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MAX4091AUA Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX4091AUA Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 16 page Single/Dual/Quad, Micropower, Single-Supply, Rail-to-Rail Op Amps 8 _______________________________________________________________________________________ Detailed Description The single MAX4091, dual MAX4092 and quad MAX4094 op amps combine excellent DC accuracy with rail-to-rail operation at both input and output. With their precision performance, wide dynamic range at low supply voltages, and very low supply current, these op amps are ideal for battery-operated equipment, indus- trial, and data acquisition and control applications. Applications Information Rail-to-Rail Inputs and Outputs The MAX4091/MAX4092/MAX4094’s input common- mode range extends 50mV beyond the positive and negative supply rails, with excellent common-mode rejection. Beyond the specified common-mode range, the outputs are guaranteed not to undergo phase reversal or latchup. Therefore, the MAX4091/MAX4092/ MAX4094 can be used in applications with common- mode signals, at or even beyond the supplies, without the problems associated with typical op amps. The MAX4091/MAX4092/MAX4094’s output voltage swings to within 15mV of the supplies with a 100k Ω load. This rail-to-rail swing at the input and the output substantially increases the dynamic range, especially in low-supply-voltage applications. Figure 1 shows the input and output waveforms for the MAX4092, config- ured as a unity-gain noninverting buffer operating from a single 3V supply. The input signal is 3.0VP-P, a 1kHz sinusoid centered at 1.5V. The output amplitude is approximately 2.98VP-P. Input Offset Voltage Rail-to-rail common-mode swing at the input is obtained by two complementary input stages in parallel, which feed a folded cascaded stage. The PNP stage is active for input voltages close to the negative rail, and the NPN stage is active for input voltages close to the positive rail. The offsets of the two pairs are trimmed. However, there is some residual mismatch between them. This mismatch results in a two-level input offset characteris- tic, with a transition region between the levels occurring at a common-mode voltage of approximately 1.3V above VEE. Unlike other rail-to-rail op amps, the transi- tion region has been widened to approximately 600mV in order to minimize the slight degradation in CMRR caused by this mismatch. The input bias currents of the MAX4091/MAX4092/ MAX4094 are typically less than 20nA. The bias current flows into the device when the NPN input stage is active, and it flows out when the PNP input stage is active. To reduce the offset error caused by input bias current flowing through external source resistances, match the effective resistance seen at each input. Connect resistor R3 between the noninverting input and ground when using the op amp in an inverting configu- ration (Figure 2a); connect resistor R3 between the noninverting input and the input signal when using the op amp in a noninverting configuration (Figure 2b). Select R3 to equal the parallel combination of R1 and R2. High source resistances will degrade noise perfor- mance, due to the the input current noise (which is mul- tiplied by the source resistance). Input Stage Protection Circuitry The MAX4091/MAX4092/MAX4094 include internal pro- tection circuitry that prevents damage to the precision input stage from large differential input voltages. This protection circuitry consists of back-to-back diodes between IN+ and IN- with two 1.7k Ω resistors in series (Figure 3). The diodes limit the differential voltage applied to the amplifiers’ internal circuitry to no more than VF, where VF is the diodes’ forward-voltage drop (about 0.7V at +25°C). Input bias current for the ICs (±20nA) is specified for small differential input voltages. For large differential input voltages (exceeding VF), this protection circuitry increases the input current at IN+ and IN-: Output Loading and Stability Even with their low quiescent current of less than 130µA per op amp, the MAX4091/MAX4092/MAX4094 are well suited for driving loads up to 1k Ω while main- taining DC accuracy. Stability while driving heavy capacitive loads is another key advantage over compa- rable CMOS rail-to-rail op amps. In op amp circuits, driving large capacitive loads increases the likelihood of oscillation. This is especially true for circuits with high-loop gains, such as a unity- gain voltage follower. The output impedance and a capacitive load form an RC network that adds a pole to the loop response and induces phase lag. If the pole frequency is low enough—as when driving a large capacitive load––the circuit phase margin is degraded, leading to either an under-damped pulse response or oscillation. The MAX4091/MAX4092/MAX4094 can drive capacitive loads in excess of 2000pF under certain conditions (Figure 4). When driving capacitive loads, the greatest potential for instability occurs when the op amp is sourcing approximately 200µA. Even in this case, sta- bility is maintained with up to 400pF of output capaci- INPUT CURRENT VV V k IN IN F = −− +− [( ) ( )] . 21 7 ✕ Ω |
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