ia3211 integration, ia3211 Datasheet - Page 3

• Manufacturer Part Number: ia3211
• Description: Optical Silicon Daa Modem/lineside Chipset
• Manufacturer: integration

The use of linear opto-isolators is a well-established technique
in instrumentation designs. Integration Associates uses this
same basic idea in the IA3211 and IA3212 chipset by using
standard, off-the-shelf linear opto-isolators with their custom
linear chips, the IA3211 and IA3212. The way this circuit
achieves such good linearity is by using matched detectors in
the dual opto-couplers with one used in the feedback circuit,
and the other used to cross the barrier.
This figure shows the method of using matched detectors. In
this circuit, the audio input signal is dropped through the input
resistor to virtual ground. The op-amp output drives a
transistor, which draws current through the LED. The source
side photodiode generates a current, which goes back to the
input source and closes the loop. The closed loop linearizes the
LED. The secondary side photodiode is an exact duplicate of
the first and is illuminated with the light from the LED. In the
output op-amp, the photocurrent 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
as shown in the hybrid and optical coupling schematics. 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 the hybrid and optical
coupling schematic. The TX audio current is generated in the
lower opto-isolator in the secondary photodiode. That current is
impressed across a 600Ω and a 1200Ω series pair of resistors.
The voltage generated across the 1200Ω resistor modulates
the current flowing through R1. This external resistor is
normally 16.5Ω, but the value can change slightly as needed
for other than 600Ω termination impedances. R1 also affects
transmit gain and trans-hybrid balance.
The receive audio signal is picked up with the induced voltage
across the 3600Ω resistor. This, of course, also has the TX
signal. 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
dependent on the matching accuracy of the resistors shown. All
resistors except for R1 are internal to the IA3212, and they are
Audio Input
+
-
Vdd
1
2
3
4
U3
linear opto
8
7
6
5
+
-
Audio Output
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 removed
from the TX signal path. If the match is perfect, then the trans-
hybrid loss is infinite. Typical IA3211 and IA3212 chipset
applications have a trans-hybrid loss of around 30 dB, which is
far better than traditional transformer circuits.
Audio AGC Compensation
Optical isolators can have gain variations that change with
temperature and age. To compensate for these variations, the
IA3211 and IA3212 chipset has an internal AGC circuit, which
adjusts the audio gain based on measured DC gain
characteristics
The AGC circuit works by adjusting the gain of a low distortion,
variable gain current amplifier so that the average photodiode
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 400ms. Increasing the
values of the capacitor will improve low frequency distortion,
but will increase settling time. The ratio of C2 to C1 should be
kept at 4.5 to 1 to provide optimum damped settling.
There are similar AGC circuits in both the IA3211 and IA3212
devices monitoring the received audio from the linear opto-
couplers, i.e., the transmit TX signal for the IA3212 and the
receive Rx signal for the IA3211.
Audio Interface
Transmit Audio Interface
The IA3211 provides a single ended transmit interface. When
used with a 120KΩ resistor in series, the IA3211 and IA3212
chipset presents an overall 0dB transmit path between the
audio source on the host side and the phone line. In addition,
the TXAMP input is biased internally to the ACREF voltage
opto
AGC
C2
Vss
IA3211 or IA3212
Band Gap
Reference
IA3211 & IA3212
C1A
C1B
C1
3