AD797
V
V
S
S
Low noise preamplification is usually performed in the non-
inverting mode (Figure 38). 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.
0.1µF
4.7µF TO 22.0µF
OR
4.7µF
0.1µF
1.1Ω TO 4.7Ω
KELVIN RETURN
KELVIN RETURN
USE SHORT
LEAD LENGTHS
(<5mm)
USE SHORT
LEAD LENGTHS
(<5mm)
C
L
LOAD
LOAD
CURRENT
CURRENT
R2
Figure 35. Recommended Power Supply Bypassing
+V
S
THE NONINVERTING CONFIGURATION
*
Ultralow noise requires very low values of the internal parasitic
resistance (rBB) 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 36) is sufficient; the
100 Ω balancing resistor (R2) is recommended but is not
required for stability. The noise penalty is minimal (eNtotal ≈
2.1 nV/√Hz), which is usually insignificant.
R1
2
3
7
6
*
AD797
R
L
4
–V
S
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 38. Low Noise Preamplifier
Table 4 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 50) while preserving low noise
performance.
R1
100Ω
+V
S
*
2
3
7
6
*
AD797
R2
100Ω
R
600Ω
L
4
Optimum flatness and stability at noise gains >1 sometimes
require a small capacitor (CL) connected across the feedback
resistor (R1 of Figure 38). Table 4 includes recommended values
of CL for several gains. In general, when R2 is greater than
100 Ω and CL is greater than 33 pF, a 100 Ω resistor should be
placed in series with CL. Source resistance matching is assumed,
and the AD797 should not be operated with unbalanced source
resistance >200 kΩ/G.
–V
S
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 36. Voltage Follower Connection
Best response flatness is obtained with the addition of a small
capacitor (CL < 33 pF) in parallel with the 100 Ω resistor
(Figure 37). The input source resistance and capacitance also
affect the response slightly, and experimentation may be
necessary for best results.
Table 4. Values for Follower with Gain Circuit
Noise
C
L
Gain
2
2
ꢀ0
20
>3±
R1
R2
CL
(Excluding RS)
3.0 nV/√Hz
ꢀ.8 nV/√Hz
ꢀ.2 nV/√Hz
ꢀ.0 nV/√Hz
0.98 nV/√Hz
ꢀ kΩ
ꢀ kΩ
≈20 pF
≈ꢀ0 pF
≈± pF
100Ω
300 Ω
33.2 Ω
ꢀ6.± Ω
ꢀ0 Ω
300 Ω
300 Ω
3ꢀ6 Ω
(G − ꢀ) × ꢀ0 Ω
+V
S
*
2
3
7
AD797
6
*
R
S
The I-to-V converter is a special case of the follower configu-
ration. When the AD797 is used in an I-to-V converter, for
example as a DAC buffer, the circuit shown in Figure 39 should
be used. The value of CL depends on the DAC, and if CL is greater
than 33 pF, a 100 Ω series resistor is required. A bypassed balancing
resistor (RS and CS) can be included to minimize dc errors.
600Ω
4
C
S
–V
S
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 37. Alternative Voltage Follower Connection
Rev. F | Page ꢀ3 of 20
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