LT1920 Single Resistor Gain Programmable, Precision Instrumentation Amplifier

FEATURES DESCRIPTIO U ■ Single Gain Set Resistor: G = 1 to 10,000 The LT ®1920 is a low power, precision instrumentation ■ Gain Error: G = 10, 0.3% Max amplifier that requires only one external resistor to set gains ■ Gain Nonlinearity: G = 10, 30ppm Max of 1 to 10,000. The low voltage noise of 7.5nV/√Hz (at 1kHz) ■ Input Offset Voltage: G = 10, 225µV Max is not compromised by low power dissipation (0.9mA typical ■ Input Offset Voltage Drift: 1µV/°C Max for ±2.3V to ±15V supplies). ■ Input Bias Current: 2nA Max The high accuracy of 30ppm maximum nonlinearity and ■ PSRR at G = 1: 80dB Min 0.3% max gain error (G = 10) is not degraded even for load ■ CMRR at G = 1: 75dB Min resistors as low as 2k (previous monolithic instrumentation ■ Supply Current: 1.3mA Max amps used 10k for their nonlinearity specifications). The ■ ± ± Wide Supply Range: 2.3V to 18V LT1920 is laser trimmed for very low input offset voltage ■ √ 1kHz Voltage Noise: 7.5nV/ Hz (125µV max), drift (1µV/°C), high CMRR (75dB, G = 1) and ■ µ 0.1Hz to 10Hz Noise: 0.28 VP-P PSRR (80dB, G = 1). Low input bias currents of 2nA max are ■ Available in 8-Pin PDIP and SO Packages achieved with the use of superbeta processing. The output ■ Meets IEC 1000-4-2 Level 4 ESD Tests with can handle capacitive loads up to 1000pF in any gain configu- Two External 5k Resistors ration while the inputs are ESD protected up to 13kV (human

body). The LT1920 with two external 5k resistors passes the U IEC 1000-4-2 level 4 specification. APPLICATIO S The LT1920, offered in 8-pin PDIP and SO packages, is a pin ■ Bridge Amplifiers for pin and spec for spec improved replacement for the ■ Strain Gauge Amplifiers AD620. The LT1920 is the most cost effective solution for ■ Thermocouple Amplifiers precision instrumentation amplifier applications. For even ■ Differential to Single-Ended Converters better guaranteed performance, see the LT1167. ■ Medical Instrumentation , LTC and LT are registered trademarks of Linear Technology Corporation.

TYPICAL APPLICATIO U Single Supply Barometer

VS Gain Nonlinearity

R5 LUCAS NOVA SENOR 392k VS

3 8 NPC-1220-015-A-3L + 1 2 – 1/2 1 – 7 1 4 1 LT1490 5k LT1634CCZ-1.25 2 5k – R1 2 4 825Ω 6 R6 LT1920 G = 60 1k 2 5k R2

5k 12Ω

8 5 + TO 6 + 3 3

4-DIGIT NONLINEARITY (100ppm/DIV) R4 RSET 4 50k DVM 5 5 R3 + OUTPUT VOLTAGE (2V/DIV) 7 50k 1/2 G = 1000 LT1490 6 RL = 1k – VOUT = ±10V 1167 TA02 R8 R7 VOLTS INCHES Hg 100k 50k 2.800 28.00 VS = 8V TO 30V 3.000 30.00 3.200 32.00 1920 TA01 1

LT1920

WW U W W U

ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATIONU

(Note 1) ORDER PART ± TOP VIEW Supply Voltage ...... 20V NUMBER Differential Input Voltage (Within the RG 1 8 RG

Supply Voltage) ...... ±40V –IN 2 – 7 +VS LT1920CN8 Input Voltage (Equal to Supply Voltage) ...... ±20V +IN 3 + 6 OUTPUT LT1920CS8

Input Current (Note 3) ...... ±20mA –VS 4 5 REF LT1920IN8 LT1920IS8 Output Short-Circuit Duration ...... Indefinite N8 PACKAGE Operating Temperature Range ...... – 40°C to 85°C 8-LEAD PDIP S8 PACKAGE Specified Temperature Range 8-LEAD PLASTIC SO S8 PART MARKING LT1920C (Note 4) ...... 0°C to 70°C TJMAX = 150°C, θJA = 130°C/ W (N8) ° θ ° 1920 LT1920I ...... – 40°C to 85°C TJMAX = 150 C, JA = 190 C/ W (S8) 1920I Storage Temperature Range ...... –65°C to 150°C Lead Temperature (Soldering, 10 sec)...... 300°C Consult factory for Military grade parts.

ELECTRICAL CHARACTERISTICS VS = ±15V, VCM = 0V, TA = 25°C, RL = 2k, unless otherwise noted.

SYMBOL PARAMETER CONDITIONS (Note 6) MIN TYP MAX UNITS

G Gain Range G = 1 + (49.4k/RG) 1 10k Gain Error G = 1 0.008 0.1 % G = 10 (Note 2) 0.010 0.3 % G = 100 (Note 2) 0.025 0.3 % G = 1000 (Note 2) 0.040 0.35 % G/T Gain vs Temperature G < 1000 (Note 2) ● 20 50 ppm/°C

Gain Nonlinearity (Note 5) VO = ±10V, G = 1 10 ppm VO = ±10V, G = 10 and 100 10 30 ppm VO = ±10V, G = 100 and 1000 20 ppm

VOST Total Input Referred Offset Voltage VOST = VOSI + VOSO/G

VOSI Input Offset Voltage G = 1000, VS = ±5V to ±15V 30 125 µV G = 1000, VS = ±5V to ±15V ● 185 µV

VOSI/T Input Offset Drift (RTI) (Note 3) ● 1 µV/°C

VOSO Output Offset Voltage G = 1, VS = ±5V to ±15V 400 1000 µV G = 1, VS = ±5V to ±15V ● 1500 µV

VOSO/T Output Offset Drift (Note 3) ● 515µV/°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 2.00 µVP-P 0.1Hz to 10Hz, G = 10 0.50 µVP-P 0.1Hz to 10Hz, G = 100 and 1000 0.28 µVP-P 2 2 Total RTI Noise = √eni + (eno/G) 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, VCM = 0V, TA = 25°C, RL = 2k, unless otherwise noted.

SYMBOL PARAMETER CONDITIONS (Note 6) MIN TYP MAX UNITS

CIN(CM) Common Mode Input Capacitance fO = 100kHz 1.6 pF

VCM Input Voltage Range G = 1, Other Input Grounded VS = ±2.3V to ±5V –VS + 1.9 +VS – 1.2 V VS = ±5V to ±18V –VS + 1.9 +VS – 1.4 V VS = ±2.3V to ±5V ● –VS + 2.1 +VS – 1.3 V VS = ±5V to ±18V ● –VS + 2.1 +VS – 1.4 V CMRR Common Mode Rejection Ratio 1k Source Imbalance, VCM = 0V to ±10V G = 1 75 95 dB G = 10 95 115 dB G = 100 110 125 dB G = 1000 110 140 dB

PSRR Power Supply Rejection Ratio VS = ±2.3 to ±18V G = 1 80 120 dB G = 10 100 135 dB G = 100 120 140 dB G = 1000 120 150 dB

IS Supply Current VS = ±2.3V to ±18V 0.9 1.3 mA

VOUT Output Voltage Swing RL = 10k VS = ±2.3V to ±5V –VS + 1.1 +VS – 1.2 V VS = ±5V to ±18V –VS + 1.2 +VS – 1.3 V VS = ±2.3V to ±5V ● –VS + 1.4 +VS – 1.3 V VS = ±5V to ±18V ● –VS + 1.6 +VS – 1.5 V

IOUT Output Current 20 27 mA BW Bandwidth G = 1 1000 kHz G = 10 800 kHz G = 100 120 kHz G = 1000 12 kHz

SR Slew Rate G = 1, VOUT = ±10V 1.2 V/µs Settling Time to 0.01% 10V Step G = 1 to 100 14 µs G = 1000 130 µs

RREFIN Reference Input Resistance 20 kΩ

IREFIN Reference Input Current VREF = 0V 50 µA

VREF Reference Voltage Range – VS + 1.6 +VS – 1.6 V

AVREF Reference Gain to Output 1 ± 0.0001 The ● denotes specifications that apply over the full specified Note 5: This parameter is measured in a high speed automatic tester that temperature range. does not measure the thermal effects with longer time constants. The Note 1: Absolute Maximum Ratings are those values beyond which the life magnitude of these thermal effects are dependent on the package used, of a device may be impaired. heat sinking and air flow conditions. Note 2: Does not include the effect of the external gain resistor RG. Note 6: Typical parameters are defined as the 60% of the yield parameter Note 3: This parameter is not 100% tested. distribution. 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.

3 LT1920

TYPICAL PERFOR A CEUW CHARACTERISTICS

Gain Nonlinearity, G = 1 Gain Nonlinearity, G = 10 Gain Nonlinearity, G = 100 NONLINEARITY (1ppm/DIV) NONLINEARITY (10ppm/DIV) NONLINEARITY (10ppm/DIV)

1167 G01 1167 G02 1167 G03 G = 1 OUTPUT VOLTAGE (2V/DIV) G = 10 OUTPUT VOLTAGE (2V/DIV) G = 100 OUTPUT VOLTAGE (2V/DIV) RL = 2k RL = 2k RL = 2k ± VOUT = ±10V VOUT = ±10V VOUT = 10V

Gain Nonlinearity, G = 1000 Gain Error vs Temperature Warm-Up Drift 0.20 14 VS = ±15V ° 0.15 TA = 25 C

V) 12

µ G = 1 S8 0.10 10 0.05 G = 1 8 0 N8 6 –0.05 VS = ±15V

GAIN ERROR (%) G = 10* NONLINEARITY (100ppm/DIV) VOUT = ±10V 4 –0.10 R = 2k L G = 100* 1167 G04 *DOES NOT INCLUDE OUTPUT VOLTAGE (2V/DIV) CHANGE IN OFFSET VOLTAGE ( 2 G = 1000 –0.15 TEMPERATURE EFFECTS G = 1000* RL = 2k ± OF RG VOUT = 10V –0.20 0 –50 –25 0 2550 75 100 0 1234 5 TEMPERATURE (°C) TIME AFTER POWER ON (MINUTES)

1920 G06 1920 G09

Input Bias Current Common Mode Rejection Ratio Negative Power Supply Rejection vs Common Mode Input Voltage vs Frequency Ratio vs Frequency 500 160 160 + VS = ±15V V = 15V 400 G = 1000 ° ° 140 TA = 25 C 140 G = 100 TA = 25 C G = 100 1k SOURCE 300 120 G = 10 IMBALANCE 120 G = 10 200 G = 1000 100 G = 1 G = 1 100 100 0 70°C 85°C 80 80 –100 60 60 ° –200 0 C 25°C 40 40 INPUT BIAS CURRENT (pA) –300 –40°C 20 –400 20 COMMON MODE REJECTION RATIO (dB) –500 0 0 –15 –12–9 –6 –3 0 3 6 9 12 15 0.1 110100 1k10k 100k NEGATIVE POWER SUPPLY REJECTION RATIO (dB) 0.1 110100 1k10k 100k COMMON MODE INPUT VOLTAGE (V) FREQUENCY (Hz) FREQUENCY (Hz)

1920 G13 1920 G14 1920 G15

4 LT1920

TYPICAL PERFOR A CEUW CHARACTERISTICS

Positive Power Supply Rejection Ratio vs Frequency Gain vs Frequency Supply Current vs Supply Voltage 160 60 1.50 V – = –15V G = 1000 ° 140 TA = 25 C 50 G = 10 G = 1000 G = 100 120 G = 100 40 1.25

100 G = 1 30 85°C G = 10 25°C 80 20 1.00

60 GAIN (dB) 10 –40°C G = 1

40 0 SUPPLY CURRENT (mA) 0.75

20 –10 VS = ±15V TA = 25°C 0 –20 0.50 POSITIVE POWER SUPPLY REJECTION RATIO (dB) 0.1 110100 1k10k 100k 0.010.1 1 10100 1000 0 5 10 15 20 FREQUENCY (Hz) FREQUENCY (kHz) SUPPLY VOLTAGE (±V) 1920 G17 1920 G16 1920 G18

Voltage Noise Density 0.1Hz to 10Hz Noise Voltage, 0.1Hz to 10Hz Noise Voltage, RTI vs Frequency G = 1 G = 1000 1000 ± VS = ±15V VS = ±15V VS = 15V ° TA = 25°C TA = 25°C TA = 25 C Hz) √ 1/fCORNER = 10Hz V/DIV)

100 V/DIV)

GAIN = 1 µ µ

1/fCORNER = 9Hz GAIN = 10 1/f = 7Hz 10 CORNER GAIN = 100, 1000 NOISE VOLTAGE (2 NOISE VOLTAGE (0.2 BW LIMIT VOLTAGE NOISE DENSITY (nV GAIN = 1000 0 1 10 100 1k10k 100k 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4 5 6 7 8 9 10 FREQUENCY (Hz) TIME (SEC) TIME (SEC)

1920 G19 1920 G20 1920 G21

Current Noise Density vs Frequency 0.1Hz to 10Hz Current Noise Short-Circuit Current vs Time 1000 50 ± V = ±15V VS = 15V VS = ±15V S T = 25°C ° 40 A TA = 25 C ° Hz) TA = –40 C

√ 30 TA = 25°C 20 ° 10 TA = 85 C 100 0

RS –10 TA = 85°C –20 OUTPUT CURRENT (mA) CURRENT NOISE (5pA/DIV)

(SINK)–30 (SOURCE) ° ° CURRENT NOISE DENSITY (fA/ TA = –40 C TA = 25 C –40 10 –50 1 10 100 1000 0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 FREQUENCY (Hz) TIME (SEC) TIME FROM OUTPUT SHORT TO GROUND (MINUTES) 1920 G22 1920 G23 1920 G24 5 LT1920

TYPICAL PERFOR A CEUW CHARACTERISTICS

Large-Signal Transient Response Small-Signal Transient Response Overshoot vs Capacitive Load 100 ± VS = 15V 90 V = ±50mV OUT∞ 80 RL = 70 60 5V/DIV

20mV/DIV 50 AV = 1 40

OVERSHOOT (%) 30 AV = 10 20 1167 G28 1167 G29 G = 1 10µs/DIV G = 1 10µs/DIV 10 ± V = ±15V VS = 15V S AV ≥ 100 RL = 2k RL = 2k 0 CL = 60pF CL = 60pF 10 100 1000 10000 CAPACITIVE LOAD (pF)

1920 G25

Large-Signal Transient Response Small-Signal Transient Response Output Impedance vs Frequency 1000 VS = ±15V TA = 25°C G = 1 TO 1000

) 100 Ω 5V/DIV

20mV/DIV 10

1 OUTPUT IMPEDANCE (

1167 G31 1167 G32 G = 10 10µs/DIV G = 10 10µs/DIV VS = ±15V VS = ±15V RL = 2k RL = 2k 0.1 CL = 60pF CL = 60pF 1 10 100 1000 FREQUENCY (kHz)

1920 G26

Undistorted Output Swing vs Frequency Large-Signal Transient Response Small-Signal Transient Response 35 VS = ±15V ° 30 G = 10, 100, 1000 TA = 25 C G = 1 25

20 5V/DIV 20mV/DIV 15

10

PEAK-TO-PEAK OUTPUT SWING (V) 5 1167 G34 1167 G35 G = 100 10µs/DIV G = 100 10µs/DIV ± VS = ±15V VS = 15V 0 RL = 2k RL = 2k 1 10 100 1000 C = 60pF CL = 60pF L FREQUENCY (kHz)

1920 G27

6 LT1920

TYPICAL PERFOR A CEUW CHARACTERISTICS

Large-Signal Transient Response Small-Signal Transient Response Settling Time vs Gain 1000 VS = ±15V TA = 25°C ∆VOUT = 10V 1mV = 0.01% s)

µ 100 5V/DIV 20mV/DIV

10 SETTLING TIME (

1167 G37 1167 G38 G = 1000 50µs/DIV G = 1000 50µs/DIV VS = ±15V VS = ±15V RL = 2k RL = 2k 1 CL = 60pF CL = 60pF 1 10 100 1000 GAIN (dB)

1920 G30

Settling Time vs Step Size Slew Rate vs Temperature 1.8 10 ± VS = ±15 TO 0.1% VS = 15V 8 G = 1 VOUT = ±10V T = 25°C G = 1 6 A 1.6 CL = 30pF TO 0.01% 4 RL = 1k s) µ V 1.4 2 0V OUT 0 +SLEW –2 0V 1.2 VOUT OUTPUT STEP (V) –4 SLEW RATE (V/ –SLEW TO 0.01% –6 1.0 –8 TO 0.1% –10 0.8 2 3114 56 78 910 12 –50 –25 0 25 50 75 100 125 SETTLING TIME (µs) TEMPERATURE (°C)

1920 G33 1920 G36

Output Voltage Swing vs Load Current

+VS VS = ±15V 85°C ° +VS – 0.5 25 C –40°C +VS – 1.0

+VS – 1.5 SOURCE

+VS – 2.0

–VS + 2.0

–VS + 1.5 SINK –VS + 1.0 OUTPUT VOLTAGE SWING (V) (REFERRED TO SUPPLY VOLTAGE) –VS + 0.5

–VS 0.010.1 1 10 100 OUTPUT CURRENT (mA)

1920 G39

7 LT1920

BLOCK DIAGRAMW

V+ VB + R5 R6 10k 10k A1 6 OUTPUT – R3 C1 400Ω –IN 2 Q1

R1 24.7k –

V–

A3 +

RG 1 V– RG 8 VB + V + R7 R8 10k 10k A2 5 REF – R4 C2 400Ω +IN 3 Q2 V– R2 24.7k 7 V+ V– 4 V–

PREAMP STAGE DIFFERENCE AMPLIFIER STAGE 1920 F01 Figure 1. Block Diagram

THEORY OF OPERATIOU The LT1920 is a modified version of the three op amp with programmed gain. Therefore, the bandwidth does not instrumentation amplifier. Laser trimming and monolithic drop proportional to gain. construction allow tight matching and tracking of circuit The input transistors Q1 and Q2 offer excellent matching, parameters over the specified temperature range. Refer to which is inherent in NPN bipolar transistors, as well as the block diagram (Figure 1) to understand the following picoampere input bias current due to superbeta process- circuit description. The collector currents in Q1 and Q2 are ing. The collector currents in Q1 and Q2 are held constant trimmed to minimize offset voltage drift, thus assuring a due to the feedback through the Q1-A1-R1 loop and high level of performance. R1 and R2 are trimmed to an Q2-A2-R2 loop which in turn impresses the differential absolute value of 24.7k to assure that the gain can be set input voltage across the external gain set resistor RG. accurately (0.3% at G = 100) with only one external Since the current that flows through R also flows through . The value of R in parallel with R1 (R2) G resistor RG G R1 and R2, the ratios provide a gained-up differential volt- determines the transconductance of the preamp stage. As age,G = (R1 + R2)/R , to the unity-gain difference amplifier is reduced for larger programmed gains, the transcon- G RG A3. The common mode voltage is removed by A3, result- ductance of the input preamp stage increases to that of the ing in a single-ended output voltage referenced to the input transistors Q1 and Q2. This increases the open-loop voltage on the REF pin. The resulting gain equation is: gain when the programmed gain is increased, reducing + – the input referred gain related errors and noise. The input VOUT – VREF = G(VIN – VIN ) voltage noise at gains greater than 50 is determined only where: by Q1 and Q2. At lower gains the noise of the difference G = (49.4kΩ /R ) + 1 amplifier and preamp gain setting resistors increase the G noise. The gain bandwidth product is determined by C1, solving for the gain set resistor gives: C2 and the preamp transconductance which increases RG = 49.4kΩ/(G – 1) 8 LT1920

THEORY OF OPERATIOU Input and Output Offset Voltage Output Offset Trimming The offset voltage of the LT1920 has two components: the The LT1920 is laser trimmed for low offset voltage so that output offset and the input offset. The total offset voltage no external offset trimming is required for most applica- referred to the input (RTI) is found by dividing the output tions. In the event that the offset needs to be adjusted, the offset by the programmed gain (G) and adding it to the circuit in Figure 2 is an example of an optional offset adjust input offset. At high gains the input offset voltage domi- circuit. The op amp buffer provides a low impedance to the nates, whereas at low gains the output offset voltage REF pin where resistance must be kept to minimum for

dominates. The total offset voltage is: best CMRR and lowest gain error. Total input offset voltage (RTI) 2 – –IN = input offset + (output offset/G) 1

Total output offset voltage (RTO) 6 R LT1920 OUTPUT = (input offset • G) + output offset G V+

8 REF

3 + 5

+IN Reference Terminal – 2 10mV

The reference terminal is one end of one of the four 10k 1 1/2 100Ω

LT1112 ±10mV + 3 resistors around the difference amplifier. The output volt- 10k ADJUSTMENT RANGE age of the LT1920 (Pin 6) is referenced to the voltage on 100Ω the reference terminal (Pin 5). Resistance in series with –10mV 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% V 1920 F02 but will lower the CMRR to 80dB. 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 The low input bias current of the LT1920 (2nA) and the potential as the negative supply (Pin 4) provided the high input impedance (200GΩ) allow the use of high output of the instrumentation amplifier remains inside the impedance sources without introducing additional offset specified operating range and that one of the inputs is at voltage errors, even when the full common mode range is least 2.5V above ground. The barometer application on the required. However, a path must be provided for the input front page of this data sheet is an example that satisfies bias currents of both inputs when a purely differential these conditions. The resistance RSET from the bridge signal is being amplified. Without this path the inputs will transducer to ground sets the operating current for the float to either rail and exceed the input common mode bridge and also has the effect of raising the input common range of the LT1920, resulting in a saturated input stage. mode voltage. The output of the LT1920 is always inside Figure 3 shows three examples of an input bias current the specified range since the barometric pressure rarely path. The first example is of a purely differential signal goes low enough to cause the output to rail (30.00 inches source with a 10kΩ input current path to ground. Since the of Hg corresponds to 3.000V). For applications that re- impedance of the signal source is low, only one resistor is quire the output to swing at or below the REF potential, the needed. Two matching resistors are needed for higher voltage on the REF pin can be level shifted. An op amp is impedance signal sources as shown in the second used to buffer the voltage on the REF pin since a parasitic example. Balancing the input impedance improves both series resistance will degrade the CMRR. The application common mode rejection and DC offset. The need for input in the back of this data sheet, Four Digit Pressure Sensor, resistors is eliminated if a center tap is present as shown is an example. in the third example. 9 LT1920

THEORY OF OPERATIOU

– – –

MICROPHONE,

THERMOCOUPLE RG LT1920 HYDROPHONE, RG LT1920 RG LT1920

ETC

+ + +

10k 200k 200k CENTER-TAP PROVIDES BIAS CURRENT RETURN 1920 F03

Figure 3. Providing an Input Common Mode Current Path

U U

APPLICATIONS INFORMATIOWU N

V V The LT1920 is a low power precision instrumentation CC CC OPTIONAL FOR HIGHEST amplifier that requires only one external resistor to accu- J1 J2 ESD PROTECTION 2N4393 2N4393

rately set the gain anywhere from 1 to 1000. The output VCC R + can handle capacitive loads up to 1000pF in any gain IN configuration and the inputs are protected against ESD OUT strikes up to 13kV (human body). RG LT1920

REF Input Protection RIN –

The LT1920 can safely handle up to ±20mA of input VEE 1920 F04 current in an overload condition. Adding an external 5k Figure 4. Input Protection input resistor in series with each input allows DC input fault voltages up to ±100V and improves the ESD immu- nity to 8kV (contact) and 15kV (air discharge), which is the sors may be connected to signal conditioning circuitry, IEC 1000-4-2 level 4 specification. If lower value input using shielded or unshielded twisted-pair cabling, the ca- resistors are needed, a clamp diode from the positive bling may act as antennae, conveying very high frequency supply to each input will maintain the IEC 1000-4-2 interference directly into the input stage of the LT1920. specification to level 4 for both air and contact discharge. The amplitude and frequency of the interference can have A 2N4393 drain/source to gate is a good low leakage diode an adverse effect on an instrumentation amplifier’s input for use with 1k resistors, see Figure 4. The input resistors stage by causing an unwanted DC shift in the amplifier’s should be carbon and not metal film or carbon film. input offset voltage. This well known effect is called RFI rectification and is produced when out-of-band interfer- RFI Reduction ence is coupled (inductively, capacitively or via radiation) In many industrial and data acquisition applications, and rectified by the instrumentation amplifier’s input tran- instrumentation amplifiers are used to accurately amplify sistors. These transistors act as high frequency signal small signals in the presence of large common mode detectors, in the same way diodes were used as RF voltages or high levels of noise. Typically, the sources of envelope detectors in early radio designs. Regardless of these very small signals (on the order of microvolts or the type of interference or the method by which it is millivolts) are sensors that can be a significant distance coupled into the circuit, an out-of-band error signal ap- from the signal conditioning circuit. Although these sen- pears in series with the instrumentation amplifier’s inputs. 10

LT1920

U U

APPLICATIONS INFORMATIOWU N To significantly reduce the effect of these out-of-band mode time constant close to the sensor’s BW also mini- signals on the input offset voltage of instrumentation mizes any noise pickup along the leads. To avoid any amplifiers, simple lowpass filters can be used at the possibility of inadvertently affecting the signal to be pro- inputs. This filter should be located very close to the input cessed, set the common mode time constant an order of pins of the circuit. An effective filter configuration is magnitude (or more) larger than the differential mode time illustrated in Figure 5, where three capacitors have been constant. To avoid any possibility of common mode to added to the inputs of the LT1920. Capacitors CXCM1 and differential mode signal conversion, match the common CXCM2 form lowpass filters with the external series resis- mode time constants to 1% or better. If the sensor is an tors RS1, 2 to any out-of-band signal appearing on each of RTD or a resistive strain gauge, then the series resistors the input traces. Capacitor CXD forms a filter to reduce any RS1, 2 can be omitted, if the sensor is in proximity to the unwanted signal that would appear across the input traces. instrumentation amplifier. 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 V+

RS1 CXCM1

0.001µF 1.6k + common mode time constants associated with the capaci- + tors are: IN

tDM(LPF) = (2)(RS)(CXD) CXD 0.1µF RG LT1920 VOUT

tCM(LPF) = (RS1, 2)(CXCM1, 2) RS2 1.6k – Setting the time constants requires a knowledge of the IN– CXCM2 frequency, or frequencies of the interference. Once this 0.001µF V– frequency is known, the common mode time constants 1920 F05 can be set followed by the differential mode time constant. EXTERNAL RFI FILTER Set the common mode time constants such that they do f(–3dB) ≈ 500Hz not degrade the LT1920’s inherent AC CMR. Then the Figure 5. Adding a Simple RC Filter at the Inputs to an differential mode time constant can be set for the band- Instrumentation Amplifier is Effective in Reducing Rectification width required for the application. Setting the differential of High Frequency Out-of-Band Signals

PACKAGE DESCRIPTIONU Dimensions in inches (millimeters) unless otherwise noted. N8 Package 8-Lead PDIP (Narrow 0.300) (LTC DWG # 05-08-1510) 0.400* (10.160) 0.300 – 0.325 0.045 – 0.065 0.130 ± 0.005 MAX (7.620 – 8.255) (1.143 – 1.651) (3.302 ± 0.127) 87 65

0.065 0.255 ± 0.015* (1.651) (6.477 ± 0.381) 0.009 – 0.015 TYP (0.229 – 0.381) 0.125 (3.175) 0.020 +0.035 MIN 0.325 (0.508) 12 34 –0.015 MIN ± ± +0.889 0.100 0.010 0.018 0.003 ()8.255 –0.381 (2.540 ± 0.254) (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 represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 11 LT1920

TYPICAL APPLICATIONU Nerve Impulse Amplifier

PATIENT/CIRCUIT 3V PROTECTION/ISOLATION 3 +IN 7 0.3Hz 8 + HIGHPASS C1 C2 3V 0.01µF R1 R3 0.47µF 30k 12k 6 5 8 RG LT1920 + 6k G = 10 R2 R4 1/2 7 OUTPUT 1M R6 30k LT1112 1V/mV 1 5 1M 6

2 –

– 4 4 – 2 R7 –3V 10k PATIENT 1 1/2 –3V R8

GROUND LT1112 100Ω + 3

AV = 101 C3

POLE AT 1kHz –IN 15nF 1920 TA03 U PACKAGE DESCRIPTION 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° 0.053 – 0.069 0.004 – 0.010 8 7 6 5 (0.254 – 0.508) (1.346 – 1.752) (0.101 – 0.254) 0.008 – 0.010 (0.203 – 0.254) 0°– 8° TYP

0.228 – 0.244 0.150 – 0.157** 0.016 – 0.050 (3.810 – 3.988) 0.014 – 0.019 0.050 (5.791 – 6.197) 0.406 – 1.270 (0.355 – 0.483) (1.270) TYP *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SO8 0996 SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD 1 2 3 4 FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE 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 Upgraded Version of the LT1920 Instrumentation Amplifier 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

Linear Technology Corporation 1920f LT/TP 0299 4K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 12 ● ● (408) 432-1900 FAX: (408) 434-0507 www.linear-tech.com  LINEAR TECHNOLOGY CORPORATION 1998