6

LT1311

Description

The LT1311 contains four identical current feedback am-

plifiers with their noninverting inputs tied together at pin

4. An external bias voltage is applied to this pin to set the

quiescent output voltage of each amplifier. Each amplifier

has an internal 20k feedback resistor between the output

and the inverting input. The amplifiers are packaged in a

14-pin SO package with all four inverting inputs on one

side and the outputs on the other. None of the inputs (or

the outputs) are on adjacent pins for excellent channel

separation.

The feedback resistors in the LT1311 are laser-trimmed at

wafer sort to set the current-to-voltage gain. The gain is set

to 20mV/

µA; the change with temperature is typically

– 70ppm/

°C. The gain matching of the four amplifiers is

ten times better. The input offset voltage and bias current

are trimmed as well. The trimming also minimizes the

resulting output offset drift. For more detailed circuit

information, please see the May 1995 (Volume 5, Number

2) issue of

Linear Technology magazine.

Supply Voltages

The LT1311 can be operated on single or split supplies.

The total supply voltage must be greater than 4V and less

than 36V. The bias voltage applied to pin 4 can be any value

from 2V above the negative supply to 2V below the positive

supply. The outputs can swing to within 1V of either

supply.

The LT1311 is trimmed while operating on a single 10V

supply with a bias voltage of 5V; this is the equivalent of

±5V supplies with the bias at ground. Operation on a

single 5V supply with a bias voltage of 2.5V results in very

similar performance. Operation on

±15V supplies results

in slightly more bandwidth and offset (see the electrical

tables and the characteristic curves).

Bypassing the supplies and bias voltage pins requires no

special care. For accurate settling, a 0.1

µFcapacitorwithin

an inch or two of the package works well.

Input Characteristics

The inputs of the LT1311 are low impedance summing

nodes. The current feedback amplifiers in the LT1311 have

an open-loop input impedance of only a few hundred

ohms and therefore the closed-loop response is fairly

independent of stray capacitance on the inputs. This is a

significant advantage over voltage feedback amplifiers

that have to be set up for a particular input capacitance.

The LT1311 settles cleanly with any input capacitance

from zero to 50pF as shown in the characteristic curves.

When the LT1311 is used to convert photo diode currents

to signal voltages, the LT1311 does not have to be located

close to the diodes.

Output Characteristics

The outputs of the LT1311 are complementary emitter

followers. The outputs will swing to within 1V of the

supplies with no load, 1.2V delivering 10mA. The outputs

are short-circuit protected with a 55mA current limit.

Voltage Gain Applications

When the LT1311 is used with external input resistors to

make an inverting voltage gain amplifier, the bandwidth

remains fairly constant for gains of 10 or less. At high

gains the bandwidth is limited by a gain bandwidth prod-

uct of about 250MHz. See the characteristic curves for

details.

The bandwidth is also influenced by any stray capacitance

in parallel with the input resistor. The parallel stray capaci-

tance results in a zero that pushes out the bandwidth. This

is particularly noticeable with large input resistors that

give gains less than one. For example, a single 100k input

resistor results in a bandwidth of 14MHz but two 50k

resistors in series result in only 10MHz bandwidth.

Overload Recovery

When one or more of the outputs is driven into the rail it

will not affect the other amplifiers. However, the output

that hit the rail will generate a glitch and take one to two

microseconds to recover. Supply current will increase

2mA to 3mA for each amplifier while it is driven into

the rail.