23
21754314fa
LTC2175-14/
LTC2174-14/LTC2173-14
applicaTions inForMaTion
DIGITAL OUTPUTS
The digital outputs of the LTC2175-14/LTC2174-14/
LTC2173-14 are serialized LVDS signals. Each channel
outputs two bits at a time (2-lane mode). At lower sam-
pling rates there is a one bit per channel option (1-lane
mode). The data can be serialized with 16, 14, or 12-bit
serialization (see timing diagrams for details). Note that
with 12-bit serialization the two LSBs are not available—
this mode is included for compatibility with the 12-bit
versions of these parts.
The output data should be latched on the rising and falling
edges of the data clock out (DCO). A data frame output
(FR) can be used to determine when the data from a new
conversion result begins. In the 2-lane, 14-bit serialization
mode, the frequency of the FR output is halved.
The maximum serial data rate for the data outputs is
1Gbps, so the maximum sample rate of the ADC will de-
pend on the serialization mode as well as the speed grade
of the ADC (see Table 1). The minimum sample rate for
all serialization modes is 5Msps.
By default the outputs are standard LVDS levels: 3.5mA
output current and a 1.25V output common mode volt-
age. An external 100 differential termination resistor
is required for each LVDS output pair. The termination
resistors should be located as close as possible to the
LVDS receiver.
The outputs are powered by OVDD and OGND which are
isolated from the A/D core power and ground.
Programmable LVDS Output Current
The default output driver current is 3.5mA. This current
can be adjusted by control register A2 in the serial pro-
gramming mode. Available current levels are 1.75mA,
2.1mA, 2.5mA, 3mA, 3.5mA, 4mA and 4.5mA. In the
parallel programming mode the SCK pin can select either
3.5mA or 1.75mA.
Optional LVDS Driver Internal Termination
In most cases using just an external 100 termination
resistor will give excellent LVDS signal integrity. In addi-
tion, an optional internal 100 termination resistor can
be enabled by serially programming mode control register
A2. The internal termination helps absorb any reflections
caused by imperfect termination at the receiver. When the
internal termination is enabled, the output driver current is
doubled to maintain the same output voltage swing. In the
parallel programming mode the SDO pin enables internal
termination. Internal termination should only be used with
1.75mA, 2.1mA or 2.5mA LVDS output current modes.
Table 1. Maximum Sampling Frequency for All Serialization Modes. Note That These Limits Are for the LTC2175-14. The Sampling
Frequency for the Slower Speed Grades Cannot Exceed 105MHz (LTC2174-14) or 80MHz (LTC2173-14).
SERIALIZATION MODE
MAXIMUM SAMPLING
FREQUENCY, fS (MHz)
DCO FREQUENCY
FR FREQUENCY
SERIAL DATA RATE
2-Lane
16-Bit Serialization
125
4 fS
fS
8 fS
2-Lane
14-Bit Serialization
125
3.5 fS
0.5 fS
7 fS
2-Lane
12-Bit Serialization
125
3 fS
fS
6 fS
1-Lane
16-Bit Serialization
62.5
8 fS
fS
16 fS
1-Lane
14-Bit Serialization
71.4
7 fS
fS
14 fS
1-Lane
12-Bit Serialization
83.3
6 fS
fS
12 fS