Datasheet

LT1991
10
1991fh
Introduction
The LT1991 may be the last op amp you ever have to stock.
Because it provides you with several precision matched
resistors, you can easily configure it into several different
classical gain circuits without adding external components.
The several pages of simple circuits in this data sheet
demonstrate just how easy the LT1991 is to use. It can
be configured into difference amplifiers, as well as
into
inverting and noninverting single ended amplifiers. The
fact that the resistors and op amp are provided together
in such a small package will often save you board space
and reduce complexity for easy probing.
The Op Amp
The op amp internal to the LT1991 is a precision device
with 15µV typical offset voltage and 3nA input bias current.
The input offset current is extremely low, so
matching the
source resistance seen by the op amp inputs will provide
for the best output accuracy. The op amp inputs are not
rail-to-rail, but extend to within 1.2V of V
CC
and 1V of
V
EE
. For many configurations though, the chip inputs will
function rail-to-rail because of effective attenuation to the
+input. The output is truly rail-to-rail, getting to
within
40mV of the supply rails. The gain bandwidth product of
the op amp is about 560kHz. In noise gains of 2 or more,
it is stable into capacitive loads up to 500pF. In noise gains
below 2, it is stable into capacitive loads up to 100pF.
The Resistors
The resistors internal to the LT1991 are very well matched
SiChrome based elements protected with barrier metal.
Although their absolute tolerance is fairly poor (±30%),
their matching is to within 0.04%. This allows the chip to
achieve a CMRR of 75dB, and gain errors within 0.04%.
The resistor values are 50k, 150k, and 2 of 450k, con-
nected to each of the inputs. The resistors have power
limitations of 1watt for the 450k resistors, 0.3watt for the
150k resistors and 0.5watt for the 50k resistors; however,
in practice, power dissipation
will be limited well below
these values by the maximum voltage allowed on the
input and REF pins. The 450k resistors connected to the
M1 and P1 inputs are isolated from the substrate, and can
therefore be taken beyond the supply voltages. The naming
of the pins “P1,” “P3,” “P9,” etc., is based on their relative
admittances. Because it has 9 times the admittance, the
voltage applied to
the P9 input has 9 times the effect of
the voltage applied to the P1 input.
Bandwidth
The bandwidth of the LT1991 will depend on the gain you
select (or more accurately the noise gain resulting from
the gain you select). In the lowest configurable gain of 1,
the –3dB bandwidth is limited to 450kHz, with peaking of
about 2dB at 280kHz. In the highest configurable gains,
bandwidth is
limited to 32kHz.
Input Noise
The LT1991 input noise is dominated by the Johnson
noise of the internal resistors (√4kTR). Paralleling all
four resistors to the +input gives a 32.1kΩ resistance,
for 23nV/√Hz of voltage noise. The equivalent network
on the –input gives another 23nV/√Hz , and taking their
RMS sum gives a total 33nV/√Hz input referred noise floor.
Output noise depends on configuration
and noise gain.
Input Resistance
The LT1991 input resistances vary with configuration,
but once configured are apparent on inspection. Note that
resistors connected to the op amp’s –input are looking
into a virtual ground, so they simply parallel. Any feedback
resistance around the op amp does not contribute to input
resistance. Resistors connected to the op amp’s +input
are looking into a high impedance, so
they add as paral-
lel or series depending on how they are connected, and
whether or not some of them are grounded. The op amp
+input itself presents a very high GΩ impedance. In the
classical noninverting op amp configuration, the LT1991
presents the high input impedance of the op amp, as is
usual for the noninverting case.
Common Mode Input Voltage Range
The LT1991 valid
common mode input range is limited
by three factors:
1. Maximum allowed voltage on the pins
2. The input voltage range of the internal op amp
3. Valid output voltage
APPLICATIONS INFORMATION