Datasheet MAX4091, MAX4092, MAX4094 (Maxim) - 9

Single/Dual/Quad, Micropower, Single-Supply, Rail-to-Rail Op Amps MAX4091/MAX4092/MAX4094
tance. If the output sources either more or less current, MAX4094, it takes some time for the voltages on the stability is increased. These devices perform well with a supply pin and the output pin of the op amp to settle. 1000pF pure capacitive load (Figure 5). Figures 6a, 6b, Supply settling time depends on the supply voltage, the and 6c show the performance with a 500pF load in par- value of the bypass capacitor, the output impedance of allel with various load resistors. the incoming supply, and any lead resistance or induc- To increase stability while driving large-capacitive tance between components. Op amp settling time loads, connect a pullup resistor to VCC at the output to depends primarily on the output voltage and is slew- decrease the current the amplifier must source. If the rate limited. With the noninverting input to a voltage fol- amplifier is made to sink current rather than source, lower held at midsupply (Figure 9), when the supply stability is further increased. steps from 0 to VCC, the output settles in approximately Frequency stability can be improved by adding an out- 2µs for VCC = 3V (Figure 10a) and 8µs for VCC = 5V put isolation resistor (RS) to the voltage-follower circuit (Figure 10b). (Figure 7). This resistor improves the phase margin of the circuit by isolating the load capacitor from the op
Power Supplies and Layout
amp’s output. Figure 8a shows the MAX4092 driving The MAX4091/MAX4092/MAX4094 operate from a sin- 5000pF (RL ≥ 100kΩ), while Figure 8b adds a 47Ω iso- gle 2.7V to 6V power supply, or from dual supplies of lation resistor. ±1.35V to ±3V. For single-supply operation, bypass the power supply with a 0.1µF capacitor. If operating from Because the MAX4091/MAX4092/MAX4094 have excel- dual supplies, bypass each supply to ground. lent stability, no isolation resistor is required, except in the most demanding applications. This is beneficial Good layout improves performance by decreasing the because an isolation resistor would degrade the low- amount of stray capacitance at the op amp’s inputs frequency performance of the circuit. and output. To decrease stray capacitance, minimize both trace lengths and resistor leads and place exter-
Power-Up Settling Time
nal components close to the op amp’s pins. The MAX4091/MAX4092/MAX4094 have a typical sup- ply current of 130µA per op amp. Although supply cur- rent is already low, it is sometimes desirable to reduce
Chip Information
it further by powering down the op amp and associated MAX4091 TRANSISTOR COUNT: 168 ICs for periods of time. For example, when using a MAX4092 to buffer the inputs of a multi-channel analog- MAX4092 TRANSISTOR COUNT: 336 to-digital converter (ADC), much of the circuitry could MAX4094 TRANSISTOR COUNT: 670 be powered down between data samples to increase PROCESS: Bipolar battery life. If samples are taken infrequently, the op amps, along with the ADC, may be powered down most of the time. When power is reapplied to the MAX4091/MAX4092/
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