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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 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769. 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 _______________________________________________________________________________________ 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 _______________________________________________________________________________________ 3 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 _______________________________________________________________________________________ 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 _______________________________________________________________________________________ 5 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 _______________________________________________________________________________________ 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 _______________________________________________________________________________________ 7 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 8 _______________________________________________________________________________________ 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 _______________________________________________________________________________________ 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) 10 ______________________________________________________________________________________ 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 ______________________________________________________________________________________ 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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