M02040-15 Datasheet, PDF (23/31 Pages) M/A-COM Technology Solutions, Inc. – 3.3 5V Limiting Amplifier for Applications to 2.125 Gbps

M02040-15
3.3/5V Limiting Amplifier for Applications to 2.125 Gbps
Rev V4
f_CUTOFF ≤ (1.25x10⁹ / 1x10³) = 1.25x10⁶
The -3 dB cutoff frequency of the low pass filter at the 50 Ω input is found as:
f_3dB = 1/ (2 * π * 50 Ω * C_AC)
so solving for C where f_3dB = f_CUTOFF
C_AC = 1/ (2 * π * 50 Ω * f_CUTOFF)
and in this case the minimum capacitor is 2.5 nF.
For 2.125 Gbps Fibre Channel, this results in a minimum capacitor of 1.5 nF.
EQ.1
4.1.3.1
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
data stream.
4.1.4
Using DC_Servo
When the DC_Servo pin (shown in Figure 3-10) is tied high, the M02040-15 disables the internal servo loop which
allows the device to pass signals with DC content resulting in instantaneous response to changes in the input data
stream (e.g. data amplitude and phase). However, there is no correction for limit amp offset when the servo loop is
disabled which results in degraded sensitivity and increased duty cycle distortion.
Because of the nature of the ESD structure on this pin, if it is driven by a device with I_OL or I_OH > 2 mA then a 1 kΩ
to 10 kΩ resistor should be used in series with the DC_Servo pin. If the internal servo loop is never going to be dis-
abled, DC_Servo should be should be connected to ground using a 1kΩ to 10kΩ resistor. If the internal servo loop
is going to be continuously disabled, the DC_Servo pin should be connected to V_CC using a 1 kΩ to 10 kΩ resistor.
4.1.5
Using RSSI_AVG
As shown in the typical applications circuit (front page), when interfacing to a TIA that features a “MON” output
such as the M02010 or M02016, the M02040-15 can reference the current sunk into the TIA “MON” output and pro-
duce a proportional current at the 2040 RSSI_AVG output. The current is sourced into resistor R_EXT to ground creat-
ing a voltage suitable for DDMI applications. R_EXT should be chosen as:
R_EXT = 1/(maximum current into RSSI_AVG)
EQ.2
This keeps the voltage at RSSI_AVG between 0 and 1 V.
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