Datasheet AD797 (Analog Devices) - 13

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Data Sheet AD797 BYPASSING CONSIDERATIONS CL
Taking full advantage of the very wide bandwidth and dynamic
100Ω
range capabilities of the AD797 requires some precautions. First, multiple bypassing is recommended in any precision
+VS
application. A 1.0 μF to 4.7 μF tantalum in parallel with 0.1 μF
*
ceramic bypass capacitors are sufficient in most applications.
2 7
When driving heavy loads, a larger demand is placed on the
AD797 6 VOUT
supply bypassing. In this case, selective use of larger values of
RS V 600Ω IN 3 4
tantalum capacitors and damping of their lead inductance with
* C
small-value (1.1 Ω to 4.7 Ω) carbon resistors can achieve an
S –V
7 improvement. Figure 36 summarizes power supply bypassing
S
03 6- 84 recommendations.
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
00 Figure 38. Alternative Voltage Follower Connection
VS VS
Low noise preamplification is usually performed in the non-
OR 0.1µF 4.7µF TO 22.0µF
inverting mode (see Figure 39). For lowest noise, the equivalent
0.1µF 4.7µF 1.1Ω TO 4.7Ω
resistance of the feedback network should be as low as possible.
KELVIN RETURN KELVIN RETURN
The 30 mA minimum drive current of the AD797 makes it easier to achieve this. The feedback resistors can be made as low as
USE SHORT USE SHORT LEAD LENGTHS LEAD LENGTHS
possible, with consideration to load drive and power consumption.
(<5mm) LOAD (<5mm) LOAD CURRENT
035
CURRENT CL
00846- Figure 36. Recommended Power Supply Bypassing
R2 THE NONINVERTING CONFIGURATION +VS
Ultralow noise requires very low values of the internal parasitic
*
resistance (rBB) for the input transistors (≈6 Ω). This implies
R1 2 7
very little damping of input and output reactive interactions. With the AD797, additional input series damping is required
AD797 6 VOUT R
for stability with direct output to input feedback. A 100 Ω
VIN L 3 4 *
resistor (R1) in the inverting input (see Figure 37) is sufficient; 8 the 100 Ω balancing resistor (R2) is recommended but is not
–VS
03 6- required for stability. The noise penalty is minimal (e 84 Ntotal ≈
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
00 2.1 nV/√Hz), which is usually insignificant. Figure 39. Low Noise Preamplifier
R1 100Ω
Table 6 provides some representative values for the AD797 when used as a low noise follower. Operation on 5 V supplies allows
+VS
the use of a 100 Ω or less feedback network (R1 + R2). Because
*
the AD797 shows no unusual behavior when operating near its
2 7
maximum rated current, it is suitable for driving the AD600/
R2 6 V AD797 OUT
AD602 (see Figure 51) while preserving low noise performance.
100Ω R V L IN 3 4 600Ω
Optimum flatness and stability at noise gains >1 sometimes require
*
a small capacitor (CL) connected across the feedback resistor (R1 of
–V
6
S
03 Figure 39). Table 6 includes recommended values of CL for several 6- 84
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
00 gains. In general, when R2 is greater than 100 Ω and CL is greater Figure 37. Voltage Follower Connection than 33 pF, a 100 Ω resistor should be placed in series with CL. Source resistance matching is assumed, and the AD797 should not Best response flatness is obtained with the addition of a small be operated with unbalanced source resistance >200 kΩ/G. capacitor (CL < 33 pF) in parallel with the 100 Ω resistor (Figure 38). The input source resistance and capacitance also
Table 6. Values for Follower with Gain Circuit
affect the response slightly, and experimentation may be
Noise
necessary for best results.
Gain R1 R2 CL (Excluding RS)
2 1 kΩ 1 kΩ ≈ 20 pF 3.0 nV/√Hz 2 300 Ω 300 Ω ≈ 10 pF 1.8 nV/√Hz 10 33.2 Ω 300 Ω ≈ 5 pF 1.2 nV/√Hz 20 16.5 Ω 316 Ω 1.0 nV/√Hz >35 10 Ω (G − 1) × 10 Ω 0.98 nV/√Hz Rev. K | Page 13 of 19 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION THERMAL RESISTANCE ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION NOISE AND SOURCE IMPEDANCE CONSIDERATIONS LOW FREQUENCY NOISE WIDEBAND NOISE BYPASSING CONSIDERATIONS THE NONINVERTING CONFIGURATION THE INVERTING CONFIGURATION DRIVING CAPACITIVE LOADS SETTLING TIME DISTORTION REDUCTION Differential Line Receiver A General-Purpose ATE/Instrumentation I/O Driver Ultrasound/Sonar Imaging Preamp Amorphous (Photodiode) Detector Professional Audio Signal Processing—DAC Buffers OUTLINE DIMENSIONS ORDERING GUIDE