LT1630 Linear Technology, LT1630 Datasheet - Page 13

• Manufacturer Part Number: LT1630
• Description: 30MHz/ 10V/us/ Dual/Quad Rail-to-Rail Input and Output Precision Op Amps
• Manufacturer: Linear Technology

APPLICATIONS
Input Offset Voltage
The offset voltage changes depending upon which input
stage is active, and the maximum offset voltages are
trimmed to less than 525 V. To maintain the precision
characteristics of the amplifier, the change of V
entire input common mode range (CMRR) is guaranteed
to be less than 525 V on a single 5V supply.
Input Bias Current
The input bias current polarity depends on the input
common mode voltage. When the PNP differential pair is
active, the input bias currents flow out of the input pins.
They flow in the opposite direction when the NPN input
stage is active. The offset voltage error due to input bias
currents can be minimized by equalizing the noninverting
and inverting input source impedance.
Output
The outputs of the LT1630/LT1631 can deliver large load
currents; the short-circuit current limit is 70mA. Take care
to keep the junction temperature of the IC below the
absolute maximum rating of 150 C (refer to the Power
Dissipation section). The output of these amplifiers have
reverse-biased diodes to each supply. If the output is
forced beyond either supply, unlimited current will flow
through these diodes. If the current is transient and limited
to several hundred mA, no damage to the part will occur.
Overdrive Protection
To prevent the output from reversing polarity when the
input voltage exceeds the power supplies, two pairs of
crossing diodes D1 to D4 are employed. When the input
voltage exceeds either power supply by approximately
700mV, D1/D2 or D3/D4 will turn on, forcing the output to
the proper polarity. For this phase reversal protection to
work properly, the input current must be limited to less
than 5mA. If the amplifier is to be severely overdriven, an
external resistor should be used to limit the overdrive
current.
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The LT1630/LT1631’s input stages are protected against
large differential input voltages by a pair of back-to-back
diodes D5/D6. When a differential voltage of more than
0.7V is applied to the inputs, these diodes will turn on,
preventing the emitter-base breakdown of the input
transistors. The current in D5/D6 should be limited to
less than 10mA. Internal 225 resistors R6 and R7 will
limit the input current for differential input signals of 4.5V
or less. For larger input levels, a resistor in series with
either or both inputs should be used to limit the current.
Worst-case differential input voltage usually occurs when
the output is shorted to ground. In addition, the amplifier
is protected against ESD strikes up to 3kV on all pins.
Capacitive Load
The LT1630/LT1631 are wideband amplifiers that can
drive capacitive loads up to 200pF on 15V supplies in a
unity-gain configuration. On a 3V supply, the capacitive
load should be kept to less than 100pF. When there is a
need to drive larger capacitive loads, a resistor of 20 to
50
capacitive load. The feedback should still be taken from the
output so that the resistor isolates the capacitive load to
ensure stability.
Feedback Components
The low input bias currents of the LT1630/LT1631 make it
possible to use the high value feedback resistors to set the
gain. However, care must be taken to ensure that the pole
formed by the feedback resistors and the total capacitance
at the inverting input does not degrade stability. For
instance, the LT1630/LT1631 in a noninverting gain of 2,
set with two 20k resistors, will probably oscillate with
10pF total input capacitance (5pF input capacitance and
5pF board capacitance). The amplifier has a 5MHz cross-
ing frequency and a 52 phase margin at 6dB of gain. The
feedback resistors and the total input capacitance form a
pole at 1.6MHz that induces a phase shift of 72 at 5MHz!
The solution is simple: either lower the value of the
resistors or add a feedback capacitor of 10pF or more.
should be connected between the output and the
LT1630/LT1631
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