MAX3747A/MAX3747B
155Mbps to 3.2Gbps, Low-Power SFP
Limiting Amplifiers
9
Maxim Integrated
Loss-of-Signal Indicator
The MAX3747A/MAX3747B are equipped with LOS cir-
cuitry that indicates when the input signal is below a pro-
grammable threshold, set by a voltage on the TH pin
(see the
Typical Operating Characteristics
). The voltage
on the TH pin is set by two resistors, one connecting
from the TH pin to V
CC
and the other connecting from TH
to V
REF
(Figure 5). An RMS power detector compares
the input signal amplitude with this threshold and feeds
the signal-detect information to the LOS output, which is
open collector. To prevent LOS chatter in the region of
the programmed threshold, approximately 2dB of hys-
teresis is built into the LOS assert/deassert function.
Once asserted, LOS is not deasserted until the input
amplitude rises to the required level. Figure 6 shows the
LOS output circuit.
Applications Information
Program the LOS Assert Threshold
Program the LOS assert threshold according to Figure
5. The combination of R
TH1
and R
TH2
should be
greater than or equal to 5kΩ, see the Assert/Deassert
vs. V
TH
graph in the
Typical Operating Characteristics
.
Select the Coupling Capacitor
When AC-coupling is desired, coupling capacitors C
IN
and C
OUT
should be selected to minimize the receiv-
er’s deterministic jitter. Jitter is decreased as the input
low-frequency cutoff (f
IN
) is decreased:
f
IN
= 1/[2π(50)(C
IN
)]
For all applications, the recommended value for C
IN
and C
OUT
is 0.1µF, which provides f
IN
equal to 32kHz.
Refer to Application Note HFAN-1.1:
Choosing AC-
Coupling Capacitors
on the Maxim website
(www.maximintegrated.com)
.
