Download DN348 - Precise Gain Without External Resistors

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Precise Gain Without External Resistors – Design Note 348
Glen Brisebois
Introduction
Inventory and manufacturing have associated costs and
logistical headaches. “If only we could simplify our stock
room and our manufacturing kits.” The LT®1991 provides
so many functions that it may be the last amplifier you ever
have to stock. This is not a limited-use single-application
difference or instrumentation amplifier. This is a flexible
part that can be configured into inverting, noninverting,
difference amplifiers, and even buffered attenuators, just
by strapping its pins. It provides you with internal precisely matched resistors and feedback capacitors, so you
can easily configure it into hundreds of different gain
circuits without external components.
The LT1991 offers the simplicity of using just one amplifier for unlimited applications—simply hook it up for type
and gain and move on. The precisely matched internal
resistors, op amp, and feedback capacitor simplify design, reduce external complexity and test time, lower pick
and place costs, and make for easy probing. All of this in
a small MSOP package.
The Resistors: 0.04% Worst Case
The LT1991 is shown in Figure 1. The internal resistors
offer 0.04% worst-case matching and 3ppm/°C MAX
matching temperature coefficient. They are nominally
50k, 150k and 450k. One end of each resistor is connected
to an op amp input, and the other is brought out to a pin.
The pins are named “M” or “P” depending on whether its
resistor goes to the “minus” or “plus” input, and numbered “M1” M3” or “P9” etcetera according to the relative
admittance of the resistor. So the “P9” pin has 9 times the
admittance (or force) of the “P1” pin. The 450k resistors
connected to the M1 and P1 inputs are not diode clamped,
and can be taken well outside the supply rails.
The Op Amp: Precision, Micropower
The op amp is exceptionally precise, with 15µV typical
input offset voltage, 3nA input bias current and 50pA of
input offset current. It operates on supplies from 2.7V to
36V with rail-to-rail outputs, and remains stable while
09/04/348
driving capacitive loads up to 500pF. Gain bandwidth product is 560kHz, while drawing only 100µA supply current.
So Easy to Use
The LT1991 is exceptionally easy to use. Drive or ground
or float the P, M, and REF inputs to set the configuration
and gain. The M1 and P1 inputs of the LT1991 are not
diode clamped, so they can withstand ±60V of common
mode voltage. There is a whole series of high input
common mode voltage circuits that can be created simply
by just strapping the pins. Figure 2 shows a few examples
of different configurations and gains. In fact, there are
over 300 unique achievable gains in the noninverting
configuration alone. Gains of up to 14 and buffered
attenuations down to 0.07 are possible.
,LTC and LT are registered trademarks of Linear Technology Corporation.
ALL THIS FITS IN
A 10-LEAD
MSOP PACKAGE
VCC
LT1991
M9
M3
M1
150k
450k
P1
P3
P9
450k
50k
4pF
–
OUT
+
450k
150k
450k
50k
4pF
REF
VEE
DN348 F01
Figure 1. The LT1991 is a Ready-to-Use Op Amp with its
Own Resistors and Internal Signal Capacitors, All in a Tiny
MSOP package. Just Wire it Up
www.BDTIC.com/Linear
VS+
VIN
8
9
10
1
2
3
VS+
7
M9
V
M3 CC
M1
LT1991 OUT
P1
REF
P3
VEE
5
P9
4
6
VOUT
VIN
8
9
10
1
2
3
VS–
6
VOUT
VS+
NONINVERTING GAIN = 14
VS+
VIN
M9
V
M3 CC
M1
LT1991 OUT
P1
REF
P3
VEE
5
P9
4
8
9
10
1
2
3
7
M9
V
M3 CC
M1
LT1991 OUT
P1
REF
P3
VEE
5
P9
4
6
VOUT = VS+/2
VS–
NONINVERTING GAIN = 5
8
9
10
1
2
3
VS+
7
MID-SUPPLY BUFFER
VS+
7
M9
V
M3 CC
M1
LT1991 OUT
P1
REF
P3
VEE
5
P9
4
VIN–
6
VOUT
VS–
VIN+
8
9
10
1
2
3
VS+
7
M9
V
M3 CC
M1
LT1991 OUT
P1
REF
P3
VEE
5
P9
4
6
8
9
10
1
2
3
VIN–
VOUT
VIN+
VS–
INVERTING GAIN = –3
–5V
DIFFERENCE GAIN = 11
7
M9
V
M3 CC
M1
6
LT1991 OUT
P1
REF
P3
VEE
5
P9
DN348 F02
4
VOUT
–5V
DIFFERENCE GAIN = 1
EXTENDED VCM = 1V TO 60V
Figure 2. Noninverting, Inverting, Difference Amplifier and Buffered Attenuators: Just a Few
Examples Achieved Simply by Connecting Pins on the LT1991. No External Resistors Required
Battery Monitor Circuit
Many batteries are composed of individual cells with
working voltages of about 1.2V each, as for example NiMH
and NiCd. Higher total voltages are achieved by placing
these in series. However, the reliability of the entire battery
pack is limited by the weakest cell, so users often like to
maintain data on individual cell charge characteristics and
histories. Figure 3 shows the LT1991 configured as a
difference amplifier in a gain of 3, applied across the
individual cells of a battery through a dual 4:1 mux.
Because of the high valued150k resistors on its M3 and P3
inputs, the error introduced by the mux impedance is
negligible. As the mux is stepped through its addresses,
the LT1991 takes each cell voltage, multiplies it by 3 and
references it to ground for easiest measurement. Note
that worst-case combinations, such as one cell much
higher than all the others, can cause the LT1991 output to
clip. Connecting the MSB line to the M1 and P1 inputs
helps reduce the effect of the wide input common mode
fluctuations from cell to cell. The low supply current of the
LT1991 makes it particularly suited to battery-powered
applications. With its 110µA maximum supply current
specification, it has about the same maximum supply
current specification as the CMOS mux!
16
V4
V3
12
14
16
11
4 BATTERY
CELLS
CELL
VOLTAGE
0.75V TO 1.7V
V2
V1
VCC
8
9
10
1
2
3
74HC4052
1
5
2
4
3
EN
LSB
MSB
7
6
VOUT
TRUTH TABLE
VEE GND
6 10 9
7
M9
V
M3 CC
M1
LT1991 OUT
P1
REF
P3
VEE
5
P9
4
8
DN348 F03
MSB LSB VOUT
L
L 3 • V4
L
H 3 • V3
H
L 3 • V2
H
H 3 • V1
Figure 3. LT1991 Applied as an Individual
Battery Cell Monitor for a 4-Cell Battery
Conclusion
The precision LT1991 is so simple to use, small, and
versatile, it is possible to stock this one amplifier and use
it for many varied applications. No external components
are needed to achieve hundreds of gains in noninverting,
inverting, difference and attenuator configurations. Just
strap the pins and go. It’s a great way to reduce inventory,
ease manufacturing, and simplify a bill of materials.
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2156
http://www.linear.com
Linear Technology Corporation
dn348f LT/TP 0904 409K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
www.BDTIC.com/Linear
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2004