Datasheet

14
LT1677
1677fa
APPLICATIO S I FOR ATIO
WUUU
Figure 6a. 0.1Hz to 10Hz Noise Test Circuit
Figure 6b. 0.1Hz to 10Hz Peak-to-Peak
Noise Tester Frequency Response
1677 F06a
10Ω
0.1µF
4.7µF
VOLTAGE GAIN
= 50,000
24.3k
100k
–
+
–
+
*
LT1677
LT1001
2k
4.3k
110k
100k
SCOPE
× 1
R
IN
= 1M
*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
1677 F06b
0.1
GAIN (dB)
Figure 7
1677 F07
100Ω
100k
–
+
LT1677
500k
500k
e
no
SOURCE RESISTANCE (kΩ)
0.1
1
10
100
1000
1 10 100
1677 F08
TOTAL NOISE DENSITY (nV/√Hz)
V
S
= ±15V
T
A
= 25°C
SOURCE RESISTANCE = 2R
R
R
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 R
S
)
2
]
1/2
Three regions can be identified as a function of source
resistance:
(i) R
S
≤ 400Ω. Voltage noise dominates
(ii) 400Ω ≤ R
S
≤ 50k at 1kHz
400Ω ≤ R
S
≤ 8k at 10Hz
(iii) R
S
> 50k at 1kHz
R
S
> 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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