LT1994
10
1994fb
Functional Description
The LT1994 is a small outline, wideband, 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
OCM
referenced
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
OCM
pin. If the pin is left open, there is an
internal resistive voltage divider, which develops a potential
halfway between the V
+
and V
–
pins. The V
OCM
pin will have
an equivalent Thevenin equivalent resistance of 40k, 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 capacitor is
used to bypass the V
OCM
pin to a 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
OCM
pin.
VV
VV
OUTCM OCM
OUT OUT
==
+
+ –
2
The outputs (OUT
+
and OUT
–
) of the LT1994 are capable
of swinging rail-to-rail. They can source or sink up to
approximately 85mA of current. Each output is rated to
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 overdriven, 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
OCM
, and
SHDN pins (Pins 2 and 7) and if exposed to voltages that
exceed either supply, they too should be current limited
to under 10mA.
SHDN Pin
If the SHDN 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
+
. If
the pin is left unconnected, an internal pull-up resistor of
120k will keep the part in normal active operation. Care
should be taken to control leakage currents at this pin to
under 1A 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
–
will each appear as open collectors with a nonlinear
capacitor in parallel, and steering diodes to either supply.
Because of the nonlinear capacitance, the outputs still have
the ability to sink and source small amounts of transient
current if exposed to signifi cant voltage transients. The
inputs (IN
+
and IN
–
) have anti-parallel diodes 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 1s but depends on the
circuit confi guration.
APPLICATIONS INFORMATION
