ad797bn Analog Devices, Inc., ad797bn Datasheet - Page 15

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

DRIVING CAPACITIVE LOADS
The capacitive load driving capabilities of the AD797 are
displayed in Figure 41. At gains over 10, usually no special
precautions are necessary. If more drive is desirable the circuit
in Figure 42 should be used. Here a 5000 pF load can be driven
cleanly at any noise gain ≥2.
SETTLING TIME
The AD797 is unique among ultralow noise amplifiers in that it
settles to 16 bits (<150 μV) in less than 800 ns. Measuring this
performance presents a challenge. A special test setup
(Figure 43) was developed for this purpose. The input signal
was obtained from a resonant reed switch pulse generator,
available from Tektronix as calibration Fixture No. 067-0608-00.
When open, the switch is simply 50 Ω to ground and settling is
purely a passive pulse decay and inherently flat. The low repe-
tition rate signal was captured on a digital oscilloscope after
being amplified and clamped twice. The selection of plug-in for
the oscilloscope was made for minimum overload recovery.
Figure 42. Recommended Circuit for Driving a High Capacitance Load
Figure 41. Capacitive Load Drive Capability vs. Closed-Loop Gain
100nF
100pF
10nF
10pF
1nF
1pF
1kΩ
1
**Use Power Supply Bypassing Shown in Figure 35
200pF
2
3
AD797
+V
–V
20pF
1kΩ
7
4
10
S
S
CLOSED-LOOP GAIN
100Ω
**
**
6
33Ω
100
C1
1k
Rev. E | Page 15 of 20
DISTORTION REDUCTION
The AD797 has distortion performance (THD < −120 dB,
@ 20 kHz, 3 V rms, R
feedback amplifiers.
At higher gains and higher frequencies, THD increases due to
reduction in loop gain. However, in contrast to most conven-
tional voltage feedback amplifiers, the AD797 provides two
effective means of reducing distortion as gain and frequency
are increased: cancellation of the output stage’s distortion, and
gain bandwidth enhancement by decompensation. By applying
these techniques, gain bandwidth can be increased to 450 MHz
at G = 1000, and distortion can be held to −100 dB at 20 kHz for
G = 100.
The unique design of the AD797 provides for cancellation of
the output stage’s distortion. To achieve this, a capacitance equal
to the effective compensation capacitance, usually 50 pF, is
connected between Pin 8 and the output (C2 in Figure 42).
Use of this feature improves distortion performance when the
closed-loop gain is more than 10 or when frequencies of interest
are greater than 30 kHz.
Bandwidth enhancement via decompensation is achieved by
connecting a capacitor from Pin 8 to ground (C1 in Figure 44)
effectively subtracting from the value of the internal
compensation capacitance (50 pF), yielding a smaller effective
compensation capacitance and, therefore, a larger bandwidth.
CALIBRATION
TEKTRONIX
FIXTURE
HP2835
2x
V
Figure 43. Settling Time Test Circuit
IN
226Ω
L
1μF
1kΩ
= 600 Ω) unequaled by most voltage
0.47μF
1kΩ
2
3
AD829
100Ω
2
3
4.26kΩ
0.1μF
A2
AD797
–V
4
A1
OSCILLOSCOPE
S
–V
PREAMP INPUT
TO TEKTRONIX
4
1kΩ
+V
7
S
S
+V
7
SECTION
1kΩ
S
6
1μF
20pF
0.47μF
6
7A26
(VIA LESS THAN 1FT
50Ω COAXIAL CABLE)
250Ω
1MΩ
NOTE:
USE CIRCUIT
BOARD
WITH GROUND
PLANE
0.1μF
51pF
AD797
2x
HP2835
20pF