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OPA347PAG4 Datasheet(PDF) 8 Page - Texas Instruments |
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OPA347PAG4 Datasheet(HTML) 8 Page - Texas Instruments |
8 / 33 page OPA347, 2347, 4347 8 SBOS167D www.ti.com COMMON-MODE REJECTION The CMRR for the OPA347 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+) – 1.7V) is given. This specifica- tion is the best indicator of the capability 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. INPUT VOLTAGE The input common-mode range extends from (V–) – 0.2V to (V+) + 0.2V. For normal operation, inputs should be limited to this range. The absolute maximum input voltage is 500mV beyond the supplies. Inputs greater than the input common-mode range but less than the maximum input voltage, while not valid, will not cause any damage to the op amp. Furthermore, if input current is limited the inputs may go beyond the power supplies without phase inversion, as shown in Figure 4, unlike some other op amps. Normally, input currents are 0.4pA. However, large inputs (greater than 500mV beyond the supply rails) can cause excessive current to flow in or out of the input pins. There- fore, as well as keeping the input voltage below the maxi- mum rating, it is also important to limit the input current to less than 10mA. This is easily accomplished with an input resistor, as shown in Figure 5. 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. How- ever, some applications exercise the amplifier through the transition region of both differential input stages. A small discontinuity may occur in this transition. Careful selection of the circuit configuration, signal levels, and biasing can often avoid this transition 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 the V+ supply and may exhibit a small discontinuity at this point. The common-mode voltage of the noninverting amplifier 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 configuration 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. OPA347—No Phase Inversion with Inputs Greater than the Power-Supply Voltage. V O V IN V B V+ Noninverting Amplifier V CM = VIN V O V B V IN V+ Inverting Amplifier V CM = VB V O V IN V+ Unity-Gain Buffer V CM = VIN = VO FIGURE 5. Input Current Protection for Voltages Exceeding the Supply Voltage. 5k Ω OPA347 10mA max +5V V IN V OUT I OVERLOAD 5.5V 0V –0.5V 200 µs/div |
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