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19-1468; Rev 0; 4/99
Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers
General Description
The MAX4194 is a variable-gain precision instrumentation amplifier that combines Rail-to-Rail(R) single-supply operation, outstanding precision specifications, and a high gain bandwidth. This amplifier is also offered in three fixed-gain versions: the MAX4195 (G = +1V/V), the MAX4196 (G = +10V/V), and the MAX4197 (G = +100V/V). The fixed-gain instrumentation amplifiers feature a shutdown function that reduces the quiescent current to 8A. A traditional three operational amplifier configuration is used to achieve maximum DC precision. The MAX4194-MAX4197 have rail-to-rail outputs and inputs that can swing to within 200mV of the negative rail and to within 1.1V of the positive rail. All parts draw only 93A and operate from a single +2.7V to +7.5V supply or from dual 1.35V to 3.75V supplies. These amplifiers are offered in 8-pin SO packages and are specified for the extended temperature range (-40C to +85C). See the MAX4198/MAX4199 data sheet for single-supply, precision differential amplifiers. o +2.7V Single-Supply Operation o Low Power Consumption 93A Supply Current 8A Shutdown Current (MAX4195/96/97) o High Common-Mode Rejection: 115dB (G = +10V/V) o Low 50V Input Offset Voltage (G +100V/V) o Low 0.01% Gain Error (G = +1V/V) o 250kHz -3dB Bandwidth (G = +1V/V, MAX4194) o Rail-to-Rail Outputs
Features
MAX4194-MAX4197
Ordering Information
PART MAX4194ESA MAX4195ESA MAX4196ESA MAX4197ESA TEMP. RANGE -40C to +85C -40C to +85C -40C to +85C -40C to +85C PIN-PACKAGE 8 SO 8 SO 8 SO 8 SO
Applications
Medical Equipment Thermocouple Amplifier 4-20mA Loop Transmitters Data-Acquisition Systems Battery-Powered/Portable Equipment Transducer Interface Bridge Amplifier
PART MAX4194 MAX4195 MAX4196 MAX4197 SHUTDOWN No Yes Yes Yes
Selector Guide
GAIN (V/V) Variable +1 +10 +100 CMRR (dB) 95 (G = +1V/V) 95 115 115
Pin Configurations
TOP VIEW
RG- 1 IN- 2
8 7
RG+ VCC OUT REF
REF 1 IN- 2 IN+ 3
8 7
SHDN VCC OUT FB
MAX4194
IN+ 3 6 5 VEE 4
MAX4195 MAX4196 MAX4197
6 5
VEE 4
SO
SO
Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd.
________________________________________________________________ Maxim Integrated Products 1
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Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers MAX4194-MAX4197
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VCC to VEE)..................................................+8V All Other Pins .................................. (VCC + 0.3V) to (VEE - 0.3V) Current into Any Pin..........................................................30mA Output Short-Circuit Duration (to VCC or VEE)........... Continuous Continuous Power Dissipation (TA = +70C) SO (derate 5.9mW/C above +70C) ........................... 471mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10sec) ............................ +300C
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VCC = +5V, VEE = 0, RL = 25k tied to VCC/2, VREF = VCC/2, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Supply Voltage Range Quiescent Current Shutdown Current SYMBOL VCC ICC I SHDN Inferred by PSR CONDITIONS Single supply Dual supplies MIN 2.7 1.35 93 8 100 75 50 50 100 75 50 50 1.0 0.5 Differential Common mode Differential Common mode VEE + 0.2 G = +1V/V G = +10V/V G = +100V/V G = +1000V/V G = +1V/V G = +10V/V G = +100V/V G = +1000V/V G = +1V/V G = +10V/V G = +100V/V 73 88 90 78 93 95 95 115 115 115 95 115 115 115 85 101 106 dB dB 1000 1000 1 4 VCC - 1.1 4.0 2.0 V/C M pF V 690 345 345 TYP MAX 7.5 3.75 110 12 450 225 225 V UNITS V A A
test
VIN+ = VIN- = VCC/2, VDIFF = 0 I SHDN = VIL, MAX4195/96/97 only G = +1V/V, VCM = VCC/2, TA = +25C G = +10V/V, VCM = VCC/2, TA = +25C G = +100V/V, VCM = VCC/2, TA = +25C G = +1000V/V, VCM = VCC/2, TA = +25C G = +1V/V, VCM = VCC/2, TA = TMIN to TMAX G = +10V/V, VCM = VCC/2, TA = TMIN to TMAX G = +100V/V, VCM = VCC/2, TA = TMIN to G = +1000V/V, VCM = VCC/2, TA = TMIN to
Input Offset Voltage
VOS
Input Offset Voltage Drift (Note 1) Input Resistance Input Capacitance Input Voltage Range
TCVOS RIN CIN VIN
G = +1V/V G +10V/V VCM = VCC/2 VCM = VCC/2 Inferred from CMR test VCM = VEE + 0.2V to VCC - 1.1V, TA = +25C, RS = 1k
DC Common-Mode Rejection
CMRDC VCM = VEE + 0.2V to VCC - 1.1V, TA = TMIN to TMAX, RS = 1k VCM = VEE + 0.2V to VCC - 1.1V, f = 120Hz
AC Common-Mode Rejection
CMRAC
2
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Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +5V, VEE = 0, RL = 25k tied to VCC/2, VREF = VCC/2, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Power-Supply Rejection Input Bias Current Input Bias Current Drift Input Offset Current Input Offset Current Drift SYMBOL PSR IB TCIB IOS TCIOS CONDITIONS +2.7V VCC +7.5V; VCM = +1.5V; VOUT = +1.5V; VREF = +1.5V; RL = 25k to +1.5V; G = +1V/V, +10V/V, +100V/V VCM = VCC/2 VCM = VCC/2 VCM = VCC/2 VCM = VCC/2 f = 10Hz G = +1V/V f = 100Hz f = 10KHz f = 0.1Hz to 10Hz f = 10Hz Input Noise Voltage en G = +10V/V f = 100Hz f = 10KHz f = 0.1Hz to 10Hz f = 10Hz G = +100V/V f = 100Hz f = 10KHz f = 0.1Hz to 10Hz f = 10Hz Input Noise Current in f = 100Hz f = 10kHz f = 0.1Hz to 10Hz RL = 25k to VCC/2 Output Voltage Swing VOH, VOL RL = 5k to VCC/2 Short-Circuit Current (Note 2) Gain Equation ISC MAX4194 only TA = +25C, VCM = VCC/2, RL = 25k, VEE + 0.1V VOUT VCC - 0.1V TA = +25C, VCM = VCC/2, RL = 5k, VEE + 0.2V VOUT VCC - 0.2V G = +1V/V G = +10V/V G = +100V/V G = +1000V/V, MAX4194 G = +1V/V G = +10V/V G = +100V/V G = +1000V/V, MAX4194 VCC - VOH VOL VCC - VOH VOL MIN 90 TYP 120 6 15 1.0 15 85 75 72 1.4 35 32 31 0.7 32 31 8.7 0.6 2.4 0.76 0.1 16 30 30 100 100 4.5 1+ (50k/RG) 0.01 0.03 0.05 0.5 0.01 0.03 0.05 0.5 0.1 0.3 0.5 0.1 0.3 0.5 % 100 100 200 200 mA mV pARMS pAHz VRMS nVHz VRMS nVHz VRMS nVHz 3.0 20 MAX UNITS dB nA pA/C nA pA/C
MAX4194-MAX4197
Gain Error
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Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers MAX4194-MAX4197
ELECTRICAL CHARACTERISTICS (continued)
(VCC = +5V, VEE = 0, RL = 25k tied to VCC/2, VREF = VCC/2, TA = TMIN to TMAX, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER Gain Temperature Coefficient (Note 1) 50k Resistance Temperature Coefficient (Note 3) Nonlinearity Capacitive Load Stability CL G = +1V/V VOUT 0.1Vp-p, G = +10V/V VCM = VCC/2 G = +100V/V G = +1000V/V Slew Rate SR VOUT = 2Vp-p, G = +1V/V G = +1V/V Settling Time tS 0.1%, VOUT = 2Vp-p G = +10V/V G = +100V/V G = +1000V/V Total Harmonic Distortion Input Logic Voltage High Input Logic Voltage Low SHDN Input Current Time to Shutdown Enable Time From Shutdown Power-Up Delay On/Off Settling Time t ON/OFF t SHDN t ENABLE THD VIH VIL VEE < V SHDN < VCC G = +1V/V, 0.1%, VOUT = +3V G = +1V/V, 0.1%, VOUT = +3.5V MAX4195/MAX4196/ MAX4197 only MAX4195/MAX4196/ MAX4197 only MAX4195/MAX4196/ MAX4197 only 0.5 0.5 1 0.5 VOUT = 2Vp-p, G = +1V/V, f = 1kHz VCC - 1.5 VCC - 2.5 0.1 MAX4194 MAX4195 MAX4194 MAX4196 MAX4194 MAX4197 MAX4194 TC50k SYMBOL CONDITIONS MAX4194/MAX4195, G = +1V/V MAX4196/MAX4197 MAX4194 VEE + 0.1V VOUT VCC - 0.1V, VCM = VCC/2, G = +1V/V, +10V/V, +100V/V, +1000V/V MIN TYP 1 1 16 0.001 300 250 220 17 34 1.5 3.1 0.147 0.06 0.05 0.04 5 7 0.001 % V V A ms ms ms ms ms V/s kHz MAX 8 15 UNITS ppm/C ppm/C % pF
-3dB Bandwidth
BW-3dB
G = +1V/V, 0.1%, VOUT = +3.5V VSHDN = VCC - 2.5V to VCC - 1.5V, G = +100V/V, 0.1%, VOUT = +3.5V
Note 1: Guaranteed by design. Note 2: Maximum output current (sinking/sourcing) in which the gain changes by less than 0.1%. Note 3: This specification represents the typical temperature coefficient of an on-chip thin film resistor. In practice, the temperature coefficient of the gain for the MAX4194 will be dominated by the temperature coefficient of the external gain-setting resistor.
4
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Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers
Typical Operating Characteristics
(VCC = +5V, VEE = 0, RL = 25k tied to VCC/2, TA = +25C, unless otherwise noted.)
MAX4194 SMALL-SIGNAL GAIN vs. FREQUENCY
MAX4194 toc01-1
MAX4194-MAX4197
MAX4195/MAX4196/MAX4197 SMALL-SIGNAL GAIN vs. FREQUENCY
MAX4194 toc02-2
0.1% SETTLING TIME vs. GAIN (VOUT = 2Vp-p)
MAX4194 toc03
4 3 NORMALIZED GAIN (dB) 2 1 0 -1 -2 -3 -4 -5 -6 100 1k 10k FREQUENCY (Hz) 100k G = +10V/V G = +100V/V G = +1V/V
4 3 NORMALIZED GAIN (dB) 2 1 0 -1 -2 -3 -4 -5 -6 G = +100V/V G = +10V/V G = +1V/V
10k
1k SETTLING TIME (s)
100
10
1M
100
1k
10k FREQUENCY (Hz)
100k
1M
1
1
10 GAIN (V/V)
100
1k
MAX4194 LARGE-SIGNAL PULSE RESPONSE (GAIN = +1V/V)
MAX4194 toc04
MAX4194 LARGE-SIGNAL PULSE RESPONSE (GAIN = +100V/V)
MAX4194 toc05
MAX4197 LARGE-SIGNAL PULSE RESPONSE (GAIN = +100V/V)
MAX4194 toc06
INPUT (500mV/div)
INPUT (5mV/div)
INPUT (5mV/div)
OUTPUT (500mV/div)
OUTPUT (500mV/div)
OUTPUT (500mV/div)
20s/div
200s/div
200s/div
MAX4194 SMALL-SIGNAL PULSE RESPONSE (GAIN = +1V/V)
MAX4194 toc07
MAX4194 SMALL-SIGNAL PULSE RESPONSE (GAIN = +100V/V)
MAX4194 toc08
MAX4197 SMALL-SIGNAL PULSE RESPONSE (GAIN = +100V/V)
MAX4194 toc09
INPUT (50mV/div)
INPUT (500V/div)
INPUT (500V/div)
OUTPUT (50mV/div)
OUTPUT (50mV/div)
OUTPUT (50mV/div)
20s/div
200s/div
200s/div
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Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers MAX4194-MAX4197
Typical Operating Characteristics (continued)
(VCC = +5V, VEE = 0, RL = 25k tied to VCC/2, TA = +25C, unless otherwise noted.)
POWER-SUPPLY REJECTION vs. FREQUENCY
MAX4194 toc10
COMMON-MODE REJECTION vs. FREQUENCY
MAX4194 toc11
VOLTAGE-NOISE DENSITY vs. FREQUENCY
MAX4194 toc12
0 -20 -40 -60 -80 G = +100V/V -100 -120 -140 1 10 100 1k 10k G = +1000V/V
-30 -40 -50 -60 CMR (dB) -70 -80 -90 -100 -110 -120 G = +10V/V G = +100V/V G = +1,000V/V G = +1V/V
1,000 VOLTAGE NOISE DENSITY (nV//Hz) G = +1V/V 100 G = +10V/V
G = +1V/V G = +10V/V
PSR (dB)
10 G = +100V/V G = +1000V/V 1 1 10 100 1k 10k 100k
100k
10
100
1k FREQUENCY (Hz)
10k
100k
FREQUENCY (Hz)
FREQUENCY (Hz)
MAX4195/MAX4196 TOTAL HARMONIC DISTORTION PLUS NOISE vs. FREQUENCY
MAX4194 toc13
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX4194 toc14
SUPPLY CURRENT vs. TEMPERATURE
G = +1000V/V G = +100V/V
MAX4194 toc15
1.000
120
98 96 SUPPLY CURRENT (A) 94 92 G = +1V/V, +10V/V 90 88 86
0.100 THD + NOISE (%)
SUPPLY CURRENT (A)
110
0.010 MAX4196 G = +10V/V 0.001 G = +1V/V MAX4195 0
100
90
80 1 10 100 FREQUENCY (Hz) 1k 10k
2
3
4 5 6 7 SUPPLY VOLTAGE (V)
8
9
84
-40
-15
10
35
60
85
TEMPERATURE (C)
MAX4194 toc16
10
SHUTDOWN CURRENT (A)
8
INPUT BIAS CURRENT (nA)
8
6
6
4
4
2
2
0 0 -40 -15 10 35 TEMPERATURE (C) 60 85 -40 -15 10 35 60 85 TEMPERATURE (C)
6
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MAX4194TOC17
MAX4195/MAX4196/MAX4197 SHUTDOWN CURRENT vs. TEMPERATURE
INPUT BIAS CURRENT vs. TEMPERATURE
10
Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers
Typical Operating Characteristics (continued)
(VCC = +5V, VEE = 0, RL = 25k tied to VCC/2, TA = +25C, unless otherwise noted.)
INPUT OFFSET CURRENT vs. TEMPERATURE
MAX4194TOC18
MAX4194-MAX4197
INPUT OFFSET VOLTAGE vs. TEMPERATURE
75 INPUT OFFSET VOLTAGE (V) 50 25 0 -25 -50 -75 MAX4194 (G = +10V/V) MAX4194 (G = +100V/V, G = +1000V/V) MAX4197 MAX4196
MAX4194TOC19
50 MAX4196 INPUT OFFSET CURRENT (pA) 0 MAX4197
100
-50
-100
MAX4195
-150
-200 -40 -15 10 35 60 85 TEMPERATURE (C)
MAX4195 -100 -40 -15 10 35 60 85 TEMPERATURE (C)
Pin Description
PIN MAX4194 1, 8 5 2 3 4 -- 6 7 -- MAX4195 MAX4196 MAX4197 -- 1 2 3 4 5 6 7 8 NAME FUNCTION FUNCTION
RG-, RG+ REF ININ+ VEE FB OUT VCC SHDN
Connection for Gain Setting Resistor Reference Voltage. Offsets output voltage. Inverting Input Noninverting Input Negative Supply Voltage Feedback. Connects to OUT. Amplifier Output Positive Supply Voltage Shutdown Control
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Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers MAX4194-MAX4197
Detailed Description
Input Stage
The MAX4194-MAX4197 family of low-power instrumentation amplifiers implements a three-amplifier topology (Figure 1). The input stage is composed of two operational amplifiers that together provide a fixed-gain differential and a unity common-mode gain. The output stage is a conventional differential amplifier that provides an overall common-mode rejection of 115dB (G =
25k 25k RG25k VCC
+10V/V). The MAX4194's gain can be externally set between +1V/V and +10,000V/V (Table 1). The MAX4195/MAX4196/MAX4197 have on-chip gain-setting resistors (Figure 2), and their gains are fixed at +1V/V, +10V/V, and +100V/V, respectively.
Input Voltage Range and Detailed Operation
The common-mode input range for all of these amplifiers is VEE + 0.2V to VCC - 1.1V. Ideally, the instrumentation amplifier (Figure 3) responds only to a differential voltage applied to its inputs, IN+ and IN-. If both inputs are at the same voltage, the output is VREF. A differential voltage at IN+ (VIN+) and IN- (VIN-) develops an identical voltage across the gain-setting resistor, causing a current (I G ) to flow. This current also flows through the feedback resistors of the two input amplifiers A1 and A2, generating a differential voltage of: VOUT2 - VOUT1 = IG * (R1 + RG + R1) where VOUT1 and VOUT2 are the output voltages of A1 and A2, RG is the gain-setting resistor (internal or external to the part), and R1 is the feedback resistor of the input amplifiers. IG is determined by the following equation: IG = (VIN+ - VIN-) / RG The output voltage (V OUT ) for the instrumentation amplifier is expressed in the following equation: VOUT = (VIN+ - VIN-) * [(2 * R1) / RG] + 1 The common-mode input range is a function of the amplifier's output voltage and the supply voltage. With a power supply of VCC, the largest output signal swing can be obtained with REF tied to VCC/2. This results in an output voltage swing of VCC/2. An output voltage swing less than full-scale increases the common-mode input range.
* R1 = R2 = 25k ** RG = INTERNAL TO MAX4195/MAX4196/MAX4197 RG = EXTERNAL TO MAX4194
IN-
MAX4194
RG+ 25k IN+ 25k 25k
OUT
REF VEE
Figure 1. MAX4194 Simplified Block Diagram
IN-
25k 25k 25k RG 25k
VCC FB
SHDN IN+
MAX4195 MAX4196 MAX4197
25k
OUT
REF 25k VEE
Figure 2. MAX4195/MAX4196/MAX4197 Simplified Block Diagram
R2*
VINIG
VOUT1 A1
R2* R1*
VIN+ - VIN-
RG** R1* IG
VOUT2 - VOUT1
A3
OUT VOUT = (VIN+ - VIN-) * 1 +
R2* A2 VOUT2 R2* REF
(
2R1 RG
)
VIN+
Figure 3. Instrumentation Amplifier Configuration
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Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers MAX4194-MAX4197
Table 1. MAX4194 External Gain Resistor Selection
GAIN (V/V) +1 +2 +5 +10 +20 +50 +100 +200 +500 +1,000 +2,000 +5,000 +10,000 CLOSEST RG (1%) CLOSEST RG (5%) () () * 49.9k 12.4k 5.62k 2.61k 1.02k 511 249 100 49.9 24.9 10 4.99 * 51k 12k 5.6k 2.7k 1.0k 510 240 100 51 24 10 5.1
COMMON-MODE INPUT VOLTAGE (V) 5 MAX4194/MAX4195 G = +1V/V REF = +2.5V/+1.5V A 3 B 2 C 1 0.03 0 1 2 D 3 4 4.97 5 E F VCC = +5V/+3V VEE = 0 TA = +25C
This disables the instrumentation amplifier and puts its output in a high-impedance state. Pulling SHDN high enables the instrumentation amplifier.
Applications Information
Setting the Gain (MAX4194)
The MAX4194's gain is set by connecting a single, external gain resistor between the two RG pins (pin 1 and pin 8), and can be described as: G = 1 + 50k / RG where G is the instrumentation amplifier's gain and RG is the gain-setting resistor. The 50k resistor of the gain equation is the sum of the two resistors internally connected to the feedback loops of the IN+ and IN- amplifiers. These embedded feedback resistors are laser trimmed, and their accuracy and temperature coefficients are included in the gain and drift specification for the MAX4194.
* Leave pins 1 and 8 open for G = +1V/V.
VCM vs. VOUT Characterization Figure 4 illustrates the MAX4194 typical common-mode input voltage range over output voltage swing at unitygain (pins 1 and 8 left floating), with a single-supply voltage of VCC = +5V and a bias reference voltage of VREF = VCC/2 = +2.5V. Points A and D show the full input voltage range of the input amplifiers (VEE + 0.2V to VCC - 1.1V) since, with +2.5V output, there is zero input differential swing. The other points (B, C, E, and F) are determined by the input voltage range of the input amps minus the differential input amplitude necessary to produce the associated VOUT. For the higher gain configurations, the VCM range will increase at the endpoints (B, C, E, and F) since a smaller differential voltage is necessary for the given output voltage. Rail-to-Rail Output Stage
The MAX4194-MAX4197's output stage incorporates a common-source structure that maximizes the dynamic range of the instrumentation amplifier. The output can drive up to a 25k (tied to VCC/2) resistive load and still typically swing within 30mV of the rails. With an output load of 5k tied to VCC/2, the output voltage swings within 100mV of the rails.
4
0
OUTPUT VOLTAGE (V)
Figure 4. Common-Mode Input Voltage vs. Output Voltage
MAX4195 OUT AC-COUPLED (VDIFF = 2V, G = +1V/V) (500mV/div)
Shutdown Mode
The MAX4195-MAX4197 feature a low-power shutdown mode. When the shutdown pin (SHDN) is pulled low, the internal amplifiers are switched off and the supply current drops to 8A typically (Figures 5a, 5b, and 5c).
50s/div
SHDN (5V/div)
Figure 5a. MAX4195 Shutdown Mode
9
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Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers MAX4194-MAX4197
VCC
MAX4196 OUT AC-COUPLED (VDIFF = 200mV, G = +10V/V) (500mV/div)
IN+ INVEE RG =
RISO VOUT RL CL
(MAX4194) (INTERNAL, MAX4195)
VREF
VREF
SHDN (5V/div) 50s/div
Figure 6a. Using a Resistor to Isolate a Capacitive Load from the Instrumentation Amplifier (G = +1V/V)
Figure 5b. MAX4196 Shutdown Mode
MAX4197 OUT AC-COUPLED (VDIFF = 20mV, G = +100V/V) (500mV/div)
INPUT (50mV/div)
OUTPUT (50mV/div)
SHDN (5V/div) 50s/div
50s/div
Figure 5c. MAX4197 Shutdown Mode
The accuracy and temperature drift of the RG resistors also influence the IC's precision and gain drift, and can be derived from the equation above. With low RG values, which are required for high-gain operation, parasitic resistances may significantly increase the gain error.
Figure 6b. Small-Signal Pulse Response with Excessive Capacitive Load (RL = 25k, CL = 1000pF)
Capacitive Load Stability
The MAX4194-MAX4197 are stable for capacitive loads up to 300pF (Figure 6a). Applications that require greater capacitive-load driving capability can use an isolation resistor (Figure 6b) between the output and the capacitive load to reduce ringing on the output signal. However, this alternative reduces gain accuracy because RISO (Figure 6c) forms a potential divider with the load resistor.
50s/div
INPUT (50mV/div)
OUTPUT (50mV/div)
Figure 6c. Small-Signal Pulse Response with Excessive Capacitive Load and Isolating Resistor (RISO = 75, RL = 25k, CL = 1000pF)
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Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers
Power-Supply Bypassing and Layout
Good layout technique optimizes performance by decreasing the amount of stray capacitance at the instrumentation amplifier's gain-setting pins. Excess capacitance will produce peaking in the amplifier's frequency response. To decrease stray capacitance, minimize trace lengths by placing external components as close to the instrumentation amplifier as possible. For best performance, bypass each power supply to ground with a separate 0.1F capacitor. identical two-element strain gauges) to the inputs of the MAX4194. The bridge contains four resistors, two of which increase and two of which decrease by the same ratio. With a fully balanced bridge, points A (IN+) and B (IN-) see half the excitation voltage (V BRIDGE ). The low impedance (120 to 350) of the strain gauges, however, could cause significant voltage drop contributions by the wires leading to the bridge, which would cause excitation variations. Output voltage VOUT can be calculated as follows: VOUT = VAB * G where G = (1 + 50k / RG) is the gain of the instrumentation amplifier. Since VAB is directly proportional to the excitation, gain errors may occur.
MAX4194-MAX4197
Transducer Applications
The MAX4194-MAX4197 instrumentation amplifiers can be used in various signal-conditioning circuits for thermocouples, PT100s, strain gauges (displacement sensors), piezoresistive transducers (PRTs), flow sensors, and bioelectrical applications. Figure 7 shows a simplified example of how to attach four strain gauges (two
RG REFERENCE RG+ IN+ VBRIDGE R VAB = VIN+ - VINR INVEE B R R VCC RGOUT REF MAX144 ADC P
R = 120 - 350 A
Figure 7. Strain Gauge Connection to the MAX4194
___________________Chip Information
TRANSISTOR COUNT: 432
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11
Micropower, Single-Supply, Rail-to-Rail, Precision Instrumentation Amplifiers MAX4194-MAX4197
Package Information
SOICN.EPS
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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