Datasheet

LT1991
11
1991fh
The maximum voltage allowed on the P3, M3, P9, and
M9 inputs includes the positive and negative supply plus
a diode drop. These pins should not be driven more than
0.2V outside of the supply rails. This is because they are
connected through diodes to internal manufacturing 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 LT1991 will be detrimmed or
damaged. The P1 and M1 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 P1 and M1 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 P1, P3, P9 and REF (see “Calculating
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.
Consider Figure 1. This shows the LT1991 configured
as a gain of 13 difference amplifier on a single supply
with the output REF connected to ground. This is a great
circuit, but it does not support V
DM
= 0V at any common
mode because the output clips into ground while trying
to produce 0V
OUT
. It can be fixed simply by declaring the
valid input
differential range not to extend below +4mV,
or by elevating the REF pin above 40mV, or by providing
a negative supply.
Calculating Input Voltage Range
Figure 2 shows the LT1991 in the generalized case of
a difference amplifier, with the inputs shorted for the
common mode calculation. The values of R
F
and R
G
are
dictated by how the P inputs and REF pin are connected.
By superposition
we can write:
V
INT
= V
EXT
• (R
F
/(R
F
+ R
G
)) + V
REF
• (R
G
/(R
F
+ R
G
))
Or, solving for V
EXT
:
V
EXT
= V
INT
• (1 + R
G
/R
F
) – V
REF
• R
G
/R
F
But valid V
INT
voltages are limited to V
CC
– 1.2V and V
EE
+ 1V, so:
MAX V
EXT
= (V
CC
– 1.2) • (1 + R
G
/R
F
) – V
REF
• R
G
/R
F
and:
MIN V
EXT
= (V
EE
+ 1) • (1 + R
G
/R
F
) – V
REF
• R
G
/R
F
These two voltages represent the high and low extremes
of the common mode input range, if the other limits have
not already been exceeded (1 and 3, above). In most
cases, the inverting inputs M1 through M9 can be taken
further than these two extremes because doing this does
not move the op amp input common mode. To
calculate
the limit on this additional range, see Figure 3. Note that,
–
+
1991 F01
50k
150k
450k
50k
150k
450k
450k
450k
REF
5V
V
CM
2.5V
V
DM
0V
–
+
8
7
6
5
4
9
10
1
2
3
LT1991
V
OUT
= 13 • V
DM
4pF
4pF
–
+
V
REF
R
F
R
F
R
G
R
G
1991 F02
V
EXT
V
INT
V
CC
V
EE
Figure 1. Difference Amplifier Cannot Produce
0V on a Single Supply. Provide a Negative
Supply, or Raise Pin 5, or Provide 4mV of V
DM
Figure 2. Calculating CM Input Voltage Range
APPLICATIONS INFORMATION