Datasheet AD8001 (Analog Devices) - 10

AD8001 THEORY OF OPERATION
Considering that additional poles contribute excess phase at A very simple analysis can put the operation of the AD8001, a high frequencies, there is a minimum feedback resistance below current feedback amplifier, in familiar terms. Being a current which peaking or oscillation may result. This fact is used to feedback amplifier, the AD8001’s open-loop behavior is expressed determine the optimum feedback resistance, RF. In practice, as transimpedance, ∆V parasitic capacitance at Pin 2 will also add phase in the feedback O/∆I–IN, or TZ. The open-loop transimped- ance behaves just as the open-loop voltage gain of a voltage loop, so picking an optimum value for RF can be difficult. feedback amplifier, that is, it has a large dc value and decreases Figure 6 illustrates this problem. Here the fine scale (0.1 dB/ at roughly 6 dB/octave in frequency. div) flatness is plotted versus feedback resistance. These plots were taken using an evaluation card which is available to cus- Since the RIN is proportional to 1/gM, the equivalent voltage tomers so that these results may readily be duplicated. gain is just T × Z gM, where the gM in question is the trans- conductance of the input stage. This results in a low open-loop Achieving and maintaining gain flatness of better than 0.1 dB at input impedance at the inverting input, a now familiar result. frequencies above 10 MHz requires careful consideration of Using this amplifier as a follower with gain, Figure 4, basic several issues. analysis yields the following result. V
0.1
O TZ S = ( ) G ×
RF =
V ( ) + × +
0 649
IN TZ S G RIN R1
RF = 698 –0.1
R1 G = 1 + R = 1/ ≈ IN gM 50 Ω
–0.2
R2
G = +2 RF = 750 –0.3 R1 –0.4 OUTPUT – dB –0.5 R2 –0.6 RIN V –0.7 OUT –0.8 VIN –0.9 1M 10M 100M FREQUENCY – Hz
Figure 4. Follower with Gain Figure 6. 0.1 dB Flatness vs. Frequency Recognizing that G × R
Choice of Feedback and Gain Resistors
IN << R1 for low gains, it can be seen to the first order that bandwidth for this amplifier is independent Because of the above-mentioned relationship between the band- of gain (G). This simple analysis in conjunction with Figure 5 width and feedback resistor, the fine scale gain flatness will, to can, in fact, predict the behavior of the AD8001 over a wide some extent, vary with feedback resistance. It, therefore, is range of conditions. recommended that once optimum resistor values have been determined, 1% tolerance values should be used if it is desired to
1M
maintain flatness over a wide range of production lots. In addition, resistors of different construction have different associated parasitic capacitance and inductance. Surface-mount resistors were used
100k
for the bulk of the characterization for this data sheet. It is not recommended that leaded components be used with the AD8001.
10k – ZT 1k 100 10 100k 1M 10M 100M 1G FREQUENCY – Hz
Figure 5. Transimpedance vs. Frequency –10– REV. D Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAMS SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE MAXIMUM POWER DISSIPATION Typical Performance Characteristics THEORY OF OPERATION Choice of Feedback and Gain Resistors Printed Circuit Board Layout Considerations Power Supply Bypassing DC Errors and Noise Driving Capacitive Loads Communications Operation as a Video Line Driver Driving A-to-D Converters Layout Considerations OUTLINE DIMENSIONS Revision History