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MAX4257EUA+ Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX4257EUA+ Datasheet(HTML) 10 Page - Maxim Integrated Products |
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10 / 16 page  UCSP, Single-Supply, Low-Noise, Low-Distortion, Rail-to-Rail Op Amps 10 Maxim Integrated MAX4249–MAX4257 Low Distortion Many factors can affect the noise and distortion that the device contributes to the input signal. The following guidelines offer valuable information on the impact of design choices on Total Harmonic Distortion (THD). Choosing proper feedback and gain resistor values for a particular application can be a very important factor in reducing THD. In general, the smaller the closed- loop gain, the smaller the THD generated, especially when driving heavy resistive loads. Large-value feed- back resistors can significantly improve distortion. The THD of the part normally increases at approximately 20dB per decade, as a function of frequency. Operating the device near or above the full-power bandwidth significantly degrades distortion. Referencing the load to either supply also improves the part’s distortion performance, because only one of the MOSFETs of the push-pull output stage drives the out- put. Referencing the load to midsupply increases the part’s distortion for a given load and feedback setting. (See the Total Harmonic Distortion vs. Frequency graph in the Typical Operating Characteristics.) For gains # 10V/V, the decompensated devices MAX4249/MAX4255/MAX4256/MAX4257 deliver the best distortion performance, since they have a higher slew rate and provide a higher amount of loop gain for a given closed-loop gain setting. Capacitive loads below 400pF, do not significantly affect distortion results. Distortion performance remains relatively con- stant over supply voltages. Low Noise The amplifier’s input-referred, noise-voltage density is dominated by flicker noise at lower frequencies, and by thermal noise at higher frequencies. Because the ther- mal noise contribution is affected by the parallel combi- nation of the feedback resistive network (RF || RG, Figure 1), these resistors should be reduced in cases where the system bandwidth is large and thermal noise is dominant. This noise contribution factor decreases, however, with increasing gain settings. For example, the input noise-voltage density of the cir- cuit with RF = 100k", RG = 11k" (AV = 10V/V) is en = 15nV/!Hz, en can be reduced to 9nV/!Hz by choosing RF = 10k", RG = 1.1k" (AV = 10V/V), at the expense of greater current consumption and potentially higher distortion. For a gain of 100V/V with RF = 100k", RG = 1.1k", the en is low (9nV/!Hz). CZ RF VOUT VIN RG 0 100mV AV = 2V/V RF = RG = 10kΩ VIN = 50mV/div VOUT = 100mV/div 2 µs/div 0 100mV AV = 2 RF = RG = 100kΩ CZ = 11pF 50mV/div 100mV/div VIN VOUT 2 µs/div Figure 1. Adding Feed-Forward Compensation Figure 2a. Pulse Response with No Feed-Forward Compensation Figure 2b. Pulse Response with 10pF Feed-Forward Compensation |
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