LT1812 Linear Technology, LT1812 Datasheet - Page 10

• Manufacturer Part Number: LT1812
• Description: 3mA/ 100MHz/ 750V/us Operational Amplifier with Shutdown
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LT1812
Layout and Passive Components
The LT1812 amplifier is more tolerant of less than ideal
layouts than other high speed amplifiers. For maximum
performance (for example, fast settling) use a ground
plane, short lead lengths and RF-quality bypass capacitors
(0.01 F to 0.1 F). For high drive current applications, use
low ESR bypass capacitors (1 F to 10 F tantalum).
The parallel combination of the feedback resistor and gain
setting resistor on the inverting input combine with the
input capacitance to form a pole that can cause peaking or
even oscillations. If feedback resistors greater than 2k are
used, a parallel capacitor of value
should be used to cancel the input pole and optimize
dynamic performance. For applications where the DC
noise gain is 1 and a large feedback resistor is used, C
should be greater than or equal to C
be an I-to-V converter.
Input Considerations
Each of the LT1812 amplifier inputs is the base of an NPN
and PNP transistor whose base currents are of opposite
polarity and provide first-order bias current cancellation.
Because of variation in the matching of NPN and PNP beta,
the polarity of the input bias current can be positive or
negative. The offset current does not depend on beta
matching and is well controlled. The use of balanced
source resistance at each input is recommended for
applications where DC accuracy must be maximized. The
inputs can withstand differential input voltages of up to 3V
without damage and need no clamping or source resis-
tance for protection.
The device should not be used as a comparator because
with sustained differential inputs, excessive power dissi-
pation may result.
Capacitive Loading
The LT1812 is stable with a 1000pF capacitive load, which
is outstanding for a 100MHz amplifier. This is accom-
plished by sensing the load induced output pole and
adding compensation at the amplifier gain node. As the
10
C
F
> R
G
• C
IN
/R
F
U
U
W
IN
. An example would
U
F
capacitive load increases, both the bandwidth and phase
margin decrease so there will be peaking in the frequency
domain and in the transient response. Coaxial cable can be
driven directly, but for best pulse fidelity, a resistor of
value equal to the characteristic impedance of the cable
(i.e., 75 ) should be placed in series with the output. The
other end of the cable should be terminated with the same
value resistor to ground.
Slew Rate
The slew rate is proportional to the differential input
voltage. Highest slew rates are therefore seen in the
lowest gain configurations. For example, a 5V output step
in a gain of 10 has a 0.5V input step, whereas in unity gain
there is a 5V input step. The LT1812 is tested for slew rate
in a gain of – 1. Lower slew rates occur in higher gain
configurations.
Shutdown
The LT1812 has a shutdown pin (SHDN, Pin 8) for
conserving power. When this pin is open or biased at
least 2V above the negative supply, the part operates
normally. When pulled down to V
drops to about 50 A. Typically, the turn-off delay is 1 s
and the turn-on delay 0.5 s. The current out of the SHDN
pin is also typically 50 A. In shutdown mode, the ampli-
fier output is not isolated from the inputs, so the LT1812
shutdown feature cannot be used for multiplexing appli-
cations. The 50 A typical shutdown current is exclusive
of any output (load) current. In order to prevent load
current (and maximize the power savings), either the
load needs to be disconnected, or the input signal needs
to be 0V. Even in shutdown mode, the LT1812 can still
drive significant current into a load. For example, in an
A
LT1812 drives 2mA into a 100
500 s for the load current to reach this value.
Power Dissipation
The LT1812 combines high speed and large output drive
in a small package. It is possible to exceed the maximum
junction temperature under certain conditions. Maximum
V
= 1 configuration, when driven with a 1V DC input, the
–
load. It takes about
, the supply current