LT1812 Просмотр технического описания (PDF) - Linear Technology

Номер в каталоге

Компоненты Описание

производитель

LT1812

3mA, 100MHz, 750V/µs Operational Amplifier with Shutdown

Linear Technology 

LT1812

APPLICATIONS INFORMATION

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

(0.01μF to 0.1μF). For high drive current applications, use

low ESR bypass capacitors (1μF to 10μF 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

CF > RG • CIN/RF

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, CF

should be greater than or equal to CIN. An example would

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 resistance 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

accomplished by sensing the load induced output pole

and adding compensation at the amplifier gain node. As

the 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., 75Ω) 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 –, the supply current

drops to about 50μA. Typically, the turn-off delay is

1μs and the turn-on delay 0.5μs. The current out of the

SHDN pin is also typically 50μA. In shutdown mode, the

amplifier output is not isolated from the inputs, so the

LT1812 shutdown feature cannot be used for multiplexing

applications. The 50μA 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 AV = 1 configuration, when driven with a 1V DC input,

the LT1812 drives 2mA into a 100Ω load. It takes about

500μs 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

1812fb

11

Share Link: