ATA06212A
APPLICATION INFORMATION
POWER SUPPLIESAND GENERALLAYOUT
CONSIDERATIONS
combination may cause clipping of subsequent
gain or decision circuits.
TheATA06212AD1C may be operated from a positive
supply as low as + 4.5 V and as high as + 6.0 V.
Below + 4.5 V, bandwidth, overload and sensitivity
will degrade, while at + 6.0 V, bandwidth, overload
and sensitivity improve (see “Bandwidth vs.
Temperature” curves). Use of surface mount
(preferably MIM type capacitors), low inductance
power supply bypass capacitors (>=56pF) are
essential for good high frequency and low noise
performance. The power supply bypass capacitors
should be mounted on or connected to a good low
inductance ground plane.
SENSITIVITY AND BANDWIDTH
In order to guarantee sensitivity, the TIA is
subjected to a comprehensive series of tests at
the die sort level (100% testing at 25 oC) to verify
the DC and AC parametric performance
(transimpedance and bandwidth).
INDIRECT MEASUREMENT OF OPTICAL
OVERLOAD
Optical overload can be defined as the maximum
optical power above which the BER (bit error rate)
increases beyond 1 error in 1010 bits. The
ATA06212AD1C is 100% tested at die sort by an
AC measurement which has excellent correlation
with a PRBS optical overload measurement.
GENERALLAYOUTCONSIDERATIONS
Since the gain stages of the transimpedance
amplifier have an open loop bandwidth in excess of
1.0 GHz, it is essential to maintain good high
frequency layout practices. To prevent oscillations, a
low inductance RF ground plane should be made
available for power supply bypassing. Traces that
can be made short should be made short, and the
utmost care should be taken to maintain very low
capacitance at the photodiode-TIA interface (IIN);
excess capacitance at this node will cause a
degradation in bandwidth and sensitivity.
INPUT REFERRED NOISE CURRENT
The “Input Noise Current” is directly related to
sensitivity . It can be defined as the output noise
voltage (VOUT) with no input signal, (including a
400 MHz lowpass filter at the output of the TIA)
divided by the AC transresistance.
IIN CONNECTION
AGC CAPACITOR
Bonding the detector cathode to IIN (and thus drawing
current from the ATA06212D1C) improves the
dynamic range. Although the detector may be used
in the reverse direction for input currents not
exceeding 25 µA, the specifications for optical
overload will not be met (refer to the equivalent circuit
diagram).
It is important to select an externalAGC capacitor
of high quality and appropriate size. The
ATA06212AD1C has an on-chip 70 kΩ resistor
with a shunt 4.5 pF capacitor to ground. Without
external capacitance the chip will provide anAGC
time constant of 315 ns. For the best performance
in a typical 622Mb/s SONET receiver, a minimum
AGC capacitor of 56pF is recommended. This will
provide the minimum amount of protection against
pattern sensitivity and pulse width distortion on
repetitive data sequences during high average
optical power conditions. TheAGC function can
be disabled by bonding CAGC to ground.
Conservative design practices should be followed
when selecting an AGC capacitor, since unit to
unit variability of the internal time constant and
various data conditions can lead to data errors if
the chosen value is too small.
VOUTCONNECTION
The output pad should be connected via a coupling
capacitor to the next stage of the receiver channel
(filter or decision circuits), as the output buffers
are not designed to drive a DC coupled 50 Ohm
load (this would require an output bias current of
approximately 36 mA to maintain a quiescent 1.8
Volts across the output load). If VOUT is connected
to a high input impedance decision circuit (>500
Ohms), then a coupling capacitor may not be
required, although caution should be exercised
since DC offsets of the photo detector/TIA
PRELIMINARY DATA SHEET - Rev 1.2
08/2001
6
ANADIGICS [ ANADIGICS, INC ]
