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OPA4344UA Datasheet(PDF) 10 Page - Texas Instruments |
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OPA4344UA Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 30 page  www.ti.com OPA344, 2344, 4344 OPA345, 2345, 4345 10 SBOS107A COMMON-MODE REJECTION The CMRR for the OPA344 and OPA345 is specified in several ways so the best match for a given application may be used. First, the CMRR of the device in the common-mode range below the transition region (VCM < (V+) – 1.8V) is given. This specification is the best indicator of the capabil- ity of the device when the application requires use of one of the differential input pairs. Second, the CMRR at VS = 5.5V over the entire common-mode range is specified. Third, the CMRR at VS = 2.7V over the entire common-mode range is provided. These last two values include the variations seen through the transition region. INPUT VOLTAGE BEYOND THE RAILS If the input voltage can go more than 0.3V below the negative power supply rail (single-supply ground), special precautions are required. If the input voltage goes suffi- ciently negative, the op amp output may lock up in an inoperative state. A Schottky diode clamp circuit will pre- vent this—see Figure 4. The series resistor prevents exces- sive current (greater than 10mA) in the Schottky diode and in the internal ESD protection diode, if the input voltage can exceed the positive supply voltage. If the signal source is limited to less than 10mA, the input resistor is not required. RAIL-TO-RAIL OUTPUT A class AB output stage with common-source transistors is used to achieve rail-to-rail output. This output stage is capable of driving 600 Ω loads connected to any potential between V+ and ground. For light resistive loads (> 50k Ω), the output voltage can typically swing to within 1mV from supply rail. With moderate resistive loads (2k Ω to 50kΩ), the output can swing to within a few tens of millivolts from the supply rails while maintaining high open-loop gain. See the typical performance curve “Output Voltage Swing vs Output Current.” V O V IN V B V+ Non-Inverting Gain V CM = VIN V O V B V IN V+ Inverting Amplifier V CM = VB V O V IN V+ G = 1 Buffer V CM = VIN = VO FIGURE 3. Design Optimization with Rail-to-Rail Input Op Amps. Rail-to-rail op amps can be used in virtually any op amp configuration. To achieve optimum performance, how- ever, applications using these special double-input-stage op amps may benefit from consideration of their special behavior. In many applications, operation remains within the com- mon-mode range of only one differential input pair. However some applications exercise the amplifier through the transition region of both differential input stages. Although the two input stages are laser trimmed for excellent matching, a small discontinuity may occur in this transition. Careful selection of the circuit configura- tion, signal levels and biasing can often avoid this transi- tion region. DESIGN OPTIMIZATION WITH RAIL-TO-RAIL INPUT OP AMPS With a unity-gain buffer, for example, signals will traverse this transition at approximately 1.3V below V+ supply and may exhibit a small discontinuity at this point. The common-mode voltage of the non-inverting ampli- fier is equal to the input voltage. If the input signal always remains less than the transition voltage, no discontinuity will be created. The closed-loop gain of this configura- tion can still produce a rail-to-rail output. Inverting amplifiers have a constant common-mode volt- age equal to VB. If this bias voltage is constant, no discontinuity will be created. The bias voltage can gener- ally be chosen to avoid the transition region. FIGURE 4. Input Current Protection for Voltages Exceed- ing the Supply Voltage. 1k Ω OPA344 10mA max V+ V IN V OUT I OVERLOAD IN5818 Schottky diode is required only if input voltage can go more than 0.3V below ground. CAPACITIVE LOAD AND STABILITY The OPA344 in a unity-gain configuration and the OPA345 in gains greater than 5 can directly drive up to 250pF pure capacitive load. Increasing the gain enhances the amplifier’s ability to drive greater capacitive loads. See the typical |
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