13
LT1632/LT1633
sn1632 16323fs
APPLICATIONS INFORMATION
WUU
U
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 LT1632/LT1633 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.
The LT1632/LT1633’s input stages are also protected
against large differential input voltages by a pair of back-
to-back diodes D5/D8. When a differential voltage of
more than 1.4V is applied to the inputs, these diodes will
turn on, preventing the emitter-base breakdown of the
input transistors. The current in D5/D8 should be limited
to less than 10mA. Internal 225Ω 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 LT1632/LT1633 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 20Ω to
50Ω should be connected between the output and the
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 LT1632/LT1633 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 LT1632/LT1633 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 6MHz cross-
ing frequency and a 55° 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 75° at 5MHz!
The solution is simple: either lower the value of the
resistors or add a feedback capacitor of 10pF or more.
TYPICAL APPLICATIONS
U
Single Supply, 40dB Gain, 550kHz Instrumentation
Amplifier
An instrumentation amplifier with a rail-to-rail output
swing, operating from a 3V supply can be constructed with
the LT1632 as shown in the first page of this data sheet.
The amplifier has a nominal gain of 100, which can be
adjusted with resistor R5. The DC output level is set by the
difference of the two inputs multiplied by the gain of 100.
The voltage gain and the DC output level can be
expressed as follows:
