LT1920 Single Resistor Gain Programmable, Precision Instrumentation Amplifier U DESCRIPTIO FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ The LT ®1920 is a low power, precision instrumentation amplifier that requires only one external resistor to set gains of 1 to 10,000. The low voltage noise of 7.5nV/√Hz (at 1kHz) is not compromised by low power dissipation (0.9mA typical for ±2.3V to ±15V supplies). The high accuracy of 30ppm maximum nonlinearity and 0.3% max gain error (G = 10) is not degraded even for load resistors as low as 2k (previous monolithic instrumentation amps used 10k for their nonlinearity specifications). The LT1920 is laser trimmed for very low input offset voltage (125µV max), drift (1µV/°C), high CMRR (75dB, G = 1) and PSRR (80dB, G = 1). Low input bias currents of 2nA max are achieved with the use of superbeta processing. The output can handle capacitive loads up to 1000pF in any gain configuration while the inputs are ESD protected up to 13kV (human body). The LT1920 with two external 5k resistors passes the IEC 1000-4-2 level 4 specification. The LT1920, offered in 8-pin PDIP and SO packages, is a pin for pin and spec for spec improved replacement for the AD620. The LT1920 is the most cost effective solution for precision instrumentation amplifier applications. For even better guaranteed performance, see the LT1167. Single Gain Set Resistor: G = 1 to 10,000 Gain Error: G = 10, 0.3% Max Gain Nonlinearity: G = 10, 30ppm Max Input Offset Voltage: G = 10, 225µV Max Input Offset Voltage Drift: 1µV/°C Max Input Bias Current: 2nA Max PSRR at G = 1: 80dB Min CMRR at G = 1: 75dB Min Supply Current: 1.3mA Max Wide Supply Range: ± 2.3V to ±18V 1kHz Voltage Noise: 7.5nV/√Hz 0.1Hz to 10Hz Noise: 0.28µVP-P Available in 8-Pin PDIP and SO Packages Meets IEC 1000-4-2 Level 4 ESD Tests with Two External 5k Resistors U APPLICATIO S ■ ■ ■ ■ ■ Bridge Amplifiers Strain Gauge Amplifiers Thermocouple Amplifiers Differential to Single-Ended Converters Medical Instrumentation , LTC and LT are registered trademarks of Linear Technology Corporation. U TYPICAL APPLICATIO Single Supply Barometer VS 3 2 2 LUCAS NOVA SENOR NPC-1220-015-A-3L 8 + 1/2 LT1490 1 LT1634CCZ-1.25 Gain Nonlinearity – 4 1 – 4 5k R6 1k 5 6 R8 100k 5k 2 + 7 R2 12Ω 5k + 3 6 LT1920 G = 60 8 3 5 TO 4-DIGIT DVM + 4 5 1/2 LT1490 – – R1 825Ω RSET R3 50k 2 1 5k 6 R4 50k VS 1 NONLINEARITY (100ppm/DIV) R5 392k OUTPUT VOLTAGE (2V/DIV) G = 1000 RL = 1k VOUT = ±10V 7 R7 50k VS = 8V TO 30V VOLTS 2.800 3.000 3.200 INCHES Hg 28.00 30.00 32.00 1167 TA02 1920 TA01 1 LT1920 W U PACKAGE/ORDER INFORMATION U W W W (Note 1) Supply Voltage ...................................................... ±20V Differential Input Voltage (Within the Supply Voltage) ..................................................... ±40V Input Voltage (Equal to Supply Voltage) ................ ±20V Input Current (Note 3) ........................................ ±20mA Output Short-Circuit Duration .......................... Indefinite Operating Temperature Range ................ – 40°C to 85°C Specified Temperature Range LT1920C (Note 4) .................................... 0°C to 70°C LT1920I .............................................. – 40°C to 85°C Storage Temperature Range ................. – 65°C to 150°C Lead Temperature (Soldering, 10 sec).................. 300°C ELECTRICAL CHARACTERISTICS ORDER PART NUMBER TOP VIEW 8 RG –IN 2 – 7 +VS +IN 3 + 6 OUTPUT 5 REF RG 1 –VS 4 LT1920CN8 LT1920CS8 LT1920IN8 LT1920IS8 N8 PACKAGE 8-LEAD PDIP S8 PACKAGE 8-LEAD PLASTIC SO S8 PART MARKING TJMAX = 150°C, θJA = 130°C/ W (N8) TJMAX = 150°C, θJA = 190°C/ W (S8) 1920 1920I Consult factory for Military grade parts. VS = ±15V, VCM = 0V, TA = 25°C, R L = 2k, unless otherwise noted. SYMBOL PARAMETER CONDITIONS (Note 6) G Gain Range G = 1 + (49.4k/RG ) Gain Error G=1 G = 10 (Note 2) G = 100 (Note 2) G = 1000 (Note 2) G/T U ABSOLUTE MAXIMUM RATINGS Gain vs Temperature G < 1000 (Note 2) Gain Nonlinearity (Note 5) VO = ±10V, G = 1 VO = ±10V, G = 10 and 100 VO = ±10V, G = 100 and 1000 MIN TYP 1 ● VOST Total Input Referred Offset Voltage VOST = VOSI + VOSO/G VOSI Input Offset Voltage G = 1000, VS = ±5V to ±15V G = 1000, VS = ±5V to ±15V ● ● VOSI/T Input Offset Drift (RTI) (Note 3) VOSO Output Offset Voltage G = 1, VS = ±5V to ±15V G = 1, VS = ±5V to ±15V ● (Note 3) ● MAX UNITS 10k 0.008 0.010 0.025 0.040 0.1 0.3 0.3 0.35 20 50 ppm/°C 10 10 20 30 ppm ppm ppm 30 125 185 µV µV 400 1000 1500 1 % % % % µV/°C µV µV VOSO /T Output Offset Drift 5 15 µV/°C IOS Input Offset Current 0.3 1 nA IB Input Bias Current 0.5 2 nA en Input Noise Voltage, RTI 0.1Hz to 10Hz, G = 1 0.1Hz to 10Hz, G = 10 0.1Hz to 10Hz, G = 100 and 1000 2.00 0.50 0.28 µVP-P µVP-P µVP-P Total RTI Noise = √eni 2 + (eno /G)2 eni Input Noise Voltage Density, RTI fO = 1kHz 7.5 nV/√Hz eno Output Noise Voltage Density, RTI fO = 1kHz 67 nV/√Hz in Input Noise Current fO = 0.1Hz to 10Hz 10 pAP-P Input Noise Current Density fO = 10Hz 124 fA/√Hz RIN Input Resistance VIN = ±10V 200 GΩ CIN(DIFF) Differential Input Capacitance fO = 100kHz 1.6 pF 2 LT1920 ELECTRICAL CHARACTERISTICS VS = ±15V, V CM = 0V, TA = 25°C, R L = 2k, unless otherwise noted. SYMBOL PARAMETER CIN(CM) Common Mode Input Capacitance fO = 100kHz VCM Input Voltage Range G = 1, Other Input Grounded VS = ±2.3V to ±5V VS = ±5V to ±18V VS = ±2.3V to ±5V VS = ±5V to ±18V CMRR PSRR Common Mode Rejection Ratio Power Supply Rejection Ratio CONDITIONS (Note 6) 120 135 140 150 dB dB dB dB IOUT Output Current BW Bandwidth G=1 G = 10 G = 100 G = 1000 SR Slew Rate Settling Time to 0.01% AVREF Reference Gain to Output V V V V 80 100 120 120 RL = 10k VS = ±2.3V to ±5V VS = ±5V to ±18V VS = ±2.3V to ±5V VS = ±5V to ±18V Reference Voltage Range +VS – 1.2 +VS – 1.4 +VS – 1.3 +VS – 1.4 VS = ±2.3 to ±18V G=1 G = 10 G = 100 G = 1000 Output Voltage Swing VREF ● ● –VS + 1.9 –VS + 1.9 –VS + 2.1 –VS + 2.1 pF dB dB dB dB VOUT Reference Input Current 1.6 UNITS 95 115 125 140 VS = ±2.3V to ±18V Reference Input Resistance MAX 75 95 110 110 Supply Current IREFIN TYP 1k Source Imbalance, VCM = 0V to ±10V G=1 G = 10 G = 100 G = 1000 IS RREFIN MIN 0.9 ● ● –VS + 1.1 –VS + 1.2 –VS + 1.4 –VS + 1.6 20 1.3 +VS – 1.2 +VS – 1.3 +VS – 1.3 +VS – 1.5 mA V V V V 27 mA 1000 800 120 12 kHz kHz kHz kHz G = 1, VOUT = ±10V 1.2 V/µs 10V Step G = 1 to 100 G = 1000 14 130 µs µs 20 kΩ 50 µA VREF = 0V The ● denotes specifications that apply over the full specified temperature range. Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Does not include the effect of the external gain resistor RG. Note 3: This parameter is not 100% tested. Note 4: The LT1920C is designed, characterized and expected to meet the industrial temperature limits, but is not tested at – 40°C and 85°C. I-grade parts are guaranteed. – VS + 1.6 +VS – 1.6 V 1 ± 0.0001 Note 5: This parameter is measured in a high speed automatic tester that does not measure the thermal effects with longer time constants. The magnitude of these thermal effects are dependent on the package used, heat sinking and air flow conditions. Note 6: Typical parameters are defined as the 60% of the yield parameter distribution. 3 LT1920 U W TYPICAL PERFOR A CE CHARACTERISTICS Gain Nonlinearity, G = 100 NONLINEARITY (10ppm/DIV) Gain Nonlinearity, G = 10 NONLINEARITY (1ppm/DIV) NONLINEARITY (10ppm/DIV) Gain Nonlinearity, G = 1 1167 G01 1167 G02 OUTPUT VOLTAGE (2V/DIV) G = 10 RL = 2k V OUT = ± 10V Gain Error vs Temperature NONLINEARITY (100ppm/DIV) Gain Nonlinearity, G = 1000 Warm-Up Drift 0.20 14 0.15 12 GAIN ERROR (%) 0.10 0.05 G=1 0 – 0.05 – 0.10 G = 1000 OUTPUT VOLTAGE (2V/DIV) RL = 2k VOUT = ±10V 1167 G04 – 0.15 VS = ±15V G = 10* VOUT = ±10V RL = 2k G = 100* *DOES NOT INCLUDE G = 1000* TEMPERATURE EFFECTS OF R G – 0.20 – 50 – 25 0 25 50 TEMPERATURE (°C) S8 10 8 N8 6 4 2 0 0 100 75 VS = ± 15V TA = 25°C G=1 1 2 3 4 TIME AFTER POWER ON (MINUTES) COMMON MODE REJECTION RATIO (dB) 160 INPUT BIAS CURRENT (pA) 400 300 200 100 0 85°C 70°C –100 – 200 0°C – 300 – 40°C 25°C – 400 – 500 –15 –12 – 9 – 6 – 3 0 3 6 9 12 15 COMMON MODE INPUT VOLTAGE (V) 1920 G13 4 140 VS = ±15V TA = 25°C 1k SOURCE IMBALANCE G = 1000 120 G = 100 G = 10 100 G=1 Negative Power Supply Rejection Ratio vs Frequency 80 60 40 20 0 0.1 1 10 1k 100 FREQUENCY (Hz) 10k 100k 1920 G14 NEGATIVE POWER SUPPLY REJECTION RATIO (dB) Common Mode Rejection Ratio vs Frequency 500 5 1920 G09 1920 G06 Input Bias Current vs Common Mode Input Voltage 1167 G03 G = 100 OUTPUT VOLTAGE (2V/DIV) RL = 2k VOUT = ±10V CHANGE IN OFFSET VOLTAGE (µV) G=1 OUTPUT VOLTAGE (2V/DIV) RL = 2k VOUT = ±10V 160 140 G = 100 120 G = 10 100 G=1 V + = 15V TA = 25°C G = 1000 80 60 40 20 0 0.1 1 10 1k 100 FREQUENCY (Hz) 10k 100k 1920 G15 LT1920 U W TYPICAL PERFOR A CE CHARACTERISTICS 160 V – = – 15V TA = 25°C 140 G=1 60 10 0 20 –10 1 10 1k 100 FREQUENCY (Hz) 10k 100k G = 10 20 40 0 0.1 G = 100 30 GAIN (dB) 80 G = 1000 40 G = 100 100 1.50 50 G = 1000 G = 10 120 Supply Current vs Supply Voltage Gain vs Frequency 60 SUPPLY CURRENT (mA) POSITIVE POWER SUPPLY REJECTION RATIO (dB) Positive Power Supply Rejection Ratio vs Frequency G=1 – 20 0.01 0.1 100 1 10 FREQUENCY (kHz) 0.75 0.50 1000 10 15 5 SUPPLY VOLTAGE (± V) 0 0.1Hz to 10Hz Noise Voltage, RTI G = 1000 VS = ±15V TA = 25°C NOISE VOLTAGE (0.2µV/DIV) VS = ±15V TA = 25°C 1/fCORNER = 10Hz GAIN = 1 1/fCORNER = 9Hz GAIN = 10 1/fCORNER = 7Hz 20 1920 G18 NOISE VOLTAGE (2µV/DIV) VOLTAGE NOISE DENSITY (nV√Hz) – 40°C 0.1Hz to 10Hz Noise Voltage, G=1 VS = ±15V TA = 25°C 10 25°C 1.00 1920 G17 Voltage Noise Density vs Frequency 100 85°C VS = ± 15V TA = 25°C 1920 G16 1000 1.25 GAIN = 100, 1000 BW LIMIT GAIN = 1000 0 1 10 100 1k FREQUENCY (Hz) 10k 100k 2 1 0 3 4 5 6 TIME (SEC) 7 8 9 1920 G19 VS = ±15V TA = 25°C 1000 1920 G22 7 8 9 10 VS = ±15V OUTPUT CURRENT (mA) (SINK) (SOURCE) 40 10 10 100 FREQUENCY (Hz) 4 5 6 TIME (SEC) Short-Circuit Current vs Time 30 20 10 TA = – 40°C TA = 25°C TA = 85°C 0 – 10 – 20 – 30 – 40 1 3 50 CURRENT NOISE (5pA/DIV) CURRENT NOISE DENSITY (fA/√Hz) VS = ±15V TA = 25°C RS 2 1920 G21 0.1Hz to 10Hz Current Noise 100 1 1920 G20 Current Noise Density vs Frequency 1000 0 10 TA = 85°C TA = – 40°C TA = 25°C – 50 0 1 2 3 4 5 6 TIME (SEC) 7 8 9 10 1920 G23 2 1 0 3 TIME FROM OUTPUT SHORT TO GROUND (MINUTES) 1920 G24 5 LT1920 U W TYPICAL PERFOR A CE CHARACTERISTICS Small-Signal Transient Response Large-Signal Transient Response Overshoot vs Capacitive Load 100 VS = ±15V VOUT = ± 50mV RL = ∞ 90 G=1 VS = ±15V RL = 2k CL = 60pF 10µs/DIV 1167 G28 OVERSHOOT (%) 5V/DIV 20mV/DIV 80 G=1 V S = ±15V R L = 2k C L = 60pF 10µs/DIV 70 60 50 AV = 1 40 30 AV = 10 20 1167 G29 10 AV ≥ 100 0 10 100 1000 CAPACITIVE LOAD (pF) 10000 1920 G25 Large-Signal Transient Response Output Impedance vs Frequency Small-Signal Transient Response 5V/DIV 20mV/DIV OUTPUT IMPEDANCE (Ω) 1000 G = 10 VS = ± 15V RL = 2k CL = 60pF 10µs/DIV 1167 G31 G = 10 VS = ±15V RL = 2k CL = 60pF 10µs/DIV VS = ± 15V TA = 25°C G = 1 TO 1000 100 10 1 1167 G32 0.1 1 10 100 FREQUENCY (kHz) 1000 1920 G26 Small-Signal Transient Response 10µs/DIV 1167 G34 35 PEAK-TO-PEAK OUTPUT SWING (V) 5V/DIV 20mV/DIV Large-Signal Transient Response G = 100 VS = ± 15V RL = 2k CL = 60pF Undistorted Output Swing vs Frequency G = 100 VS = ±15V RL = 2k CL = 60pF 10µs/DIV 1167 G35 30 G = 10, 100, 1000 G=1 25 VS = ± 15V TA = 25°C 20 15 10 5 0 1 10 100 FREQUENCY (kHz) 1000 1920 G27 6 LT1920 U W TYPICAL PERFOR A CE CHARACTERISTICS Large-Signal Transient Response Small-Signal Transient Response Settling Time vs Gain 5V/DIV 20mV/DIV SETTLING TIME (µs) 1000 1167 G37 50µs/DIV G = 1000 V S = ±15V RL = 2k C L = 60pF 50µs/DIV G = 1000 VS = ±15V RL = 2k CL = 60pF VS = ± 15V TA = 25°C ∆VOUT = 10V 1mV = 0.01% 100 10 1167 G38 1 1 10 100 1000 GAIN (dB) 1920 G30 Settling Time vs Step Size VS = ±15 G=1 TA = 25°C CL = 30pF RL = 1k 8 2 TO 0.1% 1.6 VOUT 0V 0 0V –2 VOUT –4 TO 0.01% –6 –8 –10 3 4 1.4 + SLEW 1.2 – SLEW 1.0 TO 0.1% 2 VS = ± 15V VOUT = ±10V G=1 TO 0.01% 0.8 – 50 –25 5 6 7 8 9 10 11 12 SETTLING TIME (µs) 50 0 25 75 TEMPERATURE (°C) 100 125 1920 G36 1920 G33 Output Voltage Swing vs Load Current + VS OUTPUT VOLTAGE SWING (V) (REFERRED TO SUPPLY VOLTAGE) OUTPUT STEP (V) 6 4 Slew Rate vs Temperature 1.8 SLEW RATE (V/µs) 10 VS = ± 15V 85°C 25°C – 40°C + VS – 0.5 + VS – 1.0 + VS – 1.5 SOURCE + VS – 2.0 – VS + 2.0 – VS + 1.5 SINK – VS + 1.0 – VS + 0.5 – VS 0.01 0.1 1 10 OUTPUT CURRENT (mA) 100 1920 G39 7 LT1920 W BLOCK DIAGRAM V+ VB R5 10k + R6 10k 6 OUTPUT A1 – R3 400Ω –IN C1 2 Q1 R1 24.7k V – – + A3 RG 1 RG 8 V– VB V+ + R7 10k R8 10k 5 REF A2 – R4 400Ω +IN 3 C2 V– Q2 R2 24.7k 7 V+ 4 V– V– PREAMP STAGE DIFFERENCE AMPLIFIER STAGE 1920 F01 Figure 1. Block Diagram U THEORY OF OPERATIO The LT1920 is a modified version of the three op amp instrumentation amplifier. Laser trimming and monolithic construction allow tight matching and tracking of circuit parameters over the specified temperature range. Refer to the block diagram (Figure 1) to understand the following circuit description. The collector currents in Q1 and Q2 are trimmed to minimize offset voltage drift, thus assuring a high level of performance. R1 and R2 are trimmed to an absolute value of 24.7k to assure that the gain can be set accurately (0.3% at G = 100) with only one external resistor RG. The value of RG in parallel with R1 (R2) determines the transconductance of the preamp stage. As RG is reduced for larger programmed gains, the transconductance of the input preamp stage increases to that of the input transistors Q1 and Q2. This increases the open-loop gain when the programmed gain is increased, reducing the input referred gain related errors and noise. The input voltage noise at gains greater than 50 is determined only by Q1 and Q2. At lower gains the noise of the difference amplifier and preamp gain setting resistors increase the noise. The gain bandwidth product is determined by C1, C2 and the preamp transconductance which increases 8 with programmed gain. Therefore, the bandwidth does not drop proportional to gain. The input transistors Q1 and Q2 offer excellent matching, which is inherent in NPN bipolar transistors, as well as picoampere input bias current due to superbeta processing. The collector currents in Q1 and Q2 are held constant due to the feedback through the Q1-A1-R1 loop and Q2-A2-R2 loop which in turn impresses the differential input voltage across the external gain set resistor RG. Since the current that flows through RG also flows through R1 and R2, the ratios provide a gained-up differential voltage,G = (R1 + R2)/RG, to the unity-gain difference amplifier A3. The common mode voltage is removed by A3, resulting in a single-ended output voltage referenced to the voltage on the REF pin. The resulting gain equation is: VOUT – VREF = G(VIN+ – VIN–) where: G = (49.4kΩ / RG) + 1 solving for the gain set resistor gives: RG = 49.4kΩ /(G – 1) LT1920 U THEORY OF OPERATIO Input and Output Offset Voltage Output Offset Trimming The offset voltage of the LT1920 has two components: the output offset and the input offset. The total offset voltage referred to the input (RTI) is found by dividing the output offset by the programmed gain (G) and adding it to the input offset. At high gains the input offset voltage dominates, whereas at low gains the output offset voltage dominates. The total offset voltage is: The LT1920 is laser trimmed for low offset voltage so that no external offset trimming is required for most applications. In the event that the offset needs to be adjusted, the circuit in Figure 2 is an example of an optional offset adjust circuit. The op amp buffer provides a low impedance to the REF pin where resistance must be kept to minimum for best CMRR and lowest gain error. 2 –IN – 1 Total output offset voltage (RTO) = (input offset • G) + output offset RG 3 + V+ 5 – The reference terminal is one end of one of the four 10k resistors around the difference amplifier. The output voltage of the LT1920 (Pin 6) is referenced to the voltage on the reference terminal (Pin 5). Resistance in series with the REF pin must be minimized for best common mode rejection. For example, a 2Ω resistance from the REF pin to ground will not only increase the gain error by 0.02% but will lower the CMRR to 80dB. +IN OUTPUT REF 8 Reference Terminal 6 LT1920 1 ±10mV ADJUSTMENT RANGE 1/2 LT1112 + Total input offset voltage (RTI) = input offset + (output offset/G) 2 10mV 100Ω 3 10k 100Ω –10mV V– 1920 F02 Figure 2. Optional Trimming of Output Offset Voltage Single Supply Operation Input Bias Current Return Path For single supply operation, the REF pin can be at the same potential as the negative supply (Pin 4) provided the output of the instrumentation amplifier remains inside the specified operating range and that one of the inputs is at least 2.5V above ground. The barometer application on the front page of this data sheet is an example that satisfies these conditions. The resistance RSET from the bridge transducer to ground sets the operating current for the bridge and also has the effect of raising the input common mode voltage. The output of the LT1920 is always inside the specified range since the barometric pressure rarely goes low enough to cause the output to rail (30.00 inches of Hg corresponds to 3.000V). For applications that require the output to swing at or below the REF potential, the voltage on the REF pin can be level shifted. An op amp is used to buffer the voltage on the REF pin since a parasitic series resistance will degrade the CMRR. The application in the back of this data sheet, Four Digit Pressure Sensor, is an example. The low input bias current of the LT1920 (2nA) and the high input impedance (200GΩ) allow the use of high impedance sources without introducing additional offset voltage errors, even when the full common mode range is required. However, a path must be provided for the input bias currents of both inputs when a purely differential signal is being amplified. Without this path the inputs will float to either rail and exceed the input common mode range of the LT1920, resulting in a saturated input stage. Figure 3 shows three examples of an input bias current path. The first example is of a purely differential signal source with a 10kΩ input current path to ground. Since the impedance of the signal source is low, only one resistor is needed. Two matching resistors are needed for higher impedance signal sources as shown in the second example. Balancing the input impedance improves both common mode rejection and DC offset. The need for input resistors is eliminated if a center tap is present as shown in the third example. 9 LT1920 U THEORY OF OPERATIO – RG THERMOCOUPLE – MICROPHONE, HYDROPHONE, ETC LT1920 RG – RG LT1920 + + 200k 10k LT1920 + 200k CENTER-TAP PROVIDES BIAS CURRENT RETURN 1920 F03 Figure 3. Providing an Input Common Mode Current Path U W U U APPLICATIONS INFORMATION The LT1920 is a low power precision instrumentation amplifier that requires only one external resistor to accurately set the gain anywhere from 1 to 1000. The output can handle capacitive loads up to 1000pF in any gain configuration and the inputs are protected against ESD strikes up to 13kV (human body). Input Protection The LT1920 can safely handle up to ±20mA of input current in an overload condition. Adding an external 5k input resistor in series with each input allows DC input fault voltages up to ±100V and improves the ESD immunity to 8kV (contact) and 15kV (air discharge), which is the IEC 1000-4-2 level 4 specification. If lower value input resistors are needed, a clamp diode from the positive supply to each input will maintain the IEC 1000-4-2 specification to level 4 for both air and contact discharge. A 2N4393 drain/source to gate is a good low leakage diode for use with 1k resistors, see Figure 4. The input resistors should be carbon and not metal film or carbon film. RFI Reduction In many industrial and data acquisition applications, instrumentation amplifiers are used to accurately amplify small signals in the presence of large common mode voltages or high levels of noise. Typically, the sources of these very small signals (on the order of microvolts or millivolts) are sensors that can be a significant distance from the signal conditioning circuit. Although these sen- 10 VCC VCC J1 2N4393 J2 2N4393 RIN OPTIONAL FOR HIGHEST ESD PROTECTION + RG RIN VCC LT1920 OUT REF – VEE 1920 F04 Figure 4. Input Protection sors may be connected to signal conditioning circuitry, using shielded or unshielded twisted-pair cabling, the cabling may act as antennae, conveying very high frequency interference directly into the input stage of the LT1920. The amplitude and frequency of the interference can have an adverse effect on an instrumentation amplifier’s input stage by causing an unwanted DC shift in the amplifier’s input offset voltage. This well known effect is called RFI rectification and is produced when out-of-band interference is coupled (inductively, capacitively or via radiation) and rectified by the instrumentation amplifier’s input transistors. These transistors act as high frequency signal detectors, in the same way diodes were used as RF envelope detectors in early radio designs. Regardless of the type of interference or the method by which it is coupled into the circuit, an out-of-band error signal appears in series with the instrumentation amplifier’s inputs. LT1920 U U W U APPLICATIONS INFORMATION To significantly reduce the effect of these out-of-band signals on the input offset voltage of instrumentation amplifiers, simple lowpass filters can be used at the inputs. This filter should be located very close to the input pins of the circuit. An effective filter configuration is illustrated in Figure 5, where three capacitors have been added to the inputs of the LT1920. Capacitors CXCM1 and CXCM2 form lowpass filters with the external series resistors RS1, 2 to any out-of-band signal appearing on each of the input traces. Capacitor CXD forms a filter to reduce any unwanted signal that would appear across the input traces. An added benefit to using CXD is that the circuit’s AC common mode rejection is not degraded due to common mode capacitive imbalance. The differential mode and common mode time constants associated with the capacitors are: mode time constant close to the sensor’s BW also minimizes any noise pickup along the leads. To avoid any possibility of inadvertently affecting the signal to be processed, set the common mode time constant an order of magnitude (or more) larger than the differential mode time constant. To avoid any possibility of common mode to differential mode signal conversion, match the common mode time constants to 1% or better. If the sensor is an RTD or a resistive strain gauge, then the series resistors RS1, 2 can be omitted, if the sensor is in proximity to the instrumentation amplifier. IN + V+ CXCM1 0.001µF RS1 1.6k tDM(LPF) = (2)(RS)(CXD) + CXD 0.1µF tCM(LPF) = (RS1, 2)(CXCM1, 2) Setting the time constants requires a knowledge of the frequency, or frequencies of the interference. Once this frequency is known, the common mode time constants can be set followed by the differential mode time constant. Set the common mode time constants such that they do not degrade the LT1920’s inherent AC CMR. Then the differential mode time constant can be set for the bandwidth required for the application. Setting the differential U PACKAGE DESCRIPTION IN – RG RS2 1.6k LT1920 VOUT – CXCM2 0.001µF V– 1920 F05 EXTERNAL RFI FILTER f(–3dB) ≈ 500Hz Figure 5. Adding a Simple RC Filter at the Inputs to an Instrumentation Amplifier is Effective in Reducing Rectification of High Frequency Out-of-Band Signals Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) 0.300 – 0.325 (7.620 – 8.255) 0.009 – 0.015 (0.229 – 0.381) ( +0.035 0.325 –0.015 8.255 +0.889 –0.381 ) 0.045 – 0.065 (1.143 – 1.651) 0.130 ± 0.005 (3.302 ± 0.127) 0.065 (1.651) TYP 0.100 ± 0.010 (2.540 ± 0.254) 0.400* (10.160) MAX 8 7 6 5 1 2 3 4 0.255 ± 0.015* (6.477 ± 0.381) 0.125 (3.175) 0.020 MIN (0.508) MIN 0.018 ± 0.003 (0.457 ± 0.076) N8 1197 *THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm) 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. 11 LT1920 U TYPICAL APPLICATION Nerve Impulse Amplifier 3V PATIENT/CIRCUIT PROTECTION/ISOLATION 3 8 +IN C1 0.01µF R2 1M RG 6k R4 30k – PATIENT GROUND 1 3V 6 LT1920 G = 10 1 5 R6 1M 5 2 1/2 LT1112 0.3Hz HIGHPASS 7 C2 0.47µF R3 30k R1 12k + – 2 6 4 8 + – 4 –3V –3V + AV = 101 POLE AT 1kHz U PACKAGE DESCRIPTION OUTPUT 1V/mV R7 10k R8 100Ω 3 –IN 7 1/2 LT1112 C3 15nF 1920 TA03 Dimensions in inches (millimeters) unless otherwise noted. S8 Package 8-Lead Plastic Small Outline (Narrow 0.150) (LTC DWG # 05-08-1610) 0.189 – 0.197* (4.801 – 5.004) 0.010 – 0.020 × 45° (0.254 – 0.508) 0.008 – 0.010 (0.203 – 0.254) 0.053 – 0.069 (1.346 – 1.752) 0.004 – 0.010 (0.101 – 0.254) 8 7 6 5 0°– 8° TYP 0.016 – 0.050 0.406 – 1.270 0.014 – 0.019 (0.355 – 0.483) 0.050 (1.270) TYP 0.150 – 0.157** (3.810 – 3.988) 0.228 – 0.244 (5.791 – 6.197) *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE SO8 0996 1 3 2 4 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC1100 Precision Chopper-Stabilized Instrumentation Amplifier Best DC Accuracy LT1101 Precision, Micropower, Single Supply Instrumentation Amplifier Fixed Gain of 10 or 100, IS < 105µA LT1102 High Speed, JFET Instrumentation Amplifier Fixed Gain of 10 or 100, 30V/µs Slew Rate LT1167 Single Resistor Gain Programmable Precision Instrumentation Amplifier Upgraded Version of the LT1920 LTC®1418 14-Bit, Low Power, 200ksps ADC with Serial and Parallel I/O Single Supply 5V or ±5V Operation, ±1.5LSB INL and ±1LSB DNL Max LT1460 Precision Series Reference Micropower; 2.5V, 5V, 10V Versions; High Precision LTC1562 Active RC Filter Lowpass, Bandpass, Highpass Responses; Low Noise, Low Distortion, Four 2nd Order Filter Sections LTC1605 16-Bit, 100ksps, Sampling ADC Single 5V Supply, Bipolar Input Range: ±10V, Power Dissipation: 55mW Typ 12 Linear Technology Corporation 1920f LT/TP 0299 4K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com LINEAR TECHNOLOGY CORPORATION 1998