LT1996
9
1996f
maximum voltage allowed on the input and REF pins. The
50k resistors connected to the M9 and P9 inputs are
isolated from the substrate, and can therefore be taken
beyond the supply voltages. The naming of the pins “P9,”
“P27,” “P81,” etc., is based on their admittances relative
to the feedback and REF 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 REF
input.
Bandwidth
The bandwidth of the LT1996 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 5kHz.
Input Noise
The LT1996 input noise is comprised of the Johnson noise
of the internal resistors (√4kTR), and the input voltage
noise of the op amp. Paralleling all four resistors to the
+input gives a 3.8kΩ resistance, for 8nV/√Hz of voltage
noise. The equivalent network on the –input gives another
8nV/√Hz, and the op amp 14nV/√Hz. Taking their RMS
sum gives a total 18nV/√Hz input referred noise floor.
Output noise depends on configuration and noise gain.
Input Resistance
The LT1996 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 parallel
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 LT1996
presents the high input impedance of the op amp, as is
usual for the noninverting case.
Common Mode Input Voltage Range
The LT1996 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
The maximum voltage allowed on the P27, M27, P81 and
M81 inputs includes the positive and negative supply plus
a diode drop. These pins should not be driven more than
a diode drop outside of the supply rails. This is because
they are connected through diodes to internal manufactur-
ing post-package trim circuitry, and through a substrate
diode to V
EE
. If more than 10mA is allowed to flow through
these pins, there is a risk that the LT1996 will be detrimmed
or damaged. The P9 and M9 inputs do not have clamp
diodes or substrate diodes or trim circuitry and can be
taken well outside the supply rails. The maximum allowed
voltage on the P9 and M9 pins is ±60V.
The input voltage range of the internal op amp extends to
within 1.2V of V
CC
and 1V of V
EE
. The voltage at which the
op amp inputs common mode is determined by the
voltage at the op amp’s +input, and this is determined by
the voltages on pins P9, P27, P81 and REF. (See “Calcu-
lating Input Voltage Range” section.) This is true provided
that the op amp is functioning and feedback is maintaining
the inputs at the same voltage, which brings us to the third
requirement.
For valid circuit function, the op amp output must not be
clipped. The output will clip if the input signals are attempt-
ing to force it to within 40mV of its supply voltages. This
usually happens due to too large a signal level, but it can
also occur with zero input differential and must therefore
be included as an example of a common mode problem.
APPLICATIO S I FOR ATIO
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