LT1994 Linear Technology, LT1994 Datasheet - Page 10

• Manufacturer Part Number: LT1994
• Description: Low Noise - Low Distortion Fully Differential Input/Output Amplifier/Driver
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1994
Functional Description
The LT1994 is a small outline, wide band, low noise, and
low distortion fully-differential amplifi er with accurate
output phase balancing. The LT1994 is optimized to
drive low voltage, single-supply, differential input ana-
log-to-digital converters (ADCs). The LT1994’s output is
capable of swinging rail-to-rail on supplies as low as 2.5V,
which makes the amplifi er ideal for converting ground
referenced, single-ended signals into V
differential signals in preparation for driving low voltage,
single-supply, differential input ADCs. Unlike traditional
op amps which have a single output, the LT1994 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 (like power supply noise). The
LT1994 can be used as a single ended input to differential
output amplifi er, or as a differential input to differential
output amplifi er.
The LT1994’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
open, there is an internal resistive voltage divider, which
develops a potential halfway between the V
The V
resistance of 40kΩ, and a Thevenin equivalent voltage of
half-supply. Whenever this pin is not hard tied to a low
impedance ground plane, it is recommended that a high
quality ceramic cap is used to bypass the V
low impedance ground plane (see Layout Considerations
in this document). The LT1994’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
The outputs (OUT
of swinging rail-to-rail. They can source or sink up to
approximately 85mA of current. Each output is rated to
10
V
OUTCM
OCM
pin will have an equivalent Thevenin equivalent
=
V
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+
U
and OUT
=
V
OUT
U
+
–
) of the LT1994 are capable
+
2
OCM
V
OUT
W
pin. If the pin is left
–
OCM
OCM
+
pin.
OCM
and V
referenced
U
pin to a
–
pins.
drive approximately 25pF to ground (12.5pF differentially).
Higher load capacitances should be decoupled with at least
25Ω of series resistance from each output.
Input Pin Protection
The LT1994’s input stage is protected against differential
input voltages that exceed 1V by two pairs of back-to-
back diodes that protect against emitter base breakdown
of the input transistors. In addition, the input pins have
steering diodes to either power supply. If the input pair
is over-driven, the current should be limited to under
10mA to prevent damage to the IC. The LT1994 also has
steering diodes to either power supply on the V
⎯ S ⎯ H ⎯ D ⎯ N pins (Pins 2 and 7) and if exposed to voltages that
exceed either supply, they too should be current limited
to under 10mA.
⎯ S ⎯ H ⎯ D ⎯ N Pin
If the ⎯ S ⎯ H ⎯ D ⎯ N pin (Pin 7) is pulled 2.1V below the positive
supply, an internal current is generated that is used to
power down the LT1994. The pin will have the Thevenin
equivalent impedance of approximately 55kΩ to V
the pin is left unconnected, an internal pull-up resistor of
120kΩ will keep the part in normal active operation. Care
should be taken to control leakage currents at this pin to
under 1μA to prevent leakage currents from inadvertently
putting the LT1994 into shutdown. In shutdown, all biasing
current sources are shut off, and the output pins OUT
and OUT
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 exposed to signifi cant voltage tran-
sients. The inputs (IN
that can conduct if voltage transients at the input exceed
1V. The inputs also have steering diodes to either supply.
The turn-on and turn-off time between the shutdown and
active states are on the order of 1μs but depends on the
circuit confi guration.
General Amplifi er Applications
As levels of integration have increased and, correspond-
ingly, system supply voltages decreased, there has been
–
will each appear as open collectors with a non-
+
, and IN
–
) have anti-parallel diodes
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