13

LT1113

S

APPLICATI

TYPICAL

Accelerometer Amplifier with DC Servo

Paralleling Amplifiers to Reduce Voltage Noise

8

5V TO 15V

–5V TO –15V

4

1113 • TA03

OUTPUT

R1

100M

C1

1250pF

R3

2k

C3

2

µF

R2

18k

C2

2

µF

6

5

R4

20M

1/2 LT1113

7

R5

20M

ACCELEROMETER

B & K MODEL 4381

OR EQUIVALENT

1

3

2

1/2 LT1113

R4C2 = R5C3 > R1 (1 + R2/R3) C1

OUTPUT = 0.8mV/pC* = 8.0mV/g**

DC OUTPUT

≤ 2.7mV

OUTPUT NOISE = 6nV/

√Hz AT 1kHz

*PICOCOULOMBS

**g = EARTH’S GRAVITATIONAL CONSTANT

1. ASSUME VOLTAGE NOISE OF LT1113 AND 51

Ω SOURCE RESISTOR = 4.6nV/√Hz

2. GAIN WITH n LT1113s IN PARALLEL = n • 200

3. OUTPUT NOISE =

√n • 200 • 4.6nV/√Hz

4. INPUT REFERRED NOISE =

OUTPUT NOISE

=

4.6

nV/

√Hz

n • 200

√n

5. NOISE CURRENT AT INPUT INCREASES

√n TIMES

6. IF n = 5, GAIN = 1000, BANDWIDTH = 1MHz, RMS NOISE, DC TO 1MHz =

9

µV

= 4

µV

√5

8

15V

–15V

4

8

15V

–15V

4

1113 • TA04

1k

7

6

5

1k

1k

1

2

3

–

+

A2

1/2 LT1113

–

+

1k

51

Ω

51

Ω

51

Ω

1

2

3

A1

1/2 LT1113

10k

7

5

6

1/2 LT1113

–

+

An

1/2 LT1113

1k

1k

OUTPUT