LT1991 Linear Technology, LT1991 Datasheet - Page 11

• Manufacturer Part Number: LT1991
• Description: Precision / 100uA Gain Selectable Amplifier
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
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
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
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 confi gured
EE
. If more than 10mA is allowed to fl ow through these
Figure 1. Difference Amplifi er Cannot Produce
0V on a Single Supply. Provide a Negative
Supply, or Raise Pin 5, or Provide 4mV of V
V
2.5V
CM
V
0V
DM
–
+
10
8
9
1
2
3
150k
450k
450k
150k
50k
50k
CC
and 1V of V
5V
7
–
+
4
450k
4pF
450k
EE
4pF
. The voltage at which
LT1991
1991 F01
REF
6
5
V
OUT
DM
= 13 • V
DM
as a gain of 13 difference amplifi er on a single supply
with the output REF connected to ground. This is a great
circuit, but it does not support V
mode because the output clips into ground while trying
to produce 0V
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 amplifi er, with the inputs shorted for the
common mode calculation. The values of R
dictated by how the P inputs and REF pin are connected.
By superposition we can write:
Or, solving for V
But valid V
+ 1V, so:
and:
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,
V
V
MAX V
MIN V
INT
EXT
= V
= V
Figure 2. Calculating CM Input Voltage Range
EXT
EXT
V
INT
EXT
EXT
INT
= (V
= (V
OUT
voltages are limited to V
• (R
• (1 + R
EXT
EE
. It can be fi xed simply by declaring the
CC
F
:
/(R
+ 1) • (1 + R
R
R
– 1.2) • (1 + R
G
G
G
F
V
INT
/R
+ R
F
) – V
–
+
G
)) + V
V
V
R
CC
EE
REF
F
G
DM
www.DataSheet4U.com
/R
REF
G
R
• R
F
= 0V at any common
F
/R
) – V
• (R
F
G
CC
) – V
/R
V
REF
REF
– 1.2V and V
F
G
LT1991
/(R
F
REF
• R
1991 F02
and R
F
• R
+ R
G
11
/R
G
G
G
F
/R
1991fg
))
are
EE
F