Datasheet

LTC2157-12/
LTC2156-12/LTC2155-12
20
21576512f
APPLICATIONS INFORMATION
By default the outputs are standard LVDS levels: 3.5mA
output current and a 1.25V output common mode volt-
age. An external 100Ω 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 OV
DD
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 serially programming mode control
register A3 (see Table 3). Available current levels are
1.75mA, 2.1mA, 2.5mA, 3mA, 3.5mA, 4mA and 4.5mA.
Optional LVDS Driver Internal Termination
In most cases, using just an external 100Ω termination
resistor will give excellent LVDS signal integrity. In addi-
tion, an optional internal 100Ω termination resistor can
be enabled by serially programming mode control register
A3. 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.
Overflow Bit
The overflow output bit (OF) outputs a logic high when
the analog input is either overranged or underranged. The
overflow bit has the same pipeline latency as the data bits.
The OF output is double-data rate; when CLKOUT
+
is low,
channel A’s overflow is available; when CLKOUT
+
is high,
channel B’s overflow is available.
Phase Shifting the Output Clock
To allow adequate set-up and hold time when latching the
output data, the CLKOUT
+
signal may need to be phase
shifted relative to the data output bits. Most FPGAs have
this feature; this is generally the best place to adjust the
timing.
Alternatively, the ADC can also phase shift the CLKOUT
+
/
CLKOUT
–
signals by serially programming mode control
register A2. The output clock can be shifted by 0°, 45°,
90°, or 135°. To use the phase shifting feature the clock
duty cycle stabilizer must be turned on. Another con-
trol register bit can invert the polarity of CLKOUT
+
and
CLKOUT
–
, independently of the phase shift. The combina-
tion of these two features enables phase shifts of 45° up
to 315° (Figure 11).
Figure 11. Phase Shifting CLKOUT
CLKOUT
+
D0-D11, OF
PHASE
SHIFT
0°
45°
90°
135°
180°
225°
270°
315°
CLKINV
0
0
0
0
1
1
1
1
CLKPHASE1
MODE CONTROL BITS
0
0
1
1
0
0
1
1
CLKPHASE0
0
1
0
1
0
1
0
1
21576512 F11
ENC
+