MAX4249–MAX4257 UCSP, Single-Supply, Low-Noise, Low-Distortion, Rail-to-Rail Op Amps Low Distortion Many factors can affect the noise and distortion that the device contributes to the input signal. The following CZ guidelines offer valuable information on the impact of design choices on Total Harmonic Distortion (THD). RF Choosing proper feedback and gain resistor values for a particular application can be a very important factor in reducing THD. In general, the smal er the closed- RG V loop gain, the smal er the THD generated, especial y OUT when driving heavy resistive loads. Large-value feed- back resistors can significantly improve distortion. The V THD of the part normal y increases at approximately IN 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 Figure 1. Adding Feed-Forward Compensation 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. AV = 2V/V (See the Total Harmonic Distortion vs. Frequency graph RF = RG = 10kΩ in the Typical Operating Characteristics.) V 100mV IN = 50mV/div For gains # 10V/V, the decompensated devices MAX4249/MAX4255/MAX4256/MAX4257 deliver the best distortion performance, since they have a higher 0 slew rate and provide a higher amount of loop gain for VOUT = a given closed-loop gain setting. Capacitive loads 100mV/div below 400pF, do not significantly affect distortion results. Distortion performance remains relatively con- stant over supply voltages. Low Noise 2µs/div The amplifier’s input-referred, noise-voltage density is dominated by flicker noise at lower frequencies, and by Figure 2a. Pulse Response with No Feed-Forward thermal noise at higher frequencies. Because the ther- Compensation mal noise contribution is affected by the parallel combi- nation of the feedback resistive network (RF || RG, A Figure 1), these resistors should be reduced in cases V = 2 RF = RG = 100kΩ where the system bandwidth is large and thermal noise CZ = 11pF is dominant. This noise contribution factor decreases, 100mV 50mV/div however, with increasing gain settings. VIN For example, the input noise-voltage density of the cir- cuit with R 0 F = 100k", RG = 11k" (AV = 10V/V) is en = 15nV/!Hz, en can be reduced to 9nV/!Hz by choosing 100mV/div RF = 10k", RG = 1.1k" (AV = 10V/V), at the expense of greater current consumption and potential y higher VOUT distortion. For a gain of 100V/V with RF = 100k", RG = 1.1k", the en is low (9nV/!Hz). 2µs/div Figure 2b. Pulse Response with 10pF Feed-Forward Compensation 10 Maxim Integrated
