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MAX4250 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX4250 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page UCSP, Single-Supply, Low-Noise, Low-Distortion, Rail-to-Rail Op Amps Maxim Integrated 11 MAX4249–MAX4257 Using a Feed-Forward Compensation Capacitor, CZ The amplifier’s input capacitance is 11pF. If the resis- tance seen by the inverting input is large (feedback network), this can introduce a pole within the amplifier’s bandwidth, resulting in reduced phase margin. Compensate the reduced phase margin by introducing a feed-forward capacitor (CZ) between the inverting input and the output (Figure 1). This effectively cancels the pole from the inverting input of the amplifier. Choose the value of CZ as follows: CZ = 11 x (RF / RG) [pF] In the unity-gain stable MAX4250–MAX4254, the use of a proper CZ is most important for AV = 2V/V, and AV = -1V/V. In the decompensated MAX4249/ MAX4255/MAX4256/MAX4257, CZ is most important for AV = 10V/V. Figures 2a and 2b show transient response both with and without CZ. Using a slightly smaller CZ than suggested by the for- mula above achieves a higher bandwidth at the expense of reduced phase and gain margin. As a gen- eral guideline, consider using CZ for cases where RG || RF is greater than 20k" (MAX4250–MAX4254) or greater than 5k" (MAX4249/MAX4255/MAX4256/ MAX4257). Applications Information The MAX4249–MAX4257 combine good driving capa- bility with ground-sensing input and rail-to-rail output operation. With their low distortion, low noise, and low- power consumption, these devices are ideal for use in portable instrumentation systems and other low-power, noise-sensitive applications. Ground-Sensing and Rail-to-Rail Outputs The common-mode input range of these devices extends below ground, and offers excellent common- mode rejection. These devices are guaranteed not to undergo phase reversal when the input is overdriven (Figure 3). Figure 4 showcases the true rail-to-rail output opera- tion of the amplifier, configured with AV = 10V/V. The output swings to within 8mV of the supplies with a 10k" load, making the devices ideal in low-supply- voltage applications. Output Loading and Stability Even with their low quiescent current of 400µA, these amplifiers can drive 1k" loads while maintaining excel- lent DC accuracy. Stability while driving heavy capaci- tive loads is another key feature. VOUT VIN RISO CL MAX4250 MAX4251 MAX4252 MAX4253 MAX4254 4.25V 4.45V -200mV 0 0 VOUT VIN AV = 1 VDD = 5V RL = 10kΩ 20 µs/div 0 5V VDD = 5V RL = 10kΩ AV = 10 f = 1kHz 200 µs/div VOUT 1V/div Figure 3. Overdriven Input Showing No Phase Reversal Figure 4. Rail-to-Rail Output Operation Figure 5. Capacitive-Load Driving Circuit |
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