11
2271f
LTC2271
PIN FUNCTIONS
V
CM1
(Pin 1): Common Mode Bias Output, Nominally Equal
to V
DD
/2. V
CM1
should be used to bias the common mode
of the analog inputs of channel 1. Bypass to ground with
a 1µF ceramic capacitor.
GND (Pins 2, 5, 13, 22, 45, 47, 49, Exposed Pad Pin 53):
ADC Power Ground. The exposed pad must be soldered
to the PCB ground.
A
IN1
+ (Pin 3): Channel 1 Positive Differential Analog
Input.
A
IN1
– (Pin 4): Channel 1 Negative Differential Analog
Input.
REFH (Pins 6, 8): ADC High Reference. See the Reference
section in the Applications Information for recommended
bypassing circuits for REFH and REFL.
REFL (Pins 7, 9): ADC Low Reference. See the Reference
section in the Applications Information for recommended
bypassing circuits for REFH and REFL.
PAR/SER (Pin 10): Programming Mode Selection Pin.
Connect to ground to enable the serial programming mode.
CS, SCK, SDI, SDO become a serial interface that control
the A/D operating modes. Connect to V
DD
to enable the
parallel programming mode where CS, SCK, SDI, SDO
become parallel logic inputs that control a reduced set of
the A/D operating modes. PAR/SER should be connected
directly to ground or the V
DD
of the part and not be driven
by a logic signal.
A
IN2
+ (Pin 11): Channel 2 Positive Differential Analog
Input.
A
IN2
– (Pin 12): Channel 2 Negative Differential Analog
Input.
V
CM2
(Pin 14): Common Mode Bias Output, Nominally
Equal to V
DD
/2. V
CM2
should be used to bias the common
mode of the analog inputs of channel 2. Bypass to ground
with a 1µF ceramic capacitor.
V
DD
(Pins 15, 16, 51, 52): Analog Power Supply, 1.7V
to 1.9V. Bypass to ground with 0.1µF ceramic capacitors.
Adjacent pins can share a bypass capacitor.
ENC
+
(Pin 17): Encode Input. Conversion starts on the
rising edge.
ENC
–
(Pin 18): Encode Complement Input. Conversion
starts on the falling edge. Tie to GND for single-ended
encode mode.
CS (Pin 19): In serial programming mode, (PAR/SER =
0V), CS is the serial interface chip select input. When
CS is low, SCK is enabled for shifting data on SDI into
the mode control registers. In the parallel programming
mode (PAR/SER = V
DD
), CS along with SCK selects 1-,
2- or 4-lane output mode (see Table 3). CS can be driven
with 1.8V to 3.3V logic.
SCK (Pin 20): In serial programming mode, (PAR/SER =
0V), SCK is the serial interface clock input. In the parallel
programming mode (PAR/SER = V
DD
), SCK along with CS
selects 1-, 2- or 4-lane output mode (see Table 3). SCK
can be driven with 1.8V to 3.3V logic.
SDI (Pin 21): In Serial Programming Mode, (PAR/SER =
0V), SDI is the Serial Interface Data Input. Data on SDI
is clocked into the mode control registers on the rising
edge of SCK. In the parallel programming pode (PAR/SER
= V
DD
), SDI can be used to power down the part. SDI can
be driven with 1.8V to 3.3V logic.
OGND (Pin 33): Output Driver Ground. This pin must be
shorted to the ground plane by a very low inductance path.
Use multiple vias close to the pin.
OV
DD
(Pin 34): Output Driver Supply. Bypass to ground
with a 0.1µF ceramic capacitor.
SDO (Pin 46): In serial programming mode, (PAR/SER
= 0V), SDO is the optional serial interface data output.
Data on SDO is read back from the mode control regis-
ters and can be latched on the falling edge of SCK. SDO
is an open-drain NMOS output that requires an external
2k pull-up resistor to 1.8V to 3.3V. If read back from the
mode control registers is not needed, the pull-up resistor
is not necessary and SDO can be left unconnected. In the
parallel programming mode (PAR/SER = V
DD
), SDO selects
3.5mA or 1.75mA LVDS output currents. When used as an
input, SDO can be driven with 1.8V to 3.3V logic through
a 1k series resistor.
V
REF
(Pin 48): Reference Voltage Output. Bypass to ground
with a 2.2µF ceramic capacitor. The reference output is
nominally 1.25V.
