3.3/5V Limiting Amplifier for Aplications to 2.5 Gbps
Preliminary - Rev V5P
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 1.5 nF for 2.125 Gbps Fibre Channel.
Multirate applications down to 155 Mbps
In this case, the input coupling capacitor needs to be large enough to pass 15 kHz (155x106/10,000) which results
in a capacitor value of 0.2 μF. 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 1 μF which provides
an input cutoff frequency of 3.1 kHz. This separates the two poles sufficiently to allow them to be considered inde-
pendent. This capacitor should also have a 10 nF capacitor in parallel to pass the higher frequency data (in the
multirate application) without distortion.
In all cases, a high quality coupling capacitor should be used as to pass the high frequency content of the input
Using Rate Selection
Because of the performance of PECL outputs, the M02050-15 should not be used at data rates above 2.5 Gbps.
When the RATESEL pin (shown in Figure 3-10) is driven high, the M02050-15 bandwidth is set to its maximum
which allows the M02050-15 to operate at data rates up to 2.5 Gbps. Because of the nature of the ESD structure
on this pin, if it is driven by a device with IOL or IOH > 2 mA then a 1 kΩ to 10 kΩ resistor should be used in series
with the RATESEL pin. If rate selection is not used and the part is configured for high bandwidth only, the RATESEL
pin should be connected to VCC using a 1 kΩ to 10 kΩ resistor. When operating at data rates ≤ 1.25 Gbps, then
RATESEL should be left floating (do not tie low). This enables low-rate mode which reduces the bandwidth (and
thus the noise level) of the part.
As shown in the typical applications circuit (Figure 4-1), when interfacing to a TIA that features a “MON” output
such as the M02013 or M02016, the M02050-15 can reference the current sunk into the TIA “MON” output and pro-
duce a proportional current at the M02050-15 RSSIAVG output. The current is sourced into resistor REXT to ground
creating a voltage suitable for DDMI applications. REXT should be chosen as:
REXT = 1/(maximum current into RSSIAVG)
This keeps the voltage at RSSIAVG between 0 and 1 V.
Setting the Signal Detect Level
Using Figure 4-3, the value for RST is chosen to set the LOS threshold at the desired value. The resulting hystere-
sis is also shown in Figure 4-3.
From Figure 4-3, it is apparent that small variations in RST cause significant variation in the LOS threshold level,
particularly for low input signal levels. This is because of the logarithmic relationship between the RSSI voltage and
the input signal level. It is recommended that a 1% resistor be used for RST and that allowance is provided for LOS
variation, particularly when the LOS threshold is near the sensitivity limit of the M02050-15.
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: