Datasheet LT1920 Datasheet (Linear Technology)

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FEATURES

■

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: 0.28µV

■

Available in 8-Pin PDIP and SO Packages

■

Meets IEC 1000-4-2 Level 4 ESD Tests with

P-P

Two External 5k Resistors

APPLICATIO S

■

Bridge Amplifiers

■

Strain Gauge Amplifiers

■

Thermocouple Amplifiers

■

Differential to Single-Ended Converters

■

Medical Instrumentation

LT1920

Single Resistor Gain

Programmable, Precision

Instrumentation Amplifier

DESCRIPTIO

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.

, LTC and LT are registered trademarks of Linear Technology Corporation.

TYPICAL APPLICATIO

Single Supply Barometer

LT1634CCZ-1.25

392k

R5

R8

100k

–

1/2

LT1490

–

50k

R4

R3

R6 1k

1/2

LUCAS NOVA SENOR

NPC-1220-015-A-3L

SET

R7

50k

Gain Nonlinearity

R1 825Ω

R2 12Ω

–

2 1

8 3

VOLTS

2.800

3.000

3.200

INCHES Hg

TO 4-DIGIT DVM

28.00

30.00

32.00

1920 TA01

NONLINEARITY (100ppm/DIV)

OUTPUT VOLTAGE (2V/DIV)

G = 1000

= 1k

= ±10V

OUT

1167 TA02

LT1920

G = 60

–

VS = 8V TO 30V

LT1920

ABSOLUTE MAXIMUM RATINGS

(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

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/T Gain vs Temperature G < 1000 (Note 2) ● 20 50 ppm/°C

Gain Nonlinearity (Note 5) VO = ±10V, G = 1 10 ppm

OST

OSI

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

OSI

OSO

/T Output Offset Drift (Note 3) ● 515µV/°C

OSO

Total RTI Noise = √e e

IN(DIFF)

Total Input Referred Offset Voltage V Input Offset Voltage G = 1000, VS = ±5V to ±15V 30 125 µV

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

Input Offset Current 0.3 1 nA Input Bias Current 0.5 2 nA Input Noise Voltage, RTI 0.1Hz to 10Hz, G = 1 2.00 µV

+ (eno/G)

Input Noise Voltage Density, RTI fO = 1kHz 7.5 nV/√Hz Output Noise Voltage Density, RTI fO = 1kHz 67 nV/√Hz Input Noise Current fO = 0.1Hz to 10Hz 10 pA Input Noise Current Density fO = 10Hz 124 fA/√Hz Input Resistance VIN = ±10V 200 GΩ Differential Input Capacitance fO = 100kHz 1.6 pF

PACKAGE/ORDER INFORMATION

TOP VIEW



1

–IN +IN

–V

JMAX

Consult factory for Military grade parts.

–

2

3 4

N8 PACKAGE 8-LEAD PDIP

S8 PACKAGE

8-LEAD PLASTIC SO

= 150°C, θJA = 130°C/ W (N8) = 150°C, θJA = 190°C/ W (S8)

8 7 6 5



RG



OUTPUT REF

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

G = 10 (Note 2) 0.010 0.3 % G = 100 (Note 2) 0.025 0.3 % G = 1000 (Note 2) 0.040 0.35 %

= ±10V, G = 10 and 100 10 30 ppm

= ±10V, G = 100 and 1000 20 ppm

= V

+ V

OST

OSI

G = 1000, V

G = 1, V

0.1Hz to 10Hz, G = 10 0.50 µV

0.1Hz to 10Hz, G = 100 and 1000 0.28 µV

= ±5V to ±15V ● 1500 µV

OSO

= ±5V to ±15V ● 185 µV

ORDER PART

NUMBER

LT1920CN8 LT1920CS8 LT1920IN8 LT1920IS8

S8 PART MARKING

1920 1920I

P-P P-P P-P

P-P

LT1920

ELECTRICAL CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS (Note 6) MIN TYP MAX UNITS

IN(CM)

CMRR Common Mode Rejection Ratio 1k Source Imbalance,

PSRR Power Supply Rejection Ratio VS = ±2.3 to ±18V

OUT

BW Bandwidth G = 1 1000 kHz

SR Slew Rate G = 1, V

REFIN

REF

VREF

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 R 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.

Common Mode Input Capacitance fO = 100kHz 1.6 pF Input Voltage Range G = 1, Other Input Grounded

= 0V to ±10V

G = 1 75 95 dB G = 10 95 115 dB G = 100 110 125 dB G = 1000 110 140 dB

G = 1 80 120 dB G = 10 100 135 dB G = 100 120 140 dB

G = 1000 120 150 dB Supply Current VS = ±2.3V to ±18V 0.9 1.3 mA Output Voltage Swing RL = 10k

Output Current 20 27 mA

G = 10 800 kHz G = 100 120 kHz G = 1000 12 kHz

Settling Time to 0.01% 10V Step

G = 1 to 100 14 µs

G = 1000 130 µs Reference Input Resistance 20 kΩ Reference Input Current V Reference Voltage Range –VS + 1.6 +VS – 1.6 V Reference Gain to Output 1 ± 0.0001

REF

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

= ±2.3V to ±5V –VS + 1.9 +VS – 1.2 V = ±5V to ±18V –VS + 1.9 +VS – 1.4 V = ±2.3V to ±5V ● –VS + 2.1 +VS – 1.3 V = ±5V to ±18V ● –VS + 2.1 +VS – 1.4 V

= ±2.3V to ±5V –VS + 1.1 +VS – 1.2 V = ±5V to ±18V –VS + 1.2 +VS – 1.3 V = ±2.3V to ±5V ● –VS + 1.4 +VS – 1.3 V = ±5V to ±18V ● –VS + 1.6 +VS – 1.5 V

= ±10V 1.2 V/µs

OUT

= 0V 50 µA

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.

LT1920

TYPICAL PERFOR A CE CHARACTERISTICS

Gain Nonlinearity, G = 1

NONLINEARITY (1ppm/DIV)

OUTPUT VOLTAGE (2V/DIV)G = 1

= 2k

= ±10V

OUT

Gain Nonlinearity, G = 1000

NONLINEARITY (100ppm/DIV)

OUTPUT VOLTAGE (2V/DIV)

G = 1000 R

= 2k

= ±10V

OUT

1167 G01

1167 G04

Gain Nonlinearity, G = 10

NONLINEARITY (10ppm/DIV)

G = 10 R V

OUTPUT VOLTAGE (2V/DIV)

= 2k

= ±10V

OUT

Gain Error vs Temperature

0.20

0.15

0.10

0.05

–0.05

VS = ±15V

GAIN ERROR (%)

–0.10

–0.15

–0.20

= ±10V

OUT

= 2k

*DOES NOT INCLUDE  TEMPERATURE EFFECTS  OF R

–50

–25

TEMPERATURE (°C)

1167 G02

G = 1

G = 10*

G = 100*

G = 1000*

25 100

1920 G06

Gain Nonlinearity, G = 100

NONLINEARITY (10ppm/DIV)

OUTPUT VOLTAGE (2V/DIV)

G = 100 R

= 2k

= ±10V

OUT

Warm-Up Drift

VS = ±15V

= 25°C

G = 1

CHANGE IN OFFSET VOLTAGE (µV)

12 5

TIME AFTER POWER ON (MINUTES)

1167 G03

1920 G09

Input Bias Current vs Common Mode Input Voltage

500 400 300 200 100

–100

–200

INPUT BIAS CURRENT (pA)

–300 –400 –500

–12 12

–15

COMMON MODE INPUT VOLTAGE (V)

–9

–6

85°C

0°C

–40°C

–3

70°C

25°C

Common Mode Rejection Ratio vs Frequency

160

G = 1000

140

G = 100 G = 10

120

G = 1

100

COMMON MODE REJECTION RATIO (dB)

1920 G13

110 1k

0.1 FREQUENCY (Hz)

100

VS = ±15V

= 25°C

1k SOURCE IMBALANCE

10k

1920 G14

100k

Negative Power Supply Rejection Ratio vs Frequency

160

G = 100

140

G = 10

120

G = 1

100

NEGATIVE POWER SUPPLY REJECTION RATIO (dB)

110 1k

0.1 FREQUENCY (Hz)

100

V+ = 15V

= 25°C

G = 1000

10k

1920 G15

100k

TYPICAL PERFOR A CE CHARACTERISTICS

Positive Power Supply Rejection Ratio vs Frequency

160

140

G = 10

120

G = 1

100

POSITIVE POWER SUPPLY REJECTION RATIO (dB)

0.1

G = 100

110 1k

100

FREQUENCY (Hz)

V– = –15V

= 25°C

G = 1000

10k

100k

1920 G16

Gain vs Frequency

GAIN (dB)

–10

–20

0.01 1 10 1000

VS = ±15V

= 25°C

0.1

G = 1000

G = 100

G = 10

G = 1

100

FREQUENCY (kHz)

1920 G17

Supply Current vs Supply Voltage

1.50

1.25

1.00

SUPPLY CURRENT (mA)

0.75

0.50 0

SUPPLY VOLTAGE (±V)

LT1920

85°C 25°C

–40°C

1920 G18

Voltage Noise Density vs Frequency

1000

VS = ±15V

= 25°C

1/f

CORNER

1/f

CORNER

1/f

CORNER

10 100 1k 100k10k

FREQUENCY (Hz)

VOLTAGE NOISE DENSITY (nV√Hz)

100

Current Noise Density vs Frequency

1000

100

CURRENT NOISE DENSITY (fA/√Hz)

10 100 1000

FREQUENCY (Hz)

= 10Hz

= 9Hz

= 7Hz

GAIN = 1

GAIN = 10

GAIN = 100, 1000

BW LIMIT

GAIN = 1000

1920 G19

VS = ±15V

= 25°C

1920 G22

0.1Hz to 10Hz Noise Voltage, G = 1

VS = ±15V

= 25°C

NOISE VOLTAGE (2µV/DIV)

TIME (SEC)

0.1Hz to 10Hz Current Noise

VS = ±15V

= 25°C

CURRENT NOISE (5pA/DIV)

TIME (SEC)

0.1Hz to 10Hz Noise Voltage, RTI G = 1000

VS = ±15V

= 25°C

NOISE VOLTAGE (0.2µV/DIV)

1920 G20

TIME (SEC)

1920 G21

Short-Circuit Current vs Time

50 40 30 20 10

0 –10 –20

OUTPUT CURRENT (mA)

–30

(SINK) (SOURCE)

–40 –50

1920 G23

TIME FROM OUTPUT SHORT TO GROUND (MINUTES)

TA = –40°C

= 25°C

= 85°C

TA = 85°C

TA = 25°C

VS = ±15V

1920 G24

LT1920

TYPICAL PERFOR A CE CHARACTERISTICS

Large-Signal Transient Response

5V/DIV

G = 1

= ±15V

= 2k

= 60pF

10µs/DIV

Large-Signal Transient Response

5V/DIV

1167 G28

Small-Signal Transient Response

20mV/DIV

G = 1 V

= ±15V

= 2k

= 60pF

10µs/DIV

Small-Signal Transient Response

20mV/DIV

1167 G29

Overshoot vs Capacitive Load

100

VS = ±15V

90 80 70 60

50 40

OVERSHOOT (%)

30 20 10

= ±50mV

OUT

= ∞

AV = 1

AV = 10

AV ≥ 100

100 1000 10000

CAPACITIVE LOAD (pF)

Output Impedance vs Frequency

1000

VS = ±15V

= 25°C

G = 1 TO 1000

100

1920 G25

G = 10

= ±15V

= 2k

= 60pF

10µs/DIV

Large-Signal Transient Response

5V/DIV

G = 100

= ±15V

= 2k

= 60pF

10µs/DIV

1167 G31

1167 G34

= ±15V

= 2k

= 60pF

10µs/DIVG = 10

Small-Signal Transient Response

20mV/DIV

= ±15V

= 2k

= 60pF

10µs/DIVG = 100

1167 G32

1167 G35

OUTPUT IMPEDANCE (Ω)

0.1 1

10 100 1000

FREQUENCY (kHz)

Undistorted Output Swing vs Frequency

G = 10, 100, 1000

G = 1

PEAK-TO-PEAK OUTPUT SWING (V)

10 100 1000 FREQUENCY (kHz)

1920 G26

VS = ±15V

= 25°C

1920 G27

TYPICAL PERFOR A CE CHARACTERISTICS

LT1920

Large-Signal Transient Response

5V/DIV

G = 1000

= ±15V

= 2k

= 60pF

50µs/DIV

Settling Time vs Step Size

VS = ±15

G = 1

= 25°C

= 30pF

= 1k

2 0

–2

OUTPUT STEP (V)

–4 –6 –8

–10

311

TO 0.1%

SETTLING TIME (µs)

Small-Signal Transient Response

20mV/DIV

1167 G37

TO 0.01%

G = 1000

= ±15V

= 2k

= 60pF

OUT

TO 0.01%

1920 G33

50µs/DIV

1167 G38

Settling Time vs Gain

1000

VS = ±15V

= 25°C

= 10V

∆V

OUT

1mV = 0.01%

100

SETTLING TIME (µs)

Slew Rate vs Temperature

1.8 VS = ±15V

= ±10V

OUT

G = 1

1.6

1.4

+SLEW

1.2

SLEW RATE (V/µs)

1.0

0.8

–50 –25

–SLEW

TEMPERATURE (°C)

10 100 1000

GAIN (dB)

100

125

1920 G36

1920 G30

Output Voltage Swing vs Load Current

+VS – 0.5

–V

OUTPUT VOLTAGE SWING (V) 

(REFERRED TO SUPPLY VOLTAGE)

–V

VS = ±15V

– 1.0

– 1.5

– 2.0

+ 2.0

+ 1.5

+ 1.0

+ 0.5

–V

0.01 1 10 100

0.1 OUTPUT CURRENT (mA)

85°C 25°C –40°C

SOURCE

SINK

1920 G39

LT1920

BLOCK DIAGRAM

–IN



+IN

R3

400Ω

–

R4

400Ω

–

R1

24.7k

–

R2

24.7k

R5

10k

R7

10k

R6

10k

–

R8

10k

–

DIFFERENCE AMPLIFIER STAGEPREAMP STAGE

OUTPUT

REF

–

1920 F01

Figure 1. Block Diagram

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 R determines the transconductance of the preamp stage. As

is reduced for larger programmed gains, the transcon-

ductance 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

. The value of RG in parallel with R1 (R2)

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 R Since the current that flows through R

1 and R2, the ratios provide a gained-up differential volt-

R age, G = (R1 + R2)/R

, to the unity-gain difference

also flows through

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:

OUT

– V

REF

= G(V

– V

–

)

where:

G = (49.4kΩ/RG) + 1

solving for the gain set resistor gives:

RG = 49.4kΩ/(G – 1)

THEORY OF OPERATIO

LT1920

Input and Output Offset Voltage

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:

Total input offset voltage (RTI) = input offset + (output offset/G)

Total output offset voltage (RTO) = (input offset • G) + output offset

Reference Terminal

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.

Output Offset Trimming

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.

–

–IN

+IN

2 1

Figure 2. Optional Trimming of Output Offset Voltage

LT1920

REF

±10mV

ADJUSTMENT RANGE

OUTPUT

–

1/2

LT1112

10k

10mV 100Ω

100Ω

–10mV

–

1920 F02

Single Supply Operation

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 R

from the bridge

SET

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.

Input Bias Current Return Path

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.

LT1920

THEORY OF OPERATIO

–

MICROPHONE,

THERMOCOUPLE

10k

LT1920

Figure 3. Providing an Input Common Mode Current Path

HYDROPHONE,

ETC

200k

WUU

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).

–

LT1920

CENTER-TAP PROVIDES BIAS CURRENT RETURN

J1 2N4393

J2 2N4393

–

LT1920

OPTIONAL FOR HIGHEST ESD PROTECTION

LT1920

1920 F03

OUT

REF

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-

Figure 4. Input Protection

–

1920 F04

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

WUU

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 C

form lowpass filters with the external series resis-

XCM2

tors R the input traces. Capacitor C

to any out-of-band signal appearing on each of

S1, 2

forms a filter to reduce any

unwanted signal that would appear across the input traces. An added benefit to using C

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:

DM(LPF)

CM(LPF)

= (2)(RS)(CXD)

= (R

S1, 2

)(C

XCM1, 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

XCM1

and

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

can be omitted, if the sensor is in proximity to the

S1, 2

instrumentation amplifier.

RS1

XCM1

0.001µF

1.6k

XD

0.1µF

RS2

1.6k

–

XCM2

0.001µF

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

LT1920

OUT

–

1920 F05

PACKAGE DESCRIPTION

0.300 – 0.325

(7.620 – 8.255)

0.065

(1.651)

0.009 – 0.015

(0.229 – 0.381)

+0.035

0.325

–0.015

+0.889

8.255

()

–0.381

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)

TYP

0.100 ± 0.010

(2.540 ± 0.254)

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.

Dimensions in inches (millimeters) unless otherwise noted.

N8 Package

8-Lead PDIP (Narrow 0.300)

(LTC DWG # 05-08-1510)

0.045 – 0.065

(1.143 – 1.651)

0.130 ± 0.005

(3.302 ± 0.127)

0.125

(3.175)

MIN

0.018 ± 0.003

(0.457 ± 0.076)



0.020

(0.508)

MIN

0.255 ± 0.015* (6.477 ± 0.381)



0.400* (10.160)

MAX

876

N8 1197

LT1920

TYPICAL APPLICATION

Nerve Impulse Amplifier

PATIENT/CIRCUIT

+IN

PATIENT

GROUND

–IN

PROTECTION/ISOLATION

C1

0.01µF

R1

12k

R2

–

1/2

LT1112

PACKAGE DESCRIPTION

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

*

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  

× 45°

(1.346 – 1.752)

0°– 8° TYP

0.016 – 0.050

0.406 – 1.270

R3 30k

R4 30k

AV = 101 POLE AT 1kHz

3 8

G

1 2

–

LT1920

G = 10

–3V

C2

0.47µF

0.3Hz HIGHPASS

R6 1M

R8

100Ω

Dimensions in inches (millimeters) unless otherwise noted.

S8 Package

8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

0.053 – 0.069

0.014 – 0.019

(0.355 – 0.483)

0.004 – 0.010

(0.101 – 0.254)

0.050

(1.270)

TYP

0.228 – 0.244

(5.791 – 6.197)

1/2

LT1112

–

–3V

C3

15nF

0.189 – 0.197* (4.801 – 5.004)

R7

10k

OUTPUT



1V/mV

1920 TA03

0.150 – 0.157** (3.810 – 3.988)

SO8 0996

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Power Dissipation: 55mW Typ

1920f LT/TP 0299 4K • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1998

Linear T echnology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507

●

www.linear-tech.com

Datasheet LT1920 Datasheet (Linear Technology)

Specifications and Main Features

Frequently Asked Questions

User Manual