DN348 - Precise Gain Without External Resistors

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 limiteduse 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_conv
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.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
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
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–
M9
V
M3 CC
M1
LT1991 OUT
P1
REF
P3
VEE
5
P9
4
NONINVERTING GAIN = 5
NONINVERTING GAIN = 14
+
+
8
9
10
1
2
3
VOUT
VS+
6
VOUT = VS+/2
MID-SUPPLY BUFFER
VS+
VS
7
M9
V
M3 CC
M1
LT1991 OUT
P1
REF
P3
VEE
5
P9
4
6
7
M9
V
M3 CC
M1
LT1991 OUT
P1
REF
P3
VEE
5
P9
4
8
9
10
1
2
3
VS–
VS
VIN
VS+
7
VIN–
6
VOUT
VIN+
VS–
INVERTING GAIN = –3
8
9
10
1
2
3
7
M9
V
M3 CC
M1
LT1991 OUT
P1
REF
P3
VEE
5
P9
4
6
VOUT
VIN–
VIN+
VS–
7
M9
V
M3 CC
M1
6
LT1991 OUT
P1
REF
P3
VEE
5
P9
DN348 F02
4
8
9
10
1
2
3
–5V
DIFFERENCE GAIN = 11
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
valued 150k 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!
Data Sheet Download
www.linear.com
Linear Technology Corporation
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 t7
L
H t7
H
L t7
H
H t7
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.
For applications help,
call (408) 432-1900
dn348f_conv LT/TP 0904 305K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2004
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