Datasheet LT1994 (Analog Devices) - 10

LT1994
APPLICATIONS INFORMATION Functional Description
The outputs (OUT+ and OUT–) of the LT1994 are capable of swinging rail-to-rail. They can source or sink up to The LT1994 is a small outline, wideband, low noise and approximately 85mA of current. Each output is rated to low distortion fully-differential amplifi er with accurate drive approximately 25pF to ground (12.5pF differentially). output-phase balancing. The LT1994 is optimized to Higher load capacitances should be decoupled with at least drive low voltage, single-supply, differential input ana- 25Ω of series resistance from each output. log-to-digital converters (ADCs). The LT1994’s output is capable of swinging rail-to-rail on supplies as low as 2.5V,
Input Pin Protection
which makes the amplifi er ideal for converting ground referenced, single-ended signals into V The LT1994’s input stage is protected against differential OCM referenced differential signals in preparation for driving low voltage, input voltages that exceed 1V by two pairs of back-to- single-supply, differential input ADCs. Unlike traditional back diodes that protect against emitter base breakdown op amps which have a single output, the LT1994 has two of the input transistors. In addition, the input pins have outputs to process signals differentially. This allows for steering diodes to either power supply. If the input pair two times the signal swing in low voltage systems when is overdriven, the current should be limited to under compared to single-ended output amplifi ers. The balanced 10mA to prevent damage to the IC. The LT1994 also has differential nature of the amplifi er also provides even-order steering diodes to either power supply on the VOCM, and harmonic distortion cancellation, and less susceptibility SHDN pins (Pins 2 and 7) and if exposed to voltages that to common mode noise (like power supply noise). The exceed either supply, they too should be current limited LT1994 can be used as a single-ended input to differential to under 10mA. output amplifi er, or as a differential input to differential output amplifi er.
SHDN Pin
The LT1994’s output common mode voltage, defi ned as the If the SHDN pin (Pin 7) is pulled 2.1V below the positive average of the two output voltages, is independent of the supply, an internal current is generated that is used to input common mode voltage, and is adjusted by applying power down the LT1994. The pin will have the Thevenin a voltage on the V equivalent impedance of approximately 55kΩ to V+. If OCM pin. If the pin is left open, there is an internal resistive voltage divider, which develops a potential the pin is left unconnected, an internal pull-up resistor of halfway between the V+ and V– pins. The V 120k will keep the part in normal active operation. Care OCM pin will have an equivalent Thevenin equivalent resistance of 40k, and a should be taken to control leakage currents at this pin to Thevenin equivalent voltage of half supply. Whenever this under 1μA to prevent leakage currents from inadvertently pin is not hard tied to a low impedance ground plane, it putting the LT1994 into shutdown. In shutdown, all biasing is recommended that a high quality ceramic capacitor is current sources are shut off, and the output pins OUT+ and used to bypass the V OUT– will each appear as open collectors with a nonlinear OCM pin to a low impedance ground plane (see Layout Considerations in this document). The capacitor in parallel, and steering diodes to either supply. LT1994’s internal common mode feedback path forces Because of the nonlinear capacitance, the outputs still have accurate output phase balancing to reduce even order the ability to sink and source small amounts of transient harmonics, and centers each individual output about the current if exposed to signifi cant voltage transients. The potential set by the V inputs (IN+ and IN–) have anti-parallel diodes that can OCM pin. conduct if voltage transients at the input exceed 1V. The V + V – inputs also have steering diodes to either supply. The V = V OUT OUT OUTCM OCM = + 2 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. 1994fb 10