Rainbow Electronics MAX3798 - Manual (Page 16)

MAX3798
1.0625Gbps to 10.32Gbps, Integrated, Low-
Power SFP+ Limiting Amplifier and VCSEL Driver
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Dual Path Limiter
The limiting amplifier features a high-gain mode and a
high-bandwidth mode allowing for overall system opti-
mization. Either the MSEL pin or the MODE_SEL bit can
perform the mode selection. For operating up to
4.25Gbps, the high-gain mode (MODE_SEL = 0) is rec-
ommended. For operating above 8.5Gbps, the high-
bandwidth mode (MODE_SEL = 1) is recommended.
For operations at 8.5Gbps, the mode selection is
dependent on the performance of the receiver optical
subassembly. The polarity of ROUT+/ROUT- relative to
RIN+/RIN- is programmed by the RX_POL bit.
Offset Correction Circuitry
The offset correction circuit is enabled to remove pulse-
width distortion caused by intrinsic offset voltages with-
in the differential amplifier stages. An external capacitor
(C
AZ ) connected between the CAZ1 and CAZ2 pins is
used to set the offset correction loop cutoff frequency.
The offset loop can be disabled using the AZ_EN bit.
The MAX3798 contains a feature that allows the part to
meet a 10μs mode-select switching time. The mode-
select switching time can be adjusted using the GMEN
and CAZX bits.
CML Output Stage with Deemphasis and
Slew-Rate Control
The CML output stage is optimized for differential 100 Ω
loads. The RXDE_EN bit adds analog deemphasis
compensation to the limited differential output signal for
SFP connector losses. The output stage is controlled by
a combination of the RX_EN and SQ_EN bits and the
LOS pin. See Table 1.
Amplitude of the CML output stage is controlled by an
8-bit DAC register (SET_CML). The differential output
amplitude range is from 40mV
P-P up to 1200mV P-P with
4.6mV P-P resolution (assuming an ideal 100 Ω differen-
tial load).
The lower bandwidth data path allows for reduction of
output edge speed in order to enhance EMI perfor-
mance. The SLEW_RATE bit controls the slew rate of
the output stage (see Table 2).
Loss-of-Signal (LOS) Circuitry
The input data amplitude is compared to a preset
threshold controlled by the 6-bit DAC register
SET_LOS. The LOS assert level can be programmed
from 14mV
P-P up to 77mV P-P with 1.5mV P-P resolution
(assuming an ideal 100 Ω differential source). LOS is
enabled through the LOS_EN bit and the polarity of the
LOS is controlled with the LOS_POL bit.
VCSEL Driver
The VCSEL driver inside the MAX3798 is designed to
operate from 1.0625Gbps to 10.32Gbps. The transmit-
ter contains a differential data path with pulse-width
adjustment, bias current and modulation current DACs,
output driver with programmable deemphasis, power-
on reset circuitry, BIAS monitor, VCSEL current limiter,
and eye safety circuitry. A 3-wire digital interface is
used to control the transmitter functions. The registers
that control the transmitter functionality are TXCTRL,
TXSTAT1, TXSTAT2, SET_IBIAS, SET_IMOD, IMOD-
MAX, IBIASMAX, MODINC, BIASINC, MODECTRL,
SET_PWCTRL, and SET_TXDE.
Differential Data Path
The CML input buffer is optimized for AC-coupled sig-
nals and is internally terminated with a differential 100
Ω.
Differential input data is equalized for high-frequency
losses due to SFP connectors. The TX_POL bit in the
TXCTRL register controls the polarity of TOUT+ and
TOUT- vs. TIN+ and TIN-. The SET_PWCTRL register
RX_EN SQ_EN LOS
OPERATION MODE
DESCRIPTION
0 X X CML output disabled.
1 0 X CML output enabled.
1 1 0 CML output enabled.
1 1 1 CML output disabled.
Table 1. CML Output Stage Operation Mode
MODE_SEL SLEW_RATE
OPERATION MODE
DESCRIPTION
0 0
4.25Gbps operation with
reduced output edge speed.
0 1
4.25Gbps operation with full
edge speed; 8.5Gbps
operation with high bandwidth
ROSA.
1 X
8.5Gbps with lower bandwidth
ROSA; 10.32Gbps operation.
Table 2. Slew-Rate Control for CML
Output Stage