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LT1792

Low Noise, Precision, JFET Input Op Amp

Linear Technology 

LT1792

APPLICATI S I FOR ATIO

also increases the parasitic gate capacitance, which if left

unchecked, can result in increased overshoot and ring-

ing. When the feedback around the op amp is resistive

(RF), a pole will be created with RF, the source resistance

and capacitance (RS, CS), and the amplifier input capaci-

tance (CIN = 27pF). In low gain configurations and with

RS and RF in the kilohm range (Figure 5), this pole can

create excess phase shift and even oscillation. A small

capacitor (CF) in parallel with RF eliminates this problem.

With RS(CS + CIN) = RFCF, the effect of the feedback pole

is completely removed.

RS

CS

CF

RF

–

+ CIN

Figure 5

OUTPUT

1792 F05

TYPICAL APPLICATI

S

Accelerometer Amplifier with DC Servo

C1

1250pF

10

ACCELEROMETER

B & K MODEL 4381

OR EQUIVALENT

R1

100M

R3

2k

C2

2µF

2

6

5V TO 15V

2– 7

LT1792

3

6

LT1792

3+

4

– 5V TO –15V

R2

18k

R4

20M

R5

20M

C3

2µF

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

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

DC OUTPUT ≤ 2.7mV

OUTPUT NOISE = 6nV/√Hz AT 1kHz

*PICOCOULOMBS

**g = EARTH’S GRAVITATIONAL CONSTANT

OUTPUT

1792 TA03

10Hz Fourth Order Chebyshev Lowpass Filter (0.01dB Ripple)

R1

237k

VIN

R2

C1

237k

33nF

R3

15V

249k 2 – 7

6

C2

100nF

LT1792

3+

4

–15V

R5

C3

154k

10nF

R4

R6

154k

249k 2 –

6

C4

330nF

LT1792

3+

TYPICAL OFFSET ≈ 0.8mV

1% TOLERANCES

FOR VIN = 10VP-P, VOUT = –121dB AT f > 330Hz

= – 6dB AT f = 16.3Hz

LOWER RESISTOR VALUES WILL RESULT IN LOWER THERMAL NOISE AND LARGER CAPACITORS

VOUT

1792 TA06

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