack to
improve answering machine cutoff. These zener adapters will
typically increase the answering machine off hook voltage to
above 14 V.
Appnote 71
6–98
March 07, 2001-12
A solution to the voltage ambiguity problem is to design firm-
ware, which looks for a change in line voltage, or keeps a his-
tory of line voltage to determine normal idle voltage. Then a trip
point can then be set about 30% below that idle level.
If the V/F function is not needed, the components connected to
pin 19 of the DL207 can be left out. Note that the V/F does not
function when off-hook, or in SNOOP mode.
Snooping
Asserting either the SNP input (or the inverse SNPL input) on
the DM207 puts the DAA into SNOOP mode. This is used pri-
marily for Caller ID purposes. When in SNOOP mode, the DAA
pulls about 600 uA from the loop. Line voltage must be at least
3.5 V. This current is much less than what the CO would con-
sider going off-hook, which is usually at least 5 mA. SNOOPing
does not violate FCC part 68 rules as long as it happens during
ringing, or when another telephone device is off-hook on the
same line.
SNOOP mode is an option, which requires adding an external
RC network. This is typically a 220 k resistor in series with a 4.7
nF capacitor from pin 18 of the DL207 to the negative terminal
of the bridge rectifier. When active, the AC line impedance is
roughly 220 k
Ω
.
In a typical Caller ID application, the modem firmware will
assert one of the Snoop control lines following the first ring so
as to pass the Caller ID burst of DTMF tones to the data pump
for detection.
In a Voice/FAX/Data application, a Snoop control line is asserted
during the ringing and for perhaps 15 to 60 seconds after the
telephone line is answered by an external extension telephone
(when ringing stops). The modem then listens for FAX or Data
calling tone or DTMF sequence. If a tone is recognized, then the
modem firmware can answer the call.
This steers the FAX or Data call from the answering machine,
which initially answered the call, to the modem.
Figure 7. Snoop Mode
TIP
V
DD
24
Snoop 18
DL207
V
SS
23
RING
Hook
switch
slightly open
0.6 mA
4.7 nF
220 kΩ
momentarily to indicate remote party hang-up, or for about 1
second prior to the “Please hang up now!” message.
Off-Hook Operation
The DAA is commanded to go off-hook by asserting either the
OFFHK (pin 20), or the OFFHKL (pin 21) on the DM207 This is
.
transmitted to the DL207 via the TX analog opto-isolator.
The DL207 immediately switches on the hook switch circuit.
The DAA will have an initial off-hook voltage about 2.2 V less
than the steady state value for about 200 mS. This low voltage
turn-on was designed to satisfy telephone regulations in certain
countries. It also is useful in forcing another telephone device,
such as an answering machine on the same line, to release the
line.
The AGC capacitor C2 sets this turn-on delay. The value can be
increased to get a longer turn on delay, but the settling time of
the DAA will also increase. C1 must also increase to maintain a
constant C1/C2 ratio. See the AGC description above.
Figure 8. Hook Switch Control
TIP
4
DL207
2N6520
RING
TN2540
27 nF
56 kΩ
10 –120 mA
5
V
SS
23
V
DD
24
The hook switch is a high voltage, low resistance DMOS tran-
sistor. The gate voltage must be switched between the
extreme negative terminal of the diode bridge, and V
DD
of the
DL207 When off-hook, there may be quite a high voltage across
.
the DMOS transistor. The DL207 cannot control such high volt-
ages, so it utilizes an external high voltage PNP transistor to
switch the gate. Both the DMOS and the PNP devices should
be rated above 350 V.
The turn-on and turn-off transitions of the hook switch are con-
trolled by the RC network between the gate and source of the
FET. The particular values of 27 nF and 56 k
Ω
is a compromise
,
which can pass most telephone regulations for “rotary” pulse
dialing wave forms.
Line status monitoring in the off-hook mode is similar to
SNOOP mode described above. In addition, the LSTAT pin will
pulse high for 5 to 50 mS if the loop voltage drops significantly
indicating a parallel telephone device has gone off-hook. In
order for this local phone pickup detection to occur, the line
voltage needs to drop at least 1.0 V at a rate exceeding 30 V/s.
Some telephone devices which have high holding voltages or
whose holding circuits have very slow turn on rates (high induc-
tance) may not cause the line voltage to drop sufficiently or rap-
idly enough to be detected.
Audio Interface
On the DM207 there are separate audio TX and RX ports. The
,
TX port is a high impedance input (TXAMP pin11) which oper-
,
ates with respect to the ACREF pin 18. The RX port is a single
,
ended output, RXOUT on pin 19. Figure 9 shows a sketch of the
audio interface.
Appnote 71
6–99
March 07, 2001-12
In call logging applications, Snooping should be asserted if the
V/F circuit detects the line has gone from idle to in use. This
allows the modem to monitor and detect the DTMF dialing,
then pass the dialed number to the host computer for logging.
The V/F function will also allow the host computer to measure
call duration.
Note that the V/F line monitoring circuit described above oper-
ates differently when in SNOOP or OFF-HOOK modes. Nor-
mally, the LSTAT pin goes low and stays there.
If the line current drops to zero, LSTAT will go high for as long as
there is no loop current. This can occur if the line is dead (most
common problem). It can also happen if the CO drops the line
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www.infineon.com/opto • 1-888-Infineon (1-888-463-4636)
Figure 9. Audio Interface on the DM207
15 AUDOUT
1.25 V
1 uA
from
RX opto
to
TX opto
10 kΩ
16 AUDIN
19 RXOUT
18 ACREF
10 kΩ
Differential
TX
Audio input
(optional)
10 kΩ
Differential
RX
Audio output
(optional)
RX
Audio output
Figure 10. PCB Layout Example
DM207
Isolation
barrier
DAA-
067
DAA-
067
Isolation
barrier
C
DL207
Fuse
10 kΩ
11 TXAMP
R1 121 kΩ
TX
Audio input
RJ 11
C
PCB Edge
It is important that no conductive material be inside, or under
the barrier. If the PCB is a multi-layer design, insure that ground
or voltage planes do not pass under the barrier.
Noise is the other major layout related issue. A compact layout
similar to Figure 9 is also very good at keeping induced noise to
a minimum. Being an analog circuit, the Infineon DAA2000 will
not generate significant noise by itself. However, the circuit can
(and will) pick up noise generated from nearby digital circuits.
Keeping the line side circuits inside a small, tightly designed
isolation barrier as shown will minimize noise pickup. Using a
separate ground plane for the line side components is usually
not needed. In fact, ground planes can be highly detrimental if
ground loops develop. If there is to be a ground plane, then
connect it only to the DL207 V
SS
on the line side pin.
A common problem with noise pickup is when the DM207 is
interfaced to a modem data pump IC (or other applications). Be
sure that the ACREF pin 18, is well by-passed with at least a
,
0.1 uF high frequency capacitor. In some applications, a 10 uF
tantalum may be needed. Similar bypassing of the V
DD
pin is
also good engineering practice.
In some applications, digital noise can be injected into the
DM207 via the control pins (such as OFFHK or SNP). If this hap-
pens, try adding a 3.3 k
Ω
resistor in series with the line and a
small bypass capacitor at the DM207 input pin.
Another potential problem is the need to reject local RF pickup
from the telephone line itself. The local loop, being a very long
pair of wires, is an excellent receiving antenna. Local broadcast
and TV signals will inevitably find their way into the DAA from
the loop. Prudent designers will add the necessary compo-
nents to reject such interfering signals. Techniques for RFI
reduction include adding a ferrite bead to the loop wires directly
adjacent to the RJ11 connector, and adding ferrite beads to the
DAA circuit itself, in conjunction with high voltage by-pass
capacitors. Some international telephone specifications require
such RFI filtering.
Band-Gap
Reference
17 TXBIAS
25.5 kΩ
14 V
SS
Using external resistors, plus an extra internal op-amp, the RX
output can be made differential. If this extra op-amp is not
used, leave the input and output pins open (pins 15 and 16)
The ACREF pin 18, is the internal bias supply for the DM207
,
,
and is made available should the designer wish to tie this bias
with the data pump bias. The ACREF signal itself is a 1.25 V bias
supply with very high impedance. Do not draw more than
1.0 uA of current. If a different bias voltage is being used with
the data pump, then that external reference can be connected
directly to ACREF
.
The TX drive signal can be differential if desired by adding two
external resistors as shown above. It is assumed the source
impedance is less than 100
Ω
.
The TX gain can be adjusted by changing the value of R1. The
value shown gives unity gain from the input to the line side (in
dBm, relative to 600
Ω
). On a voltage basis, the TX gain can be
calculated by:
Gain=140/(R1 + 20)
Layout and Construction
The two most important aspects to consider when designing a
DAA are noise, and isolation.
Electrical isolation is the easier topic to understand and deal
with. Most telephone specifications require very high voltage
isolation between the loop circuit and the user circuit as a
safety issue. The Infineon DAA2000 design uses opto-isolators
to achieve an isolation barrier of up to 2500 V.
This isolation can only be realized if the circuit board design and
construction is also capable of withstanding this kind of high
voltage. The line side circuit should be physically separated
from the user (modem) side circuit with at least a 0.2 inch
(5 mm) wide gap. The two opto-isolators must be the only com-
ponents to straddle the gap. Ideally, the line connection (usually
an RJ-11 jack) is mounted on the edge of the PC board and sur-
rounded by the line side components, including the DL207 The
.
amount of circuitry will depend on the specific application, and
which country requirements are being designed. All other cir-
cuitry, including the DM207 must be located on the other side
of the gap. Figure 10 shows an example.
2001 Infineon Technologies Corp. • Optoelectronics Division • San Jose, CA
www.infineon.com/opto • 1-888-Infineon (1-888-463-4636)
R1 in k
Ω
Appnote 71
6–100
March 07, 2001-12
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