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ADA4817-1ACPZ-R2 Datasheet(PDF) 13 Page - Analog Devices |
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ADA4817-1ACPZ-R2 Datasheet(HTML) 13 Page - Analog Devices |
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13 / 24 page  ADA4817-1/ADA4817-2 Rev. 0 | Page 13 of 24 THEORY OF OPERATION The ADA4817-1/ADA4817-2 are voltage feedback operational amplifiers that combine new architecture for FET input opera- tional amplifiers with the eXtra Fast Complementary Bipolar (XFCB) process from Analog Devices resulting in an outstanding combination of speed and low noise. The innovative high speed FET input stage handles common-mode signals from the nega- tive supply to within 2.3 V of the positive rail. This stage is combined with an H-bridge to attain a 870 V/μs slew rate and low distortion, in addition to 4 nV/√Hz input voltage noise. The amplifier features a high speed output stage capable of driving heavy loads sourcing and sinking up to 70 mA of linear current. Supply current and offset current are laser trimmed for optimum performance. These specifications make the ADA4817-1/ ADA4817-2 a great choice for high speed instrumentation and high resolution data acquisition systems. Its low noise, picoamp input current, precision offset, and high speed make them superb preamps for fast photodiode applications. CLOSED-LOOP FREQUENCY RESPONSE The ADA4817-1/ADA4817-2 are classic voltage feedback amplifiers with an open-loop frequency response that can be approximated as the integrator response shown in Figure 40. Basic closed-loop frequency response for inverting and nonin- verting configurations can be derived from the schematics shown in Figure 38 and Figure 39. RF A VOUT RG VIN VE Figure 38. Noninverting Configuration RF VE A VOUT RG VIN Figure 39. Inverting Configuration NONINVERTING CLOSED-LOOP FREQUENCY RESPONSE Solving for the transfer function, ( ) () G CROSSOVER G F F G CROSSOVER I O R f S R R R R f V V × × π + + + × π = 2 2 (4) where fCROSSOVER is the frequency where the amplifier’s open-loop gain equals 0 dB. At dc G G F I O R R R V V + = (5) Closed-loop −3 dB frequency G F G CROSSOVER 3dB R R R f f + × = − (6) INVERTING CLOSED-LOOP FREQUENCY RESPONSE Solving for the transfer function, () G CROSSOVER G F F CROSSOVER I O R f S R R R f V V × × π + + × × π − = 2 2 (7) At dc G F I O R R V V − = (8) Solve for closed-loop −3 dB frequency by, G F G CROSSOVER dB R R R f f + × = −3 (9) FREQUENCY (MHz) 80 60 0.1 1000 1 100 10 40 20 0 fCROSSOVER = 410MHz A = (2π × fCROSSOVER)/s Figure 40. Open-Loop Gain vs. Frequency and Basic Connections The closed-loop bandwidth is inversely proportional to the noise gain of the op amp circuit, (RF + RG)/RG. This simple model is accurate for noise gains above 2. The actual bandwidth of circuits with noise gains at or below 2 is higher than those predicted with this model due to the influence of other poles in the frequency response of the real op amp. Figure 41 shows a voltage feedback amplifier’s dc errors. For both inverting and noninverting configurations, () ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + + × − ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ + × = − + G F G OS F b G F G S b OUT R R R V R I R R R R I error V (10) RF A RG Ib– RS Ib+ +VOS – VOUT VIN Figure 41. Voltage Feedback Amplifier’s DC Errors |
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