Datasheet
Data Sheet AD797
Rev. I | Page 13 of 20
BYPASSING CONSIDERATIONS
Taking full advantage of the very wide bandwidth and dynamic
range capabilities of the AD797 requires some precautions.
First, multiple bypassing is recommended in any precision
application. A 1.0 μF to 4.7 μF tantalum in parallel with 0.1 μF
ceramic bypass capacitors are sufficient in most applications.
When driving heavy loads, a larger demand is placed on the
supply bypassing. In this case, selective use of larger values of
tantalum capacitors and damping of their lead inductance with
small-value (1.1 Ω to 4.7 Ω) carbon resistors can achieve an
improvement. Figure 36 summarizes power supply bypassing
recommendations.
USE SHORT
LEAD LENGTHS
(<5mm)
KELVIN RETURN
LOAD
CURRENT
OR
0
.1µ
F
V
S
V
S
4.7µF
00846-035
USE SHORT
LEAD LENGTHS
(<5mm)
KELVIN RETURN
LOAD
CURRENT
0.1µF 4.7µF TO 22.0µF
1.1Ω TO 4.7Ω
Figure 36. Recommended Power Supply Bypassing
THE NONINVERTING CONFIGURATION
Ultralow noise requires very low values of the internal parasitic
resistance (r
BB
) for the input transistors (≈6 Ω). This implies
very little damping of input and output reactive interactions.
With the AD797, additional input series damping is required
for stability with direct output to input feedback. A 100 Ω
resistor (R1) in the inverting input (Figure 37) is sufficient; the
100 Ω balancing resistor (R2) is recommended but is not
required for stability. The noise penalty is minimal (e
N
total ≈
2.1 nV/√Hz), which is usually insignicant.
7
4
3
R2
100Ω
R1
100Ω
*
*
V
O
U
T
V
I
N
+V
S
–V
S
AD797
00846-036
R
L
600Ω
6
2
*
USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 37. Voltage Follower Connection
Best response flatness is obtained with the addition of a small
capacitor (C
L
< 33 pF) in parallel with the 100 Ω resistor
(Figure 38). The input source resistance and capacitance also
affect the response slightly, and experimentation may be
necessary for best results.
7
*
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
*
V
O
U
T
V
I
N
+V
S
–V
S
AD797
00846-037
C
L
6
2
R
S
C
S
3
4
600Ω
100Ω
Figure 38. Alternative Voltage Follower Connection
Low noise preamplification is usually performed in the non-
inverting mode (Figure 39). For lowest noise, the equivalent
resistance of the feedback network should be as low as possible.
The 30 mA minimum drive current of the AD797 makes it easier
to achieve this. The feedback resistors can be made as low as
possible, with consideration to load drive and power consumption.
7
*
*
V
O
U
T
V
I
N
+V
S
–V
S
AD797
00846-038
R
L
C
L
R2
R1
6
2
3
4
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 39. Low Noise Preamplifier
Table 6 provides some representative values for the AD797 when
used as a low noise follower. Operation on 5 V supplies allows
the use of a 100 Ω or less feedback network (R1 + R2). Because
the AD797 shows no unusual behavior when operating near its
maximum rated current, it is suitable for driving the AD600/
AD602 (see Figure 51) while preserving low noise performance.
Optimum flatness and stability at noise gains >1 sometimes require
a small capacitor (C
L
) connected across the feedback resistor (R1 of
Figure 39). Table 6 includes recommended values of C
L
for several
gains. In general, when R2 is greater than 100 Ω and C
L
is greater
than 33 pF, a 100 Ω resistor should be placed in series with C
L
.
Source resistance matching is assumed, and the AD797 should not
be operated with unbalanced source resistance >200 kΩ/G.
Table 6. Values for Follower with Gain Circuit
Gain R1 R2 C
L
Noise
(Excluding R
S
)
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