M02044CG-31 Datasheet, PDF (15/22 Pages) M/A-COM Technology Solutions, Inc. – 3.3/5V Limiting Amplifier for Applications from 100 Mbps to 622 Mbps

Applications Information
4.1.2
Connecting V_CC and V_CC3
For 5V operation, the V_CC pin is connected to an appropriate 5V ± 7.5% supply. No potential should be applied to
the V_CC3 pin. The only connection to V_CC3 should be R_ST as shown in Figure 3-5.
When V_CC = 5V all logic outputs and the data outputs are 5V compatible. For low power operation, V_CC and V_CC3
should be connected to an appropriate 3.3V ± 7.5% supply. In this case all I/Os are 3.3V compatible.
4.1.3
Choosing an Input AC-Coupling Capacitor
When AC-coupling the input the coupling capacitor should be of sufficient value to pass the lowest frequencies of
interest, bearing in mind the number of consecutive identical bits, and the input resistance of the part. For SONET
data, a good rule of thumb is to chose a coupling capacitor that has a cut-off frequency less than 1/10,000 of the
input data rate. For example, for 622 Mbps data, the coupling capacitor should be chosen as:
f_CUTOFF ≤ (622x10⁶ / 10x10³) = 62.2 kHz
The -3 dB cutoff frequency of the low pass filter at the input is found as (assuming that the TIA output is also 50Ω
single-ended):
f_3dB = 1/ (2 * π * 100Ω * C_AC)
so solving for C where f_3dB = f_CUTOFF
C_AC = 1/ (2 * π * 100Ω * f_CUTOFF)
EQ.1
and in this case the minimum capacitor is 25.6 nF.
For Ethernet or Fibre Channel, there are less consecutive bits in the data, and the recommended cut-off frequency
is 1/(1,000) of the input data rate. This results in a minimum capacitor of 16 nF (or greater) for 100 Mbps Ethernet.
In all cases, a high quality coupling capacitor should be used as to pass the high frequency content of the input
data stream. It is also important that the ROSA bandwidth is sufficiently low and high enough to also support the
required data rate, for it’s lower and upper cutoff bandwidth impact the receiver bandwidth as much as does the
limiting amp’s lower and upper cutoff bandwidth.
4.1.4
Setting the Signal Detect Level
Using Figure 4-2, the value for R_ST is chosen to set the LOS threshold at the desired value. The resulting
hysteresis is also shown in Figure 4-2.
From Figure 4-2, it is apparent that small variations in R_ST cause significant variation in the LOS threshold level,
particularly for low input signal levels. This is because of the logarithmic relationship between the internal level
detect voltage and the input signal level. It is recommended that a 1% resistor be used for R_ST and that allowance
is provided for LOS variation, particularly when the LOS threshold is near the sensitivity limit of the M02044.
Example R_ST resistor values are given in Table 4-1.
02044-DSH-002-A
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