ad797bn Analog Devices, Inc., ad797bn Datasheet - Page 13

• Manufacturer Part Number: ad797bn
• Description: Ultralow Distortion, Ultralow Noise Op Amp
• Manufacturer: Analog Devices, Inc.

0.1μF
WIDEBAND NOISE
Due to its single stage design, the noise of the AD797 is flat over
frequencies from less than 10 Hz to beyond 1 MHz. This is not
true of most dc precision amplifiers where second stage noise
contributes to input referred noise beyond the audio frequency
range. The AD797 offers new levels of performance in wide-
band imaging applications. In sampled data systems, where
aliasing of out of band noise into the signal band is a problem,
the AD797 outperforms all previously available IC op amps.
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 be an improve-
ment. Figure 35 summarizes bypassing recommendations. The
symbol (**) is used throughout this data sheet to represent the
parallel combination of a 0.1 μF and a 4.7 μF capacitor.
USE SHORT
LEAD LENGTHS
(< 5mm)
1Ω
Figure 34. Test Setup for Measuring 0.1 Hz to 10 Hz Noise
Figure 35. Recommended Power Supply Bypassing
**Use Power Supply Bypassing Shown in Figure 35
4.7μF
2
3
KELVIN RETURN
AD797
V
LOAD
CURRENT
100kΩ
S
+V
–V
7
4
S
S
OR
**
**
6
V
OUT
0.1μF
USE SHORT
LEAD LENGTHS
(< 5mm)
1.5μF
HP 3465
DYNAMIC SIGNAL
ANALYZER
(10Hz)
1.1μF TO 4.7μF
4.7μF TO 22.0μF
KELVIN RETURN
V
LOAD
CURRENT
S
Rev. E | Page 13 of 20
THE NONINVERTING CONFIGURATION
Ultralow noise requires very low values of r
parasitic resistance) 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 input to output feedback.
A 100 Ω resistor 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 (e
2.1 nV/√Hz), which is usually insignificant. Best response
flatness is obtained with the addition of a small capacitor
(C
input source resistance and capacitance also affects the response
slightly, and experimentation may be necessary for best results.
Low noise preamplification is usually done in the noninverting
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
due consideration to load drive and power consumption.
Table 4 gives some representative values for the AD797 as a low
noise follower. Operation on 5 volt 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
(Figure 50) while preserving their low noise performance.
Optimum flatness and stability at noise gains >1 sometimes
require a small capacitor (C
resistor (R1, Figure 38). Table 4 includes recommended values
of C
100 Ω and C
placed in series with C
and the AD797 should never be operated with unbalanced
source resistance >200 kΩ/G.
L
< 33 pF) in parallel with the 100 Ω resistor (Figure 37). The
L
for several gains. In general, when R2 is greater than
**Use Power Supply Bypassing Shown in Figure 35
L
is greater than 33 pF, a 100 Ω resistor should be
100Ω
R2
Figure 36. Voltage Follower Connection
2
3
L
. Source resistance matching is assumed,
AD797
100Ω
L
) connected across the feedback
R1
+V
–V
7
4
S
S
**
6
**
BB
(the internal
R
600Ω
L
N
AD797
total ≈