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AD22057 Scheda tecnica(PDF) 7 Page - Analog Devices |
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AD22057 Scheda tecnica(HTML) 7 Page - Analog Devices |
7 / 8 page AD22057 –7– REV. A APPLICATION HINTS Frequency Compensation As are all closed-loop op amp circuits, the AD22057 is sensitive to capacitive loading at its output. However, the AD22057 is sensitive at higher output voltages due to nonlinear effects in the rail-to-rail design of the buffer amplifier (A2). In this amplifier the output stage gain increases with increasing output voltage. This behavior does not affect dc parameters such as gain accuracy or linearity; however, it can compromise ac sta- bility. When operating from a power supply of 5 V or less (and, therefore, VOUT < 5 V), the AD22057 can drive capacitive loads up to 25 pF with no external components. When operat- ing at higher supply voltages (which are associated with higher output voltages) and/or driving larger capacitive loads, an exter- nal compensation network should be used. Figure 14 shows an R-C “snubber” circuit loading the output of the AD22057. This combination, in conjunction with the internal 20 k Ω resis- tance, forms a lag network. This network attenuates the open- loop gain of the amplifier at higher frequencies. The ratio of RLAG to the load seen by the AD22057 determines the high frequency attenuation seen by the op amp. If RLAG is made 1/20th of the total load resistance ( ≈20 kΩ R L), then 26 dB of attenuation is obtained at higher frequencies. The capacitor (CLAG) is used to control the frequency of the compensation network. It should be set to form a 5 µs time constant with the resistor (RLAG). Table I shows the recommended values of RLAG and CLAG for various values of external load resistor RL. Ten percent tolerance on these components is acceptable. Alternatively, the signal may be taken from the midpoint of RLAG–CLAG. This output is particularly useful when driving CMOS analog-to-digital converters. For more information see the section Driving Charged Redistributed A/D Converters. Note that when implementing this network large signal re- sponse is compromised. This occurs because there is no active pull-down and the lag capacitor must discharge through the internal feedback resistor (20 k Ω) giving a fairly long-time constant. For example if CLAG = 0.01 µF, the large signal negative slew characteristic is a decaying exponential with a time constant of ≈200 µs. Table I. Compensation Components vs. External Load Resistor RL RLAG CLAG >100 k Ω 470 Ω 0.01 µF > 50 k Ω 390 Ω 0.01 µF > 20 k Ω 270 Ω 0.047 µF > 10 k Ω 200 Ω 0.047 µF > 5 k Ω 100 Ω 0.1 µF > 2 k Ω 47 Ω 0.22 µF Driving Charge Redistribution A/D Converters When driving CMOS ADCs, such as those embedded in popu- lar microcontrollers, the charge injection ( ∆Q) can cause a significant deflection in the AD22057 output voltage. Though generally of short duration, this deflection may persist until after the sample period of the ADC has expired. It is due to the relatively high open-loop output impedance of the AD22057. The effect can be significantly reduced by including the same R-C network recommended for improving stability (see Fre- quency Compensation section). The large capacitor in the lag network helps to absorb the additional charge, effectively lower- ing the high frequency output impedance of the AD22057. For these applications the output signal should be taken from the midpoint of the RLAG–CLAG combination as shown in Figure 15. Since the perturbations from the analog-to-digital converter are small, the output of the AD22057 will appear to be a low impedance. The transient response will, therefore, have a time constant governed by the product of the two lag compo- nents, CLAG × RLAG. For the values shown in Figure 15, this time constant is programmed at approximately 10 µs. There- fore, if samples are taken at several tens of microseconds or more, there will be negligible “stacking up” of the charge injections. 10k +VS 10k LOAD AD22057 A2 RLAG CLAG RL CL Figure 14. Using an R-C Network for Compensation 0.01 F 1k 10k +VS 10k AD22057 A2 PROCESSOR A/D IN Figure 15. Recommended Circuit for Driving CMOS A/D Converters UNDERSTANDING THE AD22057 Figure 16 shows the main elements of the AD22057. The signal inputs at Pins 1 and 8 are first applied to dual resistive attenua- tors R1 through R4, whose purpose is to reduce the common- mode voltage at the input to the preamplifier. The attenuated signal is then applied to a feedback amplifier based on the very low drift op amp, A1. The differential voltage across the inputs is accurately amplified in the presence of common-mode volt- ages of many times the supply voltage. The overall common- mode response is minimized by precise laser trimming of R3 and R4, giving the AD22057 a common-mode rejection ratio (CMRR) of at least 80 dB (10,000:1). The common-mode range of A1 extends from slightly below ground to 1 V below +VS (at the minimum temperature of –40 °C). Since an attenuation ratio of about 6 is used, the input common-mode range is –1 V to +24 V using a +5 V supply. Small filter capacitors C1 and C2 are included to minimize the effects of spurious RF signals at the inputs, which might cause dc errors due to the rectification effects at the input to A1. At high frequencies, even a small imbalance in these components would seriously degrade the CMRR, so a special high frequency trim is also carried out during manufacture. |
Codice articolo simile - AD22057_15 |
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Descrizione simile - AD22057_15 |
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