Datasheet
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MAX3946
1Gbps to 11.3Gbps, SFP+ Laser Driver
with Laser Impedance Mismatch Tolerance
Modulation Current Setting Register (SET_IMOD)
Bits 7 to 0: SET_IMOD[8:1]. The modulation current DAC is controlled by a total of 9 bits. The SET_IMOD[8:1] bits
are used to set the modulation current with even denominations from 0 to 510 bits. The LSB (SET_IMOD[0]) is con-
trolled by the MODINC register and is used to set the odd denominations in the SET_IMOD[8:0]. Any direct write to
SET_IMOD[8:1] resets the LSB.
Maximum Modulation Current Setting Register (IMODMAX)
Bits 7 to 0: IMODMAX[7:0]. The IMODMAX register is an 8-bit register that can be used to limit the maximum modula-
tion current. IMODMAX[7:0] is continuously compared to SET_IMOD[8:1]. Any attempt to modify SET_IMOD[8:1] above
IMODMAX[7:0] is ignored and flags a warning at IMODERR.
Maximum Bias Current Setting Register (IBIASMAX)
Bits 7 to 0: IBIASMAX[7:0]. The IBIASMAX register is an 8-bit register that can be used to limit the maximum bias
current. IBIASMAX[7:0] is continuously compared to SET_IBIAS[8:1]. Any attempt to modify SET_IBIAS[8:1] above
IBIASMAX[7:0] is ignored and flags a warning at IBIASERR.
Modulation Current Increment Setting Register (MODINC)
Bit 7: SET_IMOD[0]. This is the LSB of the SET_IMOD[8:0] bits. This bit can only be updated by the use of MODINC[4:0].
Bits 4 to 0: MODINC[4:0]. This string of bits is used to increment or decrement the modulation current. When written
to, the SET_IMOD[8:0] bits are updated. MODINC[4:0] are a two’s complement string.
Bit #
7 6 5 4 3 2 1 0 ADDRESS
Name
SET_IMOD
[8] (MSB)
SET_IMOD
[7]
SET_IMOD
[6]
SET_IMOD
[5]
SET_IMOD
[4]
SET_IMOD
[3]
SET_IMOD
[2]
SET_IMOD
[1]
H0x09
Default Value 0 0 0 0 0 1 0 0
Bit #
7 6 5 4 3 2 1 0 ADDRESS
Name
IMODMAX
[7] (MSB)
IMODMAX
[6]
IMODMAX
[5]
IMODMAX
[4]
IMODMAX
[3]
IMODMAX
[2]
IMODMAX
[1]
IMODMAX
[0] (LSB)
H0x0A
Default Value 0 0 1 0 0 0 0 0
Bit #
7 6 5 4 3 2 1 0 ADDRESS
Name
IBIASMAX
[7] (MSB)
IBIASMAX
[6]
IBIASMAX
[5]
IBIASMAX
[4]
IBIASMAX
[3]
IBIASMAX
[2]
IBIASMAX
[1]
IBIASMAX
[0] (LSB)
H0x0B
Default Value 0 0 1 0 0 0 0 0
Bit #
7 6 5 4 3 2 1 0 ADDRESS
Name
SET_IMOD
[0] (LSB)
X X
MODINC
[4] (MSB)
MODINC
[3]
MODINC
[2]
MODINC
[1]
MODINC
[0] (LSB)
H0x0C
Default Value 0 0 0 0 0 0 0 0