8/21/97 4:00 PM
OP297
–7–REV. D
Figure 24. Open Loop Output
Impedance vs Frequency
APPLICATIONS INFORMATION
Extremely low bias current over the full military temperature
range makes the OP297 attractive for use in sample-and-hold
amplifiers, peak detectors, and log amplifiers that must operate
over a wide temperature range. Balancing input resistances is
not necessary with the OP297 Offset voltage and TCV
OS
are
degraded only minimally by high source resistance, even when
unbalanced.
The input pins of the OP297 are protected against large differ-
ential voltage by back-to-back diodes and current-limiting resis-
tors. Common-mode voltages at the inputs are not restricted,
and may vary over the full range of the supply voltages used.
The OP297 requires very little operating headroom about the
supply rails, and is specified for operation with supplies as low
as +2 V. Typically, the common-mode range extends to within
one volt of either rail. The output typically swings to within one
volt of the rails when using a 10 kΩ load.
AC PERFORMANCE
The OP297’S AC characteristics are highly stable over its full
operating temperature range. Unity-gain small-signal response is
shown in Figure 25. Extremely tolerant of capacitive loading on
the output, the OP297 displays excellent response even with
1000 pF loads (Figure 26).
10
0%
100
90
5µs
20mV
Figure 25. Small-Signal Transient Response
(C
LOAD
= 100 pF, A
VCL
= +1)
10
0%
100
90
5µs
20mV
Figure 26. Small-Signal Transient Response
(C
LOAD
= 1000 pF, A
VCL
= +1)
10
0%
100
90
5µs
20mV
Figure 27. Large-Signal Transient Response
(A
VCL
= +1)
GUARDING AND SHIELDING
To maintain the extremely high input impedances of the
OP297, care must be taken in circuit board layout and manu-
facturing. Board surfaces must be kept scrupulously clean and
free of moisture. Conformal coating is recommended to provide
T
A
= +125°C
T
A
= –55°C
GAIN
PHASE
V
S
= ±15V
C
L
= 30pF
R
L
= 1MΩ
FREQUENCY (Hz)
OPEN-LOOP GAIN (dB)
100
80
60
40
20
0
–20
–40
100
1k 10k 100 1M 10M
90
135
180
225
PHASE SHIFT (DEG)
+EDGE
–EDGE
T
A
= +25°C
V
S
= ±15V
A
VCL
= +1
V
OUT
= 100mV
p-p
OVERSHOOT (%)
70
60
50
40
30
20
10
0
LOAD CAPACITANCE (pF)
10
100
1000
10000
T
A
= +25°C
V
S
= ±15V
FREQUENCY (Hz)
OUTPUT IMPEDANCE (Ω)
1000
100
10
1
0.1
0.01
0.001
10 100 1k 10k 100k
1M
Figure 22. Open Loop Gain,
Phase vs. Frequency
Figure 23. Small Signal Overshoot
vs. Capacitance Load
