LTC6404
19
6404f
APPLICATIONS INFORMATION
Functional Description
The LTC6404 is a small outline, wide band, low noise,
and low distortion fully-differential amplifi er with accurate
output phase balancing. The LTC6404 is optimized to drive
low voltage, single-supply, differential input 14-bit to 18-bit
analog-to-digital converters (ADCs). The LTC6404’s output
is capable of swinging rail-to-rail on supplies as low as
2.7V, which makes the amplifi er ideal for converting ground
referenced, single-ended signals into DC level-shifted
differential signals in preparation for driving low voltage,
single-supply, differential input ADCs. Unlike traditional
op amps which have a single output, the LTC6404 has
two outputs to process signals differentially. This allows
for two times the signal swing in low voltage systems
when compared to single-ended output amplifi ers. The
balanced differential nature of the amplifi er also provides
even-order harmonic distortion cancellation, and less
susceptibility to common mode noise (e.g., power supply
noise). The LTC6404 can be used as a single-ended input
to differential output amplifi er, or as a differential input to
differential output amplifi er.
The LTC6404’s output common mode voltage, defi ned
as the average of the two output voltages, is independent
of the input common mode voltage, and is adjusted by
applying a voltage on the V
OCM
pin. If the pin is left open,
there is an internal resistive voltage divider that develops
a potential halfway between the V
+
and V
–
pins. Whenever
this pin is not hard tied to a low impedance ground plane,
it is recommended that a high quality ceramic capacitor is
used to bypass the V
OCM
pin to a low impedance ground
plane (See Layout Considerations in this document). The
LTC6404’s internal common mode feedback path forces
accurate output phase balancing to reduce even order
harmonics, and centers each individual output about the
potential set by the V
OCM
pin.
VV
VV
OUTCM OCM
OUT OUT
==
+
+ –
2
The outputs (OUT
+
and OUT
–
) of the LTC6404 are capable
of swinging rail-to-rail. They can source or sink up to ap-
proximately 65mA of current.
Additional outputs (OUTF
+
and OUTF
–
) are available that
provide fi ltered versions of the OUT
+
and OUT
–
outputs. An
on-chip single pole RC passive fi lter band limits the fi ltered
outputs to a –3dB frequency of 88.5MHz. The user has a
choice of using the unfi ltered outputs, the fi ltered outputs,
or modifying the fi ltered outputs to adjust the frequency
response by adding additional components.
In applications where the full bandwidth of the LTC6404 is
desired, the unfi ltered outputs (OUT
+
and OUT
–
) should be
used. The unfi ltered outputs OUT
+
and OUT
–
are designed
to drive 10pF to ground (or 5pF differentially). Capacitances
greater than 10pF will produce excess peaking, and can
be mitigated by placing at least 25Ω in series with each
output pin.
Input Pin Protection
The LTC6404’s input stage is protected against differential
input voltages which exceed 1.4V by two pairs of back-
to-back diodes connected in anti-parallel series between
IN
+
and IN
–
(Pins 6 and 15). In addition, the input pins
have steering diodes to either power supply. If the input
pair is overdriven, the current should be limited to under
10mA to prevent damage to the IC. The LTC6404 also has
steering diodes to either power supply on the V
OCM
and
SHDN pins (Pins 4 and 1), and if forced to voltages which
exceed either supply, they too, should be current-limited
to under 10mA.
SHDN Pin
If the SHDN pin (Pin 1) is pulled 2.1V below the posi-
tive supply, circuitry is activated which powers down
the LTC6404. The pin will have the Thevenin equivalent
impedance of approximately 66kΩ to V
+
. If the pin is left
unconnected, an internal pull-up resistor of 150k will
keep the part in normal active operation. Care should
be taken to control leakage currents at this pin to under
1µA to prevent inadvertently putting the LTC6404 into
shutdown. In shutdown, all biasing current sources are
shut off, and the output pins, OUT
+
and OUT
–
, will each
appear as open collectors with a non-linear capacitor in
parallel and steering diodes to either supply. Because of
the non-linear capacitance, the outputs still have the ability
to sink and source small amounts of transient current if
driven by signifi cant voltage transients. The inputs (IN
+
,
and IN
–
) appear as anti-parallel diodes which can conduct
