3.3/5V Limiting Amplifier for Applications from 100 Mbps to 1.25 Gbps
Connecting VCC and VCC3
For 5V operation, the VCC pin is connected to an appropriate 5V ± 7.5% supply. No potential should be applied to
the VCC3 pin. The only connection to VCC3 should be RST as shown in Figure 3-6.
When VCC = 5V all logic outputs and the data outputs are 5V compatible. For low power operation, VCC and VCC3
should be connected to an appropriate 3.3V ± 7.5% supply. In this case all I/Os are 3.3V compatible.
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 1.25 Gbps data, the coupling capacitor should be chosen as:
fCUTOFF ≤ (1.25x109 / 10x103) = 125 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
f3dB = 1/ (2 * π * 100Ω * CAC)
so solving for C where f3dB = fCUTOFF
CAC = 1/ (2 * π * 100Ω * fCUTOFF)
and in this case the minimum capacitor is 12.8 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 1.6 nF (or greater) for 1 Gbps Ethernet.
Multirate applications down to 100 Mbps
In this case, the input coupling capacitor needs to be large enough to pass 15 kHz (155x106/10,000, a condition
more stringent than 100 Mbps Ethernet) which results in a capacitor value of 100 nF. However, because this low
pass frequency is close to the 25 kHz low pass frequency of the internal DC servo loop, it is preferable to use a
larger input coupling capacitor such as 200 nF which provides an input cutoff frequency of 7.5 kHz. This separates
the two poles sufficiently to allow them to be considered independent.
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.
Setting the Signal Detect Level
Using Figure 4-2, the value for RST is chosen to set the LOS threshold at the desired value. The resulting
hysteresis is also shown in Figure 4-2.
M/A-COM Technology Solutions Inc. (MACOM) and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice.
Visit www.macom.com for additional data sheets and product information.
For further information and support please visit: