LT1678CS8 查看數據表（PDF） - Linear Technology

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LT1678CS8

Dual/Quad Low Noise, Rail-to-Rail, Precision Op Amps

Linear Technology 

LT1678/LT1679

APPLICATIO S I FOR ATIO

0.1µF

100k

10Ω

–

*

LT1678

+

2k

4.7µF

VOLTAGE GAIN

= 50,000

*DEVICE UNDER TEST

NOTE: ALL CAPACITOR VALUES ARE FOR

NONPOLARIZED CAPACITORS ONLY

+

4.3k

LT1001

–

100k

24.3k

0.1µF

22µF

2.2µF

SCOPE

×1

RIN = 1M

110k

16789 F03

Figure 3. 0.1Hz to 10Hz Noise Test Circuit

100

90

80

70

60

50

40

30

0.01

0.1

1

10

FREQUENCY (Hz)

100

16789 F04

Figure 4. 0.1Hz to 10Hz Peak-to-Peak

Noise Tester Frequency Response

3. Sudden motion in the vicinity of the device can also

“feedthrough” to increase the observed noise.

Current noise is measured in the circuit shown in Figure 5

and calculated by the following formula:

Total Noise = [(op amp voltage noise)2 + (resistor noise)2

+ (current noise RS)2]1/2

Three regions can be identified as a function of source

resistance:

( ) ( ) ⎡ 2

2 ⎤1/ 2

⎢ eno − 130nV • 101 ⎥

in = ⎣

⎦

(1MΩ)(101)

100k

100Ω

500k –

LT1678

500k +

eno

16789 F05

Figure 5.

The LT1678/LT1679 achieve their low noise, in part, by

operating the input stage at 100µA versus the typical 10µA

of most other op amps. Voltage noise is inversely propor-

tional while current noise is directly proportional to the

square root of the input stage current. Therefore, the

LT1678/LT1679’s current noise will be relatively high. At

low frequencies, the low 1/f current noise corner fre-

quency (≈ 200Hz) minimizes current noise to some extent.

In most practical applications, however, current noise will

not limit system performance. This is illustrated in the

Total Noise vs Source Resistance plot (Figure 6) where:

12

(i) RS ≤ 400Ω. Voltage noise dominates

(ii) 400Ω ≤ RS ≤ 50k at 1kHz Resistor Noise

400Ω ≤ RS ≤ 8k at 10Hz Dominates

(iii) RS > 50k at 1kHz Current Noise

RS > 8k at 10Hz Dominates

Clearly the LT1678/LT1679 should not be used in region

(iii), where total system noise is at least six times higher

than the voltage noise of the op amp, i.e., the low voltage

noise specification is completely wasted. In this region the

LT1113 or LT1169 are better choices.

1000

R

R

VS = ±15V

TA = 25°C

SOURCE RESISTANCE = 2R

100

AT 1kHz

AT 10Hz

10

RESISTOR

NOISE ONLY

1

0.1

1

10

100

SOURCE RESISTANCE (kΩ)

16789 F06

Figure 6. Total Noise vs Source Resistance

sn16789 16789fs

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