LT1492 Linear Technology, LT1492 Datasheet - Page 12

• Manufacturer Part Number: LT1492
• Description: 5MHz/ 3V/us/ Low Power Single Supply/ Dual and Quad Precision Op Amps
• Manufacturer: Linear Technology

APPLICATIONS
LT1492/LT1493
Feedback Components
Because the input currents of the LT1492/LT1493 are less
than 100nA, it is possible to use high value feedback
resistors to set the gain. However, care must be taken to
insure that the pole that is formed by the feedback resis-
tors and the input capacitance does not degrade the
stability of the amplifier. For example, if a single supply,
noninverting gain of 2 is set with two 62k resistors, the
LT1492/LT1493 will probably oscillate. This is because
the amplifier goes open-loop at 1MHz (6dB of gain) and
has 50 of phase margin. The feedback resistors and the
10pF input capacitance generate a pole at 500kHz that
introduces 63 of phase shift at 1MHz! The solution is
simple, lower the values of the resistors or add a feedback
capacitor of 10pF or more.
Capacitive Loading
The LT1492/LT1493 are optimized for low voltage, single
supply operation. The Maximum Capacitive Load vs Load
Current typical performance curve shows the circuit per-
formance without any load resistance. When the output is
sinking current or driving heavy loads with high supplies,
capacitive load handling is degraded.
12
10000
Maximum Capacitive Load vs Load Current
1000
100
10
0.1
SOURCING
SINKING
V
V
S
U
S
= 2.5V
= 15V
LOAD CURRENT (mA)
V
S
1
= 2.5V
INFORMATION
V
U
S
= 15V
10
W
1492/93 AI01
100
U
Comparator Applications
Sometimes it is desirable to use an op amp as a compara-
tor. When operating the LT1492/LT1493 on a single 3.3V
or 5V supply, the output interfaces directly with most TTL
and CMOS logic.
The response time of the LT1492/LT1493 is a strong
function of the amount of input overdrive as shown in the
following photos. These amplifiers are unity-gain stable
op amps and not fast comparators, therefore, the logic
being driven may oscillate due to the long transition time.
The output can be speeded up by adding 20mV or more of
hysteresis (positive feedback), but the offset is then a
function of the input direction.
100
100
4
2
0
0
4
2
0
0
V
R
V
R
S
S
L
L
= 5V
=
= 5V
=
20mV, 10mV, 5mV, 2mV Overdrives
20mV, 10mV, 5mV, 2mV Overdrives
Comparator Response (+)
Comparator Response (–)
20 s/DIV
20 s/DIV
1492/93 AI02
1492/93 AI02