LT1677CS8 datasheet(14/20 Pages) LINER

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LT1677

1677fa

APPLICATIO S I FOR ATIO

Figure 6a. 0.1Hz to 10Hz Noise Test Circuit

Figure 6b. 0.1Hz to 10Hz Peak-to-Peak

Noise Tester Frequency Response

1677 F06a

Ω

0.1

µF

4.7

µF

VOLTAGE GAIN

= 50,000

24.3k

100k

–

LT1677

LT1001

4.3k

110k

100k

SCOPE

× 1

RIN = 1M

*DEVICE UNDER TEST

NOTE: ALL CAPACITOR VALUES ARE FOR

NONPOLARIZED CAPACITORS ONLY

2.2

µF

0.1

µF

FREQUENCY (Hz)

100

0.01

100

1677 F06b

0.1

Figure 7

1677 F07

100

Ω

100k

LT1677

500k

eno

SOURCE RESISTANCE (k

Ω)

0.1

100

1000

100

1677 F08

VS = ±15V

TA = 25°C

SOURCE RESISTANCE = 2R

AT 1kHz

AT 10Hz

RESISTOR

NOISE ONLY

Figure 8. Total Noise vs Source Resistance

root of the input stage current. Therefore, the LT1677’s

current noise will be relatively high. At low frequencies, the

low 1/f current noise corner frequency (

≈90Hz) mini-

mizes 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 8) where:

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:

(i) RS ≤ 400Ω. Voltage noise dominates

(ii) 400

Ω ≤ RS ≤ 50k at 1kHz

400

Ω ≤ RS ≤ 8k at 10Hz

(iii) RS > 50k at 1kHz

RS > 8k at 10Hz

Clearly the LT1677 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

LT1792 or LT1793 is the best choice.

} Resistor noise

dominates

} Current noise

dominates

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