Datasheet AD815 (Analog Devices) - 10

AD815 Choice of Feedback and Gain Resistors G = –1
The fine scale gain flatness will, to some extent, vary with
R SIDE A F = 562
⍀ feedback resistance. It therefore is recommended that once
RL = 100
⍀ optimum resistor values have been determined, 1% tolerance values should be used if it is desired to maintain flatness over
SIDE B
a wide range of production lots. Table I shows optimum values for several useful configurations. These should be used as starting point in any application.
Table I. Resistor Values RF (
⍀
) RG (
⍀
) 1V 20ns
G = +1 562 ⬁ –1 499 499 Figure 42. 4 V Step Response, G = –1 +2 499 499 +5 499 125
THEORY OF OPERATION
+10 1 k 110 The AD815 is a dual current feedback amplifier with high (500 mA) output current capability. Being a current feedback amplifier, the AD815’s open-loop behavior is expressed
PRINTED CIRCUIT BOARD LAYOUT
as transimpedance, ∆VO/∆I–IN, or TZ. The open-loop
CONSIDERATIONS
transimpedance behaves just as the open-loop voltage gain As to be expected for a wideband amplifier, PC board parasitics of a voltage feedback amplifier, that is, it has a large dc value can affect the overall closed-loop performance. Of concern are and decreases at roughly 6 dB/octave in frequency. stray capacitances at the output and the inverting input nodes. If Since R a ground plane is to be used on the same side of the board as IN is proportional to 1/gM, the equivalent voltage gain is just T the signal traces, a space (5 mm min) should be left around the Z × gM, where the gM in question is the transconductance of the input stage. Using this amplifier as a follower with gain, signal lines to minimize coupling. Figure 43, basic analysis yields the following result:
POWER SUPPLY BYPASSING
V T S ( ) O Z Adequate power supply bypassing can be critical when optimizing = G × V T S + R the performance of a high frequency circuit. Inductance in the ( )+G × R IN Z IN F power supply leads can form resonant circuits that produce where: peaking in the amplifier’s response. In addition, if large current transients must be delivered to the load, then bypass capacitors R G F = 1 + (typically greater than 1 µF) will be required to provide the best RG settling time and lowest distortion. A parallel combination of RIN = 1/gM ≈ 25 Ω 10.0 µF and 0.1 µF is recommended. Under some low frequency applications, a bypass capacitance of greater than 10
R
µF may be
F
necessary. Due to the large load currents delivered by the
RG
AD815, special consideration must be given to careful bypassing. The ground returns on both supply bypass capacitors as well as
RIN V
signal common must be “star” connected as shown in Figure 44.
OUT RN +VS VIN +IN +OUT R
Figure 43. Current Feedback Amplifier Operation
F RG
Recognizing that G × R
R
IN << RF for low gains, it can be seen to
(OPTIONAL) F
the first order that bandwidth for this amplifier is independent
–OUT
of gain (G). Considering that additional poles contribute excess phase at
–IN
high frequencies, there is a minimum feedback resistance below
–V
which peaking or oscillation may result. This fact is used to
S
determine the optimum feedback resistance, RF. In practice parasitic capacitance at the inverting input terminal will also add Figure 44. Signal Ground Connected in “Star” phase in the feedback loop, so picking an optimum value for RF Configuration can be difficult. Achieving and maintaining gain flatness of better than 0.1 dB at frequencies above 10 MHz requires careful consideration of several issues. –10– REV. D Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION 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 POWER CONSIDERATIONS Other Power Considerations Parallel Operation Differential Operation Creating Differential Signals Direct Single-Ended-to-Differential Conversion Twelve Channel Video Distribution Amplifier OUTLINE DIMENSIONS Ordering Guide Revision History