LT1678/LT1679

12

sn16789 16789fs

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:

i

e

nV

M

n

no

=

⎡

⎣

⎢

⎤

⎦

⎥

()( )

2

2

12

130

101

1

101

•

/

Ω

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:

APPLICATIO S I FOR ATIO

16789

F03

10Ω

0.1µF

4.7µF

VOLTAGE GAIN

= 50,000

24.3k

100k

–

+

*

LT1678

LT1001

2k

4.3k

110k

100k

SCOPE

× 1

*DEVICE UNDER TEST

NOTE: ALL CAPACITOR VALUES ARE FOR

NONPOLARIZED CAPACITORS ONLY

2.2µF

0.1µF

22µF

FREQUENCY (Hz)

100

90

80

70

60

50

40

30

0.01

1

10

100

16789

F04

0.1

16789

F05

100Ω

100k

LT1678

500k

500k

Figure 3. 0.1Hz to 10Hz Noise Test Circuit

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

Noise Tester Frequency Response

Figure 5.

Three regions can be identified as a function of source

resistance:

Resistor Noise

Dominates

Current Noise

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.

SOURCE RESISTANCE (kΩ)

0.1

1

10

100

1000

1

10

100

16789

F06

SOURCE RESISTANCE = 2R

R

R

AT 1kHz

AT 10Hz

RESISTOR

NOISE ONLY

Figure 6. Total Noise vs Source Resistance