LT1311 Datasheet, PDF (6/8 Pages) Linear Technology – Quad 12MHz, 145ns Settling Precision Current-to-Voltage Converter for Optical Disk Drives

LT1311
APPLICATIONS INFORMATION
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 0.1µF capacitor within
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.
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