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LT1991 Precision, 100A Gain Selectable Amplifier
FEATURES

DESCRIPTIO

Pin Configurable as a Difference Amplifier, Inverting and Noninverting Amplifier Difference Amplifier Gain Range 1 to 13 CMRR >75dB Noninverting Amplifier Gain Range 0.07 to 14 Inverting Amplifier Gain Range -0.08 to -13 Gain Error <0.04% Gain Drift < 3ppm/C Wide Supply Range: Single 2.7V to Split 18V Micropower: 100A Supply Current Precision: 50V Maximum Input Offset Voltage 560kHz Gain Bandwidth Product Rail-to-Rail Output Space Saving 10-Lead MSOP and DFN Packages
The LT(R)1991 combines a precision operational amplifier with eight precision resistors to form a one-chip solution for accurately amplifying voltages. Gains from -13 to 14 with a gain accuracy of 0.04% can be achieved using no external components. The device is particularly well suited for use as a difference amplifier, where the excellent resistor matching results in a common mode rejection ratio of greater than 75dB. The amplifier features a 50V maximum input offset voltage and a gain bandwidth product of 560kHz. The device operates from any supply voltage from 2.7V to 36V and draws only 100A supply current on a 5V supply. The output swings to within 40mV of either supply rail. The resistors have excellent matching, 0.04% over temperature for the 450k resistors. The matching temperature coefficent is guaranteed less than 3ppm/C. The resistors are extremely linear with voltage, resulting in a gain nonlinearity of less than 10ppm. The LT1991 is fully specified at 5V and 15V supplies and from -40C to 85C. The device is available in space saving 10-lead MSOP and low profile (0.8mm) 3mm x 3mm DFN packages.
, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Patent Pending.
APPLICATIO S

Handheld Instrumentation Medical Instrumentation Strain Gauge Amplifiers Differential to Single-Ended Conversion
TYPICAL APPLICATIO
Rail-to-Rail Gain = 1 Difference Amplifier
5V 50k 150k VM(IN) VIN VP(IN) 450k VOUT = VREF + VIN SWING 40mV TO EITHER RAIL ROUT <0.1 40 35
PERCENTAGE OF UNITS (%)
- +
450k
-
450k 150k 50k
4pF
+
LT1991 450k
INPUT RANGE -0.5V TO 5.1V RIN = 900k
4pF VREF = 2.5V
1991 TA01
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Distribution of Resistor Matching
450k RESISTORS LT1991A 30 25 20 15 10 5 0 - 0.04 0 - 0.02 0.02 RESISTOR MATCHING (%) 0.04
1991TA01b
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LT1991
ABSOLUTE
(Note 1)
AXI U RATI GS
Maximum Junction Temperature DD Package ...................................................... 125C MS Package ..................................................... 150C Storage Temperature Range DD Package .......................................-65C to 125C MS Package ......................................-65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
Total Supply Voltage (V + to V -) ............................... 40V Input Voltage (Pins P1/M1, Note 2) ....................... 60V Input Voltage (Other inputs Note 2).............. V + + 0.2V to V - - 0.2V Output Short-Circuit Duration (Note 3) ............ Indefinite Operating Temperature Range (Note 4) ...-40C to 85C Specified Temperature Range (Note 5) ....-40C to 85C
PACKAGE/ORDER I FOR ATIO
TOP VIEW P1 P3 P9 VEE REF 1 2 3 4 5 10 M1 9 M3 8 M9 7 VCC 6 OUT
ORDER PART NUMBER
TOP VIEW
LT1991CDD LT1991IDD LT1991ACDD LT1991AIDD DD PART MARKING* LBMM
DD PACKAGE 10-LEAD (3mm x 3mm) PLASTIC DFN
EXPOSED PAD CONNECTED TO VEE PCB CONNECTION OPTIONAL TJMAX = 125C, JA = 160C/W
*Temperature and electrical grades are identified by a label on the shipping container. Consult LTC Marketing for parts specified with wider operating temperature ranges.
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. Difference amplifier configuration, VS = 5V, 0V or 15V; VCM = VREF = half supply, unless otherwise noted.
SYMBOL G PARAMETER Gain Error CONDITIONS VS = 15V, VOUT = 10V; RL = 10k G = 1; LT1991A G = 1; LT1991 G = 3 or 9; LT1991A G = 3 or 9; LT1991 VS = 15V; VOUT = 10V; RL = 10k VS = 15V; VOUT = 10V; RL = 10k VS = 15V; VCM = 15.2V G = 9; LT1991A G = 3; LT1991A G = 1; LT1991A Any Gain; LT1991 P1/M1 Inputs VS = 15V; VREF = 0V VS = 5V, 0V; VREF = 2.5V VS = 3V, 0V; VREF = 1.25V

ELECTRICAL CHARACTERISTICS
GNL G/T CMRR
Gain Nonlinearity Gain Drift vs Temperature (Note 6) Common Mode Rejection Ratio, Referred to Inputs (RTI)
VCM
Input Voltage Range (Note 7)
2
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ORDER PART NUMBER
P1 P3 P9 VEE REF 1 2 3 4 5 10 9 8 7 6 M1 M3 M9 VCC OUT
MS PACKAGE 10-LEAD PLASTIC MSOP TJMAX = 150C, JA = 230C/W
LT1991CMS LT1991IMS LT1991ACMS LT1991AIMS MS PART MARKING* LTQD
MIN
TYP
MAX 0.04 0.08 0.06 0.12
UNITS % % % % ppm ppm/C dB dB dB dB
1 0.3 80 75 75 60 -28 -0.5 0.75 100 93 90 70
10 3
27.6 5.1 2.35
V V V
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LT1991
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. Difference amplifier configuration, VS = 5V, 0V or 15V; VCM = VREF = half supply, unless otherwise noted.
SYMBOL VCM PARAMETER Input Voltage Range (Note 7) CONDITIONS P1/M1 Inputs, P9/M9 Connected to REF VS = 15V; VREF = 0V VS = 5V, 0V; VREF = 2.5V VS = 3V, 0V; VREF = 1.25V P3/M3 Inputs VS = 15V; VREF = 0V VS = 5V, 0V; VREF = 2.5V VS = 3V, 0V; VREF = 1.25V P9/M9 Inputs VS = 15V; VREF = 0V VS = 5V, 0V; VREF = 2.5V VS = 3V, 0V; VREF = 1.25V VOS Op Amp Offset Voltage (Note 8) LT1991AMS, VS = 5V, 0V

ELECTRICAL CHARACTERISTICS
MIN -60 -14 -1.5 -15.2 0.5 0.95 -15.2 0.85 1.0
TYP
MAX 60 16.8 7.3 15.2 4.2 1.95 15.2 3.9 1.9
UNITS V V V V V V V V V V V V V V V V V V/C nA nA pA pA pA pA VP-P VRMS VP-P VRMS nV/Hz nV/Hz
15 15
50 135 80 160 100 200 150 250 1 5 7.5 500 750 1000 1500
LT1991AMS, VS = 15V LT1991MS
25 25

LT1991DD VOS/T IB IOS Op Amp Offset Voltage Drift (Note 6) Op Amp Input Bias Current (Note 11) Op Amp Input Offset Current (Note 11) LT1991A
0.3 2.5 50 50
LT1991
Op Amp Input Noise Voltage
0.01Hz to 1Hz 0.01Hz to 1Hz 0.1Hz to 10Hz 0.1Hz to 10Hz G = 1; f = 1kHz G = 9; f = 1kHz P1 (M1 = Ground) P3 (M3 = Ground) P9 (M9 = Ground) M1 (P1 = Ground) M3 (P3 = Ground) M9 (P9 = Ground)

0.35 0.07 0.25 0.05 180 46 630 420 350 315 105 35 900 600 500 450 150 50 0.01 0.02 0.02 0.04 0.3 -30 105 135 2.4 2.7 1170 780 650 585 195 65 0.04 0.06 0.08 0.12 3
en RIN
Input Noise Voltage Density Input Impedance (Note 10)
k k k k k k % % % % ppm/C ppm/C dB V
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Resistor Matching (Note 9)
450k Resistors, LT1991A Other Resistors, LT1991A 450k Resistors, LT1991 Other Resistors, LT1991 Resistor Matching Absolute Value VS = 1.35V to 18V (Note 8)
R/T PSRR
Resistor Temperature Coefficient (Note 6) Power Supply Rejection Ratio Minimum Supply Voltage
3
LT1991
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. Difference amplifier configuration, VS = 5V, 0V or 15V; VCM = VREF = half supply, unless otherwise noted.
SYMBOL VOUT PARAMETER Output Voltage Swing (to Either Rail) CONDITIONS No Load VS = 5V, 0V VS = 5V, 0V VS = 15V 1mA Load VS = 5V, 0V VS = 5V, 0V VS = 15V ISC Output Short-Circuit Current (Sourcing) Output Short-Circuit Current (Sinking) BW -3dB Bandwidth Drive Output Positive; Short Output to Ground Drive Output Negative; Short Output to VS or Midsupply G=1 G=3 G=9 f = 10kHz G = 1; 0.1V Step; 10% to 90% G = 9; 0.1V Step; 10% to 90% G = 1; VS = 5V, 0V; 2V Step G = 1; VS = 5V, 0V; -2V Step G = 1; VS = 15V, 10V Step G = 1; VS = 15V, -10V Step VS = 5V, 0V; VOUT = 1V to 4V VS = 15V; VOUT = 10V; VMEAS = 5V VS = 5V, 0V

ELECTRICAL CHARACTERISTICS
MIN
TYP 40
MAX 55 65 110 225 275 300
UNITS mV mV mV mV mV mV mA mA mA mA kHz kHz kHz kHz s s s s s s V/s V/s

150

8 4 8 4
12 21 110 78 40 560 3 8 42 48 114 74
GBWP tr, tf ts
Op Amp Gain Bandwidth Product Rise Time, Fall Time Settling Time to 0.01%
SR IS
Slew Rate Supply Current
0.06 0.08
0.12 0.12 100 130 110 150 160 210
A A A A
VS = 15V
Note 1: Absolute Maximum Ratings are those beyond which the life of the device may be impaired. Note 2: The P3/M3 and P9/M9 inputs should not be taken more than 0.2V beyond the supply rails. The P1/M1 inputs can withstand 60V if P9/M9 are grounded and VS = 15V (see Applications Information section about "High Voltage CM Difference Amplifiers"). Note 3: A heat sink may be required to keep the junction temperature below absolute maximum ratings. Note 4: Both the LT1991C and LT1991I are guaranteed functional over the -40C to 85C temperature range. Note 5: The LT1991C is guaranteed to meet the specified performance from 0C to 70C and is designed, characterized and expected to meet specified performance from -40C to 85C but is not tested or QA sampled at these temperatures. The LT1991I is guaranteed to meet specified performance from -40C to 85C. Note 6: This parameter is not 100% tested.
Note 7: Input voltage range is guaranteed by the CMRR test at VS = 15V. For the other voltages, this parameter is guaranteed by design and through correlation with the 15V test. See the Applications Information section to determine the valid input voltage range under various operating conditions. Note 8: Offset voltage, offset voltage drift and PSRR are defined as referred to the internal op amp. You can calculate output offset as follows. In the case of balanced source resistance, VOS,OUT = VOS * NOISEGAIN + IOS * 450k + IB * 450k * (1- RP/RN) where RP and RN are the total resistance at the op amp positive and negative terminal respectively. Note 9: Applies to resistors that are connected to the inverting inputs. Resistor matching is not tested directly, but is guaranteed by the gain error test. Note 10: Input impedance is tested by a combination of direct measurements and correlation to the CMRR and gain error tests. Note 11: IB and IOS are tested at VS = 5V, 0V only.
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LT1991 TYPICAL PERFOR A CE CHARACTERISTICS
Supply Current vs Supply Voltage
200 175
OUTPUT VOLTAGE SWING (mV)
TA = 25C TA = -40C
OUTPUT VOLTAGE (mV)
SUPPLY CURRENT (A)
150 125 100 75 50 25 0 0 2
TA = 85C
4
6 8 10 12 14 16 18 20 SUPPLY VOLTAGE (V)
1991 G01
Output Voltage Swing vs Load Current (Output High)
VCC -100 OUTPUT SHORT-CIRCUIT CURRENT (mA) VS = 5V, 0V
OUTPUT VOLTAGE SWING (mV)
-200 -300 -400 -500 -600 -700 -800 -900 -1000 0 1 2 TA = 85C TA = 25C
TA = -40C
INPUT OFFSET VOLTAGE (V)
34567 LOAD CURRENT (mA)
Output Offset Voltage vs Difference Gain
1000
OUTPUT OFFSET VOLTAGE (V)
750 500
VS = 5V, 0V REPRESENTATIVE PARTS
SLEW RATE (V/s)
GAIN ERROR (%)
250 0 -250 -500 -750
-1000
1
2
3
4
5
6 7 8 9 10 11 12 13 GAIN (V/V)
1991 G07
UW
8 9
1991 G04
(Difference Amplifier Configuration) Output Voltage Swing vs Load Current (Output Low)
VCC -20
Output Voltage Swing vs Temperature
VS = 5V, 0V NO LOAD OUTPUT HIGH (RIGHT AXIS)
1400 1200 1000 800
VS = 5V, 0V TA = 85C
-40 -60
TA = 25C 600 TA = -40C 400 200 VEE
60 40 20 OUTPUT LOW (LEFT AXIS)
VEE -50 -25
0
25
50
75
100
125
0
1
2
TEMPERATURE (C)
1991 G02
34567 LOAD CURRENT (mA)
8
9
10
1991 G03
Output Short-Circuit Current vs Temperature
25 VS = 5V, 0V SINKING 20
100 50 0 -50 -100 -150 150
Input Offset Voltage vs Difference Gain
VS = 5V, 0V REPRESENTATIVE PARTS
15
10
SOURCING
5
10
0 -50 -25
50 25 0 75 TEMPERATURE (C)
100
125
1
2
3
4
5
1991 G05
6 7 8 9 10 11 12 13 GAIN (V/V) 1991 G06
Gain Error vs Load Current
0.04 0.03 0.02 0.01 0 -0.01 -0.02 -0.03 -0.04 0 REPRESENTATIVE UNITS 1 3 LOAD CURRENT (mA) 2 4 5
1991 G08
Slew Rate vs Temperature
0.30 0.25 0.20 SR- (FALLING EDGE) 0.15 SR+ (RISING EDGE) 0.10 0.05 0 -50 -25 GAIN = 1 VS = 15V VOUT = 10V
GAIN = 1 VS = 15V VOUT = 10V TA = 25C
50 25 75 0 TEMPERATURE (C)
100
125
1991 G09
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LT1991 TYPICAL PERFOR A CE CHARACTERISTICS
Bandwidth vs Gain
120 100
-3dB BANDWIDTH (kHz)
80
CMRR (dB)
70 60 50 40 30 20 10 0
PSRR (dB)
60 40 20 0
12
34
5 6 7 8 9 10 11 12 13 GAIN SETTING (V/V)
1991 G10
Output Impedance vs Frequency
1000 VS = 5V, 0V TA = 25C
120 100
100
OUTPUT IMPEDANCE ()
GAIN ERROR (%)
10 GAIN = 3 1
CMRR (dB)
GAIN = 9
GAIN = 1
0.1
0.01 1 10 100 1k FREQUENCY (Hz) 10k 100k
1991 G13
Gain vs Frequency
30 GAIN = 9 VS = 5V, 0V TA = 25C
20
0 -1
GAIN -45
PHASE (deg)
GAIN (dB)
GAIN (dB)
10
GAIN = 3
-2 -3 -4 -5 -135 -90
0
GAIN = 1
-10
-6 -7 -180 1 10 FREQUENCY (kHz) 100 400
1991 G17
-20
1
10 100 FREQUENCY (kHz)
600
1991 G16
-8 0.5
OP AMP VOLTAGE NOISE (100nV/DIV)
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UW
VS = 5V, 0V TA = 25C
(Difference Amplifier Configuration) PSRR vs Frequency
CMRR vs Frequency
120 110 100 90 80 GAIN = 1 GAIN = 3 GAIN = 9 VS = 5V, 0V TA = 25C
120 110 100 90 80 70 60 50 40 30 20 10 0
VS = 5V, 0V TA = 25C GAIN = 9 GAIN = 1 GAIN = 3
10
100
1k 10k FREQUENCY (Hz)
100k
1M
1991 G11
10
100
1k 10k FREQUENCY (Hz)
100k
1991 G12
CMRR vs Temperature
GAIN = 1 VS = 15V
Gain Error vs Temperature
0.030 0.025 0.020 0.015 0.010 0.005 GAIN = 1 VS = 15V
80 60 40 20 0 -50 -25 REPRESENTATIVE UNITS 50 25 75 0 TEMPERATURE (C) 100 125
0 -50 -25
REPRESENTATIVE UNITS 50 25 75 0 TEMPERATURE (C) 100 125
1991 G14
1991 G15
Gain and Phase vs Frequency
2 1 PHASE VS = 5V, 0V TA = 25C 0 GAIN = 1
0.01Hz to 1Hz Voltage Noise
VS = 15V TA = 25C MEASURED IN G =13 REFERRED TO OP AMP INPUTS
0
10 20 30 40 50 60 70 80 90 100 TIME (s)
1991 G21
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LT1991 TYPICAL PERFOR A CE CHARACTERISTICS
Small Signal Transient Response
GAIN = 1
50mV/DIV
5s/DIV
PI FU CTIO S
(Difference Amplifier Configuration)
P1 (Pin 1): Noninverting Gain-of-1 input. Connects a 450k internal resistor to the op amp's noninverting input. P3 (Pin 2): Noninverting Gain-of-3 input. Connects a 150k internal resistor to the op amp's noninverting input. P9 (Pin 3): Noninverting Gain-of-9 input. Connects a 50k internal resistor to the op amp's noninverting input. VEE (Pin 4): Negative Power Supply. Can be either ground (in single supply applications), or a negative voltage (in split supply applications). REF (Pin 5): Reference Input. Sets the output level when difference between inputs is zero. Connects a 450k internal resistor to the op amp's noninverting input.
BLOCK DIAGRA
UW
Small Signal Transient Response
GAIN = 3
Small Signal Transient Response
GAIN = 9
50mV/DIV
50mV/DIV
1991 G18
5s/DIV
1991 G19
5s/DIV
1991 G20
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OUT (Pin 6): Output. VOUT = VREF + 1 * (VP1 - VM1) + 3 * (VP3 - VM3) + 9 * (VP9 - VM9). VCC (Pin 7): Positive Power Supply. Can be anything from 2.7V to 36V above the VEE voltage. M9 (Pin 8): Inverting Gain-of-9 input. Connects a 50k internal resistor to the op amp's inverting input. M3 (Pin 9): Inverting Gain-of-3 input. Connects a 150k internal resistor to the op amp's inverting input. M1 (Pin 10): Inverting Gain-of-1 input. Connects a 450k internal resistor to the op amp's inverting input. Exposed Pad: Must be soldered to PCB.
M1 10
M3 9
M9 8 50k
VCC 7 450k
OUT 6
150k 4pF 450k INM 450k INP LT1991 150k 50k 4pF 1 P1 2 P3 3 P9 4 VEE 5 REF
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OUT
450k
7
LT1991
APPLICATIO S I FOR ATIO
Introduction
The LT1991 may be the last op amp you ever have to stock. Because it provides you with several precision matched resistors, you can easily configure it into several different classical gain circuits without adding external components. The several pages of simple circuits in this data sheet demonstrate just how easy the LT1991 is to use. It can be configured into difference amplifiers, as well as into inverting and noninverting single ended amplifiers. The fact that the resistors and op amp are provided together in such a small package will often save you board space and reduce complexity for easy probing. The Op Amp The op amp internal to the LT1991 is a precision device with 15V typical offset voltage and 3nA input bias current. The input offset current is extremely low, so matching the source resistance seen by the op amp inputs will provide for the best output accuracy. The op amp inputs are not rail-to-rail, but extend to within 1.2V of VCC and 1V of VEE. For many configurations though, the chip inputs will function rail-to-rail because of effective attenuation to the +input. The output is truly rail-to-rail, getting to within 40mV of the supply rails. The gain bandwidth product of the op amp is about 560kHz. In noise gains of 2 or more, it is stable into capacitive loads up to 500pF. In noise gains below 2, it is stable into capacitive loads up to 100pF. The Resistors The resistors internal to the LT1991 are very well matched SiChrome based elements protected with barrier metal. Although their absolute tolerance is fairly poor (30%), their matching is to within 0.04%. This allows the chip to achieve a CMRR of 75dB, and gain errors within 0.04%. The resistor values are 50k, 150k, and 2 of 450k, connected to each of the inputs. The resistors have power limitations of 1watt for the 450k resistors, 0.3watt for the 150k resistors and 0.5watt for the 50k resistors; however, in practice, power dissipation will be limited well below these values by the maximum voltage allowed on the input and REF pins. The 450k resistors connected to the M1 and P1 inputs are isolated from the substrate, and can therefore be taken beyond the supply voltages. The naming of the pins "P1," "P3," "P9," etc., is based on their relative
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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 P1 input. Bandwidth The bandwidth of the LT1991 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 32kHz. Input Noise The LT1991 input noise is dominated by the Johnson noise of the internal resistors (4kTR). Paralleling all four resistors to the +input gives a 32.1k resistance, for 23nV/Hz of voltage noise. The equivalent network on the -input gives another 23nV/Hz, and taking their RMS sum gives a total 33nV/Hz input referred noise floor. Output noise depends on configuration and noise gain. Input Resistance The LT1991 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 G impedance. In the classical noninverting op amp configuration, the LT1991 presents the high input impedance of the op amp, as is usual for the noninverting case. Common Mode Input Voltage Range The LT1991 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
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LT1991
APPLICATIO S I FOR ATIO
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 postpackage trim circuitry, and through a substrate diode to VEE. 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 VCC and 1V of VEE. 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 attempting 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
5V 7 8 50k 450k 4pF 9 10 VDM 0V+ VCM 2.5V 150k 450k
-
-
6 VOUT = 13 * VDM
1 2
450k 150k 50k
+
4pF
3
450k
REF 5 LT1991
4
1991 F01
Figure 1. Difference Amplifier Cannot Produce 0V on a Single Supply. Provide a Negative Supply, or Raise Pin 5, or Provide 4mV of VDM
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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 VDM = 0V at any common mode because the output clips into ground while trying to produce 0VOUT. 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 RF and RG are dictated by how the P inputs and REF pin are connected. By superposition we can write: VINT = VEXT * (RF/(RF + RG)) + VREF * (RG/(RF + RG)) Or, solving for VEXT: VEXT = VINT * (1 + RG/RF) - VREF * RG/RF But valid VINT voltages are limited to VCC - 1.2V and VEE + 1V, so: MAX VEXT = (VCC - 1.2) * (1 + RG/RF) - VREF * RG/RF and: MIN VEXT = (VEE + 1) * (1 + RG/RF) - VREF * RG/RF
RF RG VEXT RG VCC
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VINT
+
VEE RF VREF
1991 F02
Figure 2. Calculating CM Input Voltage Range
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, with VMORE = 0, the op amp output is at VREF. From the max
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LT1991
APPLICATIO S I FOR ATIO
VEXT (the high cm limit), as VMORE goes positive, the op amp output will go more negative from VREF by the amount VMORE * RF/RG, so: VOUT = VREF - VMORE * RF/RG Or: VMORE = (VREF - VOUT) * RG/RF The most negative that VOUT can go is VEE + 0.04V, so: Max VMORE = (VREF - VEE - 0.04V) * RG/RF (should be positive) The situation where this function is negative, and therefore problematic, when VREF = 0 and VEE = 0, has already been dealt with in Figure 1. The strength of the equation is demonstrated in that it provides the three solutions suggested in Figure 1: raise VREF, lower VEE, or provide some negative VMORE. Likewise, from the lower common mode extreme, making the negative input more negative will raise the output voltage, limited by VCC - 0.04V. MIN VMORE = (VREF - VCC + 0.04V) * RG/RF (should be negative)
RF
RG VMORE VEXT MAX OR MIN VINT RG
VCC
- +
VEE RF VREF
1991 F03
Figure 3. Calculating Additional Voltage Range of Inverting Inputs
Again, the additional input range calculated here is only available provided the other remaining constraint is not violated, the maximum voltage allowed on the pin. The Classical Noninverting Amplifier: High Input Z Perhaps the most common op amp configuration is the noninverting amplifier. Figure 4 shows the textbook
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representation of the circuit on the top. The LT1991 is shown on the bottom configured in a precision gain of 5.5. One of the benefits of the noninverting op amp configuration is that the input impedance is extremely high. The LT1991 maintains this benefit. Given the finite number of available feedback resistors in the LT1991, the number of gain configurations is also finite. The complete list of such Hi-Z input noninverting gain configurations is shown in Table 1. Many of these are also represented in Figure 5 in schematic form. Note that the P-side resistor inputs have been connected so as to match the source impedance seen by the internal op amp inputs. Note also that gain and noise gain are identical, for optimal precision.
RF RG
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-
VOUT
VIN
+
VOUT = GAIN * VIN GAIN = 1 + RF/RG
CLASSICAL NONINVERTING OP AMP CONFIGURATION. YOU PROVIDE THE RESISTORS.
8
50k
450k 4pF
9 10
150k 450k
-
6 VOUT
1 2
450k 150k 50k
+
4pF
3
450k LT1991 5
VIN CLASSICAL NONINVERTING OP AMP CONFIGURATION IMPLEMENTED WITH LT1991. RF = 225k, RG = 50k, GAIN = 5.5. GAIN IS ACHIEVED BY GROUNDING, FLOATING OR FEEDING BACK THE AVAILABLE RESISTORS TO ARRIVE AT DESIRED RF AND RG. WE PROVIDE YOU WITH <0.1% RESISTORS.
1991 F04
Figure 4. The LT1991 as a Classical Noninverting Op Amp
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LT1991
APPLICATIO S I FOR ATIO
Table 1. Configuring the M Pins for Simple Noninverting Gains. The P Inputs are driven as shown in the examples on the next page
Gain 1 1.077 1.1 1.25 1.273 1.3 1.4 2 2.5 2.8 3.25 3.5 4 5 5.5 7 10 11 13 14 M9 Output Output Output Float Output Output Output Float Float Ground Ground Ground Float Float Ground Ground Ground Ground Ground Ground M9, M3, M1 Connection M3 Output Output Float Output Ground Ground Ground Float Ground Output Output Output Ground Ground Float Ground Float Float Ground Ground M1 Output Ground Ground Ground Output Float Ground Ground Output Output Float Ground Float Ground Output Output Float Ground Float Ground
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LT1991
APPLICATIO S I FOR ATIO
VS+ 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS - GAIN = 1 VS+ 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- VIN GAIN = 4 VS+ 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 7 VS+ 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- OUT REF 5 OUT REF 5 6 VOUT VIN OUT REF 5 6 VOUT OUT REF 5 6 VOUT VIN
VIN
VIN
VIN
VIN
GAIN = 13
Figure 5. Some Implementations of Classical Noninverting Gains Using the LT1991. High Input Z Is Maintained
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VS+ 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 V S- GAIN = 2 VS+ 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 V S- VIN GAIN = 5 VS+ 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 V S- VIN GAIN = 10 VS + 8 M9 9 M3 10 M1 VOUT 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 V S- VIN GAIN = 14
1991 F05
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VS+ 8 M9 9 M3 10 M1 VOUT 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS - GAIN = 3.25 VS+ 8 M9 9 M3 10 M1 VOUT 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 V S- GAIN = 5.5 VS+ 8 M9 9 M3 10 M1 VOUT 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 V S- GAIN = 11 OUT REF 5 6 VOUT OUT REF 5 6 VOUT OUT REF 5 6 VOUT
OUT REF 5
6
VIN
OUT REF 5
6
OUT REF 5
6
6
OUT REF 5
6
VOUT
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LT1991
APPLICATIO S I FOR ATIO
Attenuation Using the P Input Resistors
Attenuation happens as a matter of fact in difference amplifier configurations, but it is also used for reducing peak signal level or improving input common mode range even in single ended systems. When signal conditioning indicates a need for attenuation, the LT1991 resistors are ready at hand. The four precision resistors can provide several attenuation levels, and these are tabulated in Table 2 as a design reference.
VIN RA VINT RG VIN OKAY UP TO 60V 1 2 450k 150k 50k VINT
+
4pF
VINT = A * VIN A = RG/(RA + RG)
3
450k LT1991 5
CLASSICAL ATTENUATOR
LT1991 ATTENUATING TO THE +INPUT BY DRIVING AND GROUNDING AND FLOATING INPUTS RA = 450k, RG = 50k, SO A = 0.1.
1991 F06
Figure 6. LT1991 Provides for Easy Attenuation to the Op Amp's +Input. The P1 Input Can Be Taken Well Outside of the Supplies
Because the attenuations and the noninverting gains are set independently, they can be combined. This provides high gain resolution, about 340 unique gains between 0.077 and 14, as plotted in Figure 7. This is too large a number to tabulate, but the designer can calculate achievable gain by taking the vector product of the gains and attenuations in Tables 1 and 2, and seeking the best match. Average gain resolution is 1.5%, with a worst case of 7%.
100
10
GAIN
1
0.1
0.01
0
50
100
150 200 COUNT
250
300
350
1991 F07
Figure 7. Over 346 Unique Gain Settings Achievable with the LT1991 by Combining Attenuation with Noninverting Gain
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Table 2. Configuring the P Pins for Various Attenuations. Those Shown in Bold Are Functional Even When the Input Drive Exceeds the Supplies
A 0.0714 0.0769 0.0909 0.1 0.143 0.182 0.2 0.214 0.231 0.25 0.286 0.308 0.357 0.4 0.5 0.6 0.643 0.692 0.714 0.75 0.769 0.786 0.8 0.818 0.857 0.9 0.909 0.923 0.929 1 P9 Ground Ground Ground Ground Ground Ground Float Ground Ground Float Ground Ground Ground Float Float Float Drive Drive Drive Float Drive Drive Float Drive Drive Drive Drive Drive Drive Drive P9, P3, P1, REF Connection P3 P1 Ground Ground Float Float Ground Float Ground Drive Drive Ground Drive Drive Drive Ground Float Drive Ground Ground Ground Drive Ground Ground Drive Float Drive Float Float Drive Drive Drive Drive Drive Drive Drive Drive Drive Drive Ground Float Drive Drive Drive Drive Drive Drive Ground Ground Float Drive Float Drive Drive Drive Ground Ground Float Drive Float Drive Drive REF Ground Float Ground Float Drive Drive Ground Ground Ground Float Ground Float Drive Drive Ground Ground Ground Ground Ground Ground Float Drive Ground Ground Ground Ground Ground Ground Ground Drive
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LT1991
APPLICATIO S I FOR ATIO
Inverting Configuration
The inverting amplifier, shown in Figure 8, is another classical op amp configuration. The circuit is actually identical to the noninverting amplifier of Figure 4, except that VIN and GND have been swapped. The list of available gains is shown in Table 3, and some of the circuits are shown in Figure 9. Noise gain is 1+|Gain|, as is the usual case for inverting amplifiers. Again, for the best DC performance, match the source impedance seen by the op amp inputs.
RF
RG VIN
-
VOUT
+
VOUT = GAIN * VIN GAIN = - RF/RG
CLASSICAL INVERTING OP AMP CONFIGURATION. YOU PROVIDE THE RESISTORS.
VIN (DRIVE)
8
50k
450k 4pF
9 10
150k 450k
-
6 VOUT
1 2
450k 150k 50k
+
4pF
3
450k LT1991 5
CLASSICAL INVERTING OP AMP CONFIGURATION IMPLEMENTED WITH LT1991. RF = 225k, RG = 50k, GAIN = -4.5. GAIN IS ACHIEVED BY GROUNDING, FLOATING OR FEEDING BACK THE AVAILABLE RESISTORS TO ARRIVE AT DESIRED RF AND RG. WE PROVIDE YOU WITH <0.1% RESISTORS.
1991 F08
Figure 8. The LT1991 as a Classical Inverting Op Amp. Note the Circuit Is Identical to the Noninverting Amplifier, Except that VIN and Ground Have Been Swapped
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Table 3. Configuring the M Pins for Simple Inverting Gains
Gain -0.077 -0.1 -0.25 -0.273 -0.3 -0.4 -1 -1.5 -1.8 -2.25 -2.5 -3 -4 -4.5 -6 -9 -10 -12 -13 M9 Output Output Float Output Output Output Float Float Drive Drive Drive Float Float Drive Drive Drive Drive Drive Drive M9, M3, M1 Connection M3 Output Float Output Drive Drive Drive Float Drive Output Output Output Drive Drive Float Drive Float Float Drive Drive M1 Drive Drive Drive Output Float Drive Drive Output Output Float Drive Float Drive Output Output Float Drive Float Drive
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APPLICATIO S I FOR ATIO
VS + 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = -0.25 VS + VIN 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = -3 VS + VIN 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = -6 VS + VIN 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = -12 OUT REF 5 OUT REF 5 6 VOUT VIN OUT REF 5 6 VOUT OUT REF 5 6 VOUT
VIN
VIN
VIN
6
Figure 9. It Is Simple to Get Precision Inverting Gains with the LT1991. Input Impedance Varies from 45k (Gain = -13) to 450k (Gain = -1)
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VS + 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = -1 VS + 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = -4 VS + 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = -9 VS + 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = -13
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VS+ VIN 6 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = -2.25 VS+ VIN 6 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = -4.5 VS+ VIN 6 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = -10 OUT REF 5 6 VOUT OUT REF 5 6 VOUT OUT REF 5 6 VOUT
OUT REF 5
VOUT
OUT REF 5
VOUT
OUT REF 5
VOUT
VIN VOUT
OUT REF 5
6
VOUT
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LT1991
APPLICATIO S I FOR ATIO
Difference Amplifiers
The resistors in the LT1991 allow it to easily make difference amplifiers also. Figure 10 shows the basic 4-resistor difference amplifier and the LT1991. A difference gain of 3 is shown, but notice the effect of the additional dashed connections. By connecting the 450k resistors in parallel, the gain is reduced by a factor of 2. Of course, with so many resistors, there are many possible gains. Table 4 shows the difference gains and how they are achieved. Note that, as for inverting amplifiers, the noise gain is 1 more than the signal gain.
Table 4. Connections Giving Difference Gains for the LT1991
Gain 0.077 0.1 0.25 0.273 0.3 0.4 1 1.5 1.8 2.25 2.5 3 4 4.5 6 9 10 12 13 VIN
+
VIN
-
Output M3, M9 M9 M3 M1, M9 M9 M9 M1 M1, M3 M3 M3
P1 P1 P1 P3 P3 P1, P3 P1 P3 P9 P9 P1, P9 P3 P1, P3 P9 P3, P9 P9 P1, P9 P3, P9 P1, P3, P9
M1 M1 M1 M3 M3 M1, M3 M1 M3 M9 M9 M1, M9 M3 M1, M3 M9 M3, M9 M9 M1, M9 M3, M9 M1, M3, M9
M1 M1
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RF VIN- VIN+ RG
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VOUT
RG
+
RF
VOUT = GAIN * (VIN+ - VIN-) GAIN = RF/RG
CLASSICAL DIFFERENCE AMPLIFIER USING THE LT1991
8 M9 50k 9 M3 150k 10 M1 450k PARALLEL TO CHANGE R F, R G
450k 4pF
GND (REF) P3, P9 P9 P3 P1, P9 P9 P9 P1 P1, P3 P3 P3
VIN-
-
6 VOUT
1 P1 2 P3 3 P9
450k 150k 50k
+
4pF
VIN+
450k LT1991
5
CLASSICAL DIFFERENCE AMPLIFIER IMPLEMENTED WITH LT1991. RF = 450k, RG = 150k, GAIN = 3. ADDING THE DASHED CONNECTIONS CONNECTS THE TWO 450k RESISTORS IN PARALLEL, SO RF IS REDUCED TO 225k. GAIN BECOMES 225k/150k = 1.5.
1991 F10
P1 P1
Figure 10. Difference Amplifier Using the LT1991. Gain Is Set Simply by Connecting the Correct Resistors or Combinations of Resistors. Gain of 3 Is Shown, with Dashed Lines Modifying It to Gain of 1.5. Noise Gain Is Optimal
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LT1991
APPLICATIO S I FOR ATIO
VS + 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 0.25 VS + VIN- 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 3 VS + VIN- 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 6 VS + VIN- 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 12 OUT REF 5 OUT REF 5 6 VOUT VIN- OUT REF 5 6 VOUT VIN+ VIN- OUT REF 5 6 VOUT VIN
+
VIN VIN
-
VIN
-
+
VIN+
VIN+
VIN+
VIN+
Figure 11. Many Difference Gains Are Achievable Just by Strapping the Pins
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VS + 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 1 VS + 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 4 VS + 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 9 VS + VIN- 6 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 13
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VS+ VIN- 6 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 2.25 VS+ VIN- 6 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 4.5 VS+ VIN- 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 10 OUT REF 5 6 VOUT OUT REF 5 6 VOUT OUT REF 5 6 VOUT
OUT REF 5
VOUT
VIN+
OUT REF 5
VOUT
VIN+
OUT REF 5
6
VOUT
VIN+
VOUT
OUT REF 5
6
VOUT
VIN+
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LT1991
APPLICATIO S I FOR ATIO
RF
VIN- VIN+
RG
-
VOUT
RG
+
RF
VOUT = GAIN * (VIN+ - VIN-) GAIN = RF/RG
CLASSICAL DIFFERENCE AMPLIFIER
Figure 12. Another Method of Selecting Difference Gain Is "Cross-Coupling." The Additional Method Means the LT1991 Provides All Integer Gains from 1 to 13
Difference Amplifier: Additional Integer Gains Using Cross-Coupling Figure 12 shows the basic difference amplifier as well as the LT1991 in a difference gain of 3. But notice the effect of the additional dashed connections. This is referred to as "cross-coupling" and has the effect of reducing the differential gain from 3 to 2. Using this method, additional integer gains are achievable, as shown in Table 5 below, so that all integer gains from 1 to 13 are achieved with the LT1991. Note that the equations can be written by inspection from the VIN+ connections, and that the VIN- connections are simply the opposite (swap P for M and M for P). Noise gain, bandwidth, and input impedance specifications for the various cases are also tabulated, as these are not obvious. Schematics are provided in Figure 13.
Table 5. Connections Using Cross-Coupling. Note That Equations Can Be Written by Inspection of the VIN+ Column
Gain 2 5 6* 7 8 VIN+ P3, M1 P9, M3 P9, M1 VIN- M3, P1 M9, P3 M9, P1 Noise -3dB BW RIN RIN Equation Gain kHz Typ k Typ k 3-1 9-3 9-1 5 13 11 70 32 35 32 38 32 281 97 122 121 248 242 141 49 49 44 50 37
+ -
P9, M3, M1 M9, P3, P1 9 - 3 - 1 14 P9, P1, M3 M9, M1, P3 9 + 1 - 3 14
11 P9, P3, M1 M9, M3, P1 9 + 3 - 1 14
*Gain of 6 is better implemented as shown previously, but is included here for completeness.
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8 M9 50k 9 M3 150k 10 M1 450k CROSSCOUPLING 450k 4pF VIN-
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6 VOUT
1 P1 2 P3 3 P9
450k 150k 50k
+
4pF
VIN+
450k LT1991
5
CLASSICAL DIFFERENCE AMPLIFIER IMPLEMENTED WITH LT1991. RF = 450k, RG = 150k, GAIN = 3. GAIN CAN BE ADJUSTED BY "CROSS COUPLING." MAKING THE DASHED CONNECTIONS REDUCE THE GAIN FROM 3 T0 2. WHEN CROSS COUPLING, SEE WHAT IS CONNECTED TO THE VIN+ VOLTAGE. CONNECTING P3 AND M1 GIVES +3 -1 = 2. CONNECTIONS TO VIN- ARE SYMMETRIC: M3 AND P1.
1991 F10
VS+ VIN- 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 2 VS+ VIN- 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 7 VS+ VIN- 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 VS- GAIN = 11 OUT REF 5 6 VOUT OUT REF 5 6 VOUT VIN- 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 OUT REF 5 6 VOUT VIN- 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9
VS+ 7 VCC LT1991 VEE 4 VS- GAIN = 5 VS+ 7 VCC LT1991 VEE 4 VS- GAIN = 8 OUT REF 5 6 VOUT OUT REF 5 6 VOUT
VIN
+
VIN+
VIN+
VIN+
VIN+
1991 F13
Figure 13. Integer Gain Difference Amplifiers Using Cross-Coupling
LT1991
APPLICATIO S I FOR ATIO
High Voltage CM Difference Amplifiers
This class of difference amplifier remains to be discussed. Figure 14 shows the basic circuit on the top. The effective input voltage range of the circuit is extended by the fact that resistors RT attenuate the common mode voltage seen by the op amp inputs. For the LT1991, the most useful resistors for RG are the M1 and P1 450k resistors, because they do not have diode clamps to the supplies and therefore can be taken outside the supplies. As before, the input CM of the op amp is the limiting factor and is set by the voltage at the op amp +input, VINT. By superposition we can write: VINT = VEXT * (RF||RT)/(RG + RF||RT) + VREF * (RG||RT)/ (RF + RG||RT) + VTERM * (RF||RG)/(RT + RF||RG) Solving for VEXT: VEXT = (1 + RG/(RF||RT)) * (VINT - VREF * (RG||RT)/ (RF + RG||RT) - VTERM * (RF||RG)/(RT + RF||RG)) Given the values of the resistors in the LT1991, this equation has been simplified and evaluated, and the resulting equations provided in Table 6. As before, substituting VCC - 1.2 and VEE + 1 for VLIM will give the valid upper and lower common mode extremes respectively. Following are sample calculations for the case shown in Figure 14, right-hand side. Note that P9 and M9 are terminated so row 3 of Table 6 provides the equation: MAX VEXT = 11 * (VCC - 1.2V) - VREF - 9 * VTERM = 11 * (10.8V) - 2.5 - 9 * 12 = 8.3V and: MIN VEXT = 11 * (VEE + 1V) - VREF - 9 * VTERM = 11 * (1V) - 2.5 - 9 * 12 = -99.5V but this exceeds the 60V absolute maximum rating of the P1, M1 pins, so -60V becomes the de facto negative common mode limit. Several more examples of high CM circuits are shown in Figures 15, 16, 17 for various supplies.
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Table 6. HighV CM Connections Giving Difference Gains for the LT1991
Noise Gain 2 P3, M3 P9, M9 P3||P9 M3||M9 5 11 14 Max, Min VEXT (Substitute VCC - 1.2, VEE + 1 for VLIM) 2 * VLIM - VREF 5 * VLIM - VREF - 3 * VTERM 11 * VLIM - VREF - 9 * VTERM 14 * VLIM - VREF - 12 * VTERM Gain 1 1 1 1 VIN+ P1 P1 P1 P1 VIN- M1 M1 M1 M1 RT
RF VCC VIN- VIN+ (= VEXT) RG
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VOUT
RG
+
RT RT
VOUT = GAIN * (VIN+ - VIN-) VEE GAIN = RF/RG RF VREF
VTERM HIGH CM VOLTAGE DIFFERENCE AMPLIFIER INPUT CM TO OP AMP IS ATTENUATED BY RESISTORS RT CONNECTED TO VTERM. 12V 8 M9 50k 9 M3 150k 10 M1 450k 7 450k 4pF
-
6 VOUT
1 P1 VIN+ VIN- INPUT CM RANGE = -60V TO 8.3V 2 P3 3 P9
450k 150k 50k
+
4pF
450k 4
REF 5 LT1991
2.5V
HIGH NEGATIVE CM VOLTAGE DIFFERENCE AMPLIFIER IMPLEMENTED WITH LT1991. RF = 450k, RG = 450k, RT 50k, GAIN = 1 VTERM = VCC = 12V, VREF = 2.5V, VEE = GROUND.
1991 F14
Figure 14. Extending CM Input Range
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APPLICATIO S I FOR ATIO
3V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 6 VOUT OUT REF 5 1.25V VIN - VIN + VIN - VIN +
VCM = 0.8V TO 2.35V
3V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 1.25V VCM = 0V TO 4V 7 VCC LT1991 VEE 4 6 VOUT OUT REF 5 1.25V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9
VIN - VIN +
VIN - VIN +
3V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 1.25V VCM = -1.5V TO 7.2V 7 VCC LT1991 VEE 4 6 VOUT OUT REF 5 1.25V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9
VIN - VIN +
VIN - VIN +
3V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 1.25V VCM = -2.25V TO 8.95V 7 VCC LT1991 VEE 4 6 VOUT OUT REF 5 1.25V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9
VIN - VIN +
VIN - VIN +
Figure 15. Common Mode Ranges for Various LT1991 Configurations on VS = 3V, 0V; with Gain = 1
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3V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 OUT REF 5 6 VOUT 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 3V 7 VCC LT1991 VEE 4 OUT REF 5 3V 6 VOUT VIN - VIN + VCM = 2V TO 3.6V VDM > 40mV 3V 7 VCC LT1991 VEE 4 6 VOUT OUT REF 5 1.25V 3V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 VCM = -1V TO 0.6V VDM <-40mV 3V 7 VCC LT1991 VEE 4 6 VOUT OUT REF 5 1.25V VIN - VIN + VCM = 3.8V TO 7.75V VCM = -5V TO -1.25V 3V 7 VCC LT1991 VEE 4 6 VOUT OUT REF 5 1.25V 3V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 3V 7 VCC LT1991 VEE 4 6 VOUT OUT REF 5 1.25V VIN - VIN + VCM = 9.8V TO 18.55V VCM = -17.2V TO -8.45V 3V 7 VCC LT1991 VEE 4 6 VOUT OUT REF 5 1.25V 3V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 3V 7 VCC LT1991 VEE 4 6 VOUT OUT REF 5 1.25V VIN - VIN + VCM = 12.75V TO 23.95V VCM = -23.2V TO -12V
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LT1991
APPLICATIO S I FOR ATIO
5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 OUT REF 5 2.5V 6 VOUT VIN - VIN +
VIN - VIN +
VCM = -0.5V TO 5.1V
5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 2.5V VCM = -5V TO 9V 7 VCC LT1991 VEE 4 OUT REF 5 2.5V 6 VOUT 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9
VIN - VIN +
VIN - VIN +
5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 2.5V VCM = -14V TO 16.8V 7 VCC LT1991 VEE 4 OUT REF 5 2.5V 6 VOUT 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9
VIN - VIN +
VIN - VIN +
5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 2.5V VCM = -18.5V TO 20.7V 7 VCC LT1991 VEE 4 OUT REF 5 2.5V 6 VOUT 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9
VIN - VIN +
VIN - VIN +
Figure 16. Common Mode Ranges for Various LT1991 Configurations on VS = 5V, 0V; with Gain = 1
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5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 OUT REF 5 6 VOUT 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 5V 7 VCC LT1991 VEE 4 OUT REF 5 3V 6 VOUT VIN - VIN + VCM = 2V TO 7.6V VDM > 40mV 5V 7 VCC LT1991 VEE 4 OUT REF 5 2.5V 6 VOUT 5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 VCM = -3V TO 2.6V VDM <-40mV 5V 7 VCC LT1991 VEE 4 OUT REF 5 2.5V 6 VOUT VIN - VIN + VCM = 2.5V TO 16.5V VCM = -12.5V TO 1.5V 5V 7 VCC LT1991 VEE 4 OUT REF 5 2.5V 6 VOUT 5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 5V 7 VCC LT1991 VEE 4 OUT REF 5 2.5V 6 VOUT VIN - VIN + VCM = 8.5V TO 39.3V VCM = -36.5V TO -5.7V 5V 7 VCC LT1991 VEE 4 OUT REF 5 2.5V 6 VOUT 5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 5V 7 VCC LT1991 VEE 4 OUT REF 5 2.5V 6 VOUT VIN - VIN + VCM = 11.5V TO 50.7V VCM = -48.5V TO -9.3V
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APPLICATIO S I FOR ATIO
5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 -5V VCM = -8V TO 7.6V OUT REF 5 6 VOUT VIN - VIN +
VIN - VIN +
5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 2.5V 7 VCC LT1991 VEE 4 -5V VCM = -20V TO 19V OUT REF 5 6 VOUT 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9
VIN - VIN +
VIN - VIN +
5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 -5V VCM = -44V TO 41.8V -5V OUT REF 5 6 VOUT 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9
VIN - VIN +
VIN - VIN +
5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 -5V VCM = -56V TO 53.2V -5V OUT REF 5 6 VOUT 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9
VIN - VIN +
VIN - VIN +
Figure 17. Common Mode Ranges for Various LT1991 Configurations on VS = 5V, with Gain = 1
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5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 VCC LT1991 VEE 4 OUT REF 5 -5V 6 VOUT 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 5V 7 VCC LT1991 VEE 4 OUT REF 5 -5V 6 VOUT VIN - VIN + -5V VCM = -3V TO 12.6V VDM > 40mV 5V 7 VCC LT1991 VEE 4 -5V VCM = -5V TO 34V OUT REF 5 6 VOUT 5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 -5V VCM = -13V TO 2.6V VDM <-40mV 5V 7 VCC LT1991 VEE 4 -5V VCM = -35V TO 4V OUT REF 5 6 VOUT VIN - VIN + 5V 7 VCC LT1991 VEE 4 -5V VCM = 1V TO 60V OUT REF 5 6 VOUT 5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 5V 7 VCC LT1991 VEE 4 -5V VCM = -60V TO -3.2V OUT REF 5 6 VOUT VIN - VIN + 5V 7 VCC LT1991 VEE 4 -5V VCM = 4V TO 60V OUT REF 5 6 VOUT 5V 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 5V 7 VCC LT1991 VEE 4 -5V VCM = -60V TO -6.8V
1991 F17
W
UU
VIN - VIN +
OUT REF 5
6
VOUT
1991fc
LT1991
PACKAGE DESCRIPTIO
5.23 (.206) MIN
0.50 0.305 0.038 (.0197) (.0120 .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT
0.254 (.010) GAUGE PLANE
0.18 (.007) SEATING PLANE
NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
MS Package 10-Lead Plastic MSOP
(Reference LTC DWG # 05-08-1661)
0.889 0.127 (.035 .005) 3.20 - 3.45 (.126 - .136)
3.00 0.102 (.118 .004) (NOTE 3) 10 9 8 7 6 0.497 0.076 (.0196 .003) REF
DETAIL "A" 0 - 6 TYP
4.90 0.152 (.193 .006)
3.00 0.102 (.118 .004) (NOTE 4)
12345 0.53 0.152 (.021 .006)
DETAIL "A"
1.10 (.043) MAX
0.86 (.034) REF
0.17 - 0.27 (.007 - .011) TYP
0.50 (.0197) BSC
0.127 0.076 (.005 .003)
MSOP (MS) 0603
1991fc
23
LT1991
TYPICAL APPLICATIO
VP
Bidirectional Current Source
VS+ 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7
VIN - VIN + R2* 10k
LT1991 5 4 VS
-
*SHORT R2 FOR LOWEST OUTPUT OFFSET CURRENT. INCLUDE R2 FOR HIGHEST OUTPUT IMPEDANCE.
RELATED PARTS
PART NUMBER LT1990 LT1991 LT1995 LT6010/LT6011/LT6012 LT6013/LT6014 LTC6910-X DESCRIPTION High Voltage, Gain Selectable Difference Amplifier Precision Gain Selectable Difference Amplifier High Speed, Gain Selectable Difference Amplifier Single/Dual/Quad 135A 14nV/Hz Rail-to-Rail Out Precision Op Amp Single/Dual 145A 8nV/Hz Rail-to-Rail Out Precision Op Amp Programmable Gain Amplifiers COMMENTS 250V Common Mode, Micropower, Pin Selectable Gain = 1, 10 Micropower, Pin Selectable Gain = -13 to 14 30MHz, 1000V/s, Pin Selectable Gain = -7 to 8 Similar Op Amp Performance as Used in LT1991 Difference Amplifier Lower Noise AV 5 Version of LT1991 Type Op Amp 3 Gain Configurations, Rail-to-Rail Input and Output
24
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
+
VM
1/2 LT6011
1/2 LT6011
U
Micropower AV = 10 Instrumentation Amplifier
10 9 8 7 6 VOUT
+
- -
1 2 3
- +
4pF
LT1991
4pF 4 5
1991 TA02
Single Supply AC Coupled Amplifier
VS = 2.7V TO 36V 1F 8 M9 9 M3 10 M1 1 P1 2 P3 3 P9 7 6 5 4
6 R1 10k VIN + - VIN - 10k VCC VIN ILOAD = 0.1F
LT1991
VOUT
GAIN = 12 BW = 7Hz TO 32kHz
1991 TA03
1991fc LT/LWI/LT 0505 REV C * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2004

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