LT1677 Datasheet, PDF (11/16 Pages) Linear Technology – Low Noise, Rail-to-Rail Precision Op Amp

APPLICATIO S I FOR ATIO
0.1µF
100k
10Ω
–
*
LT1677
+
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
R_IN = 1M
110k
1677 F06a
Figure 6a. 0.1Hz to 10Hz Noise Test Circuit
LT1677
100
90
80
70
60
50
40
30
0.01
0.1
1
10
FREQUENCY (Hz)
100
1677 F06b
Figure 6b. 0.1Hz to 10Hz Peak-to-Peak
Noise Tester Frequency Response
Current noise is measured in the circuit shown in Figure 7
and calculated by the following formula:
( ) ( ) ^{ 2}
2 ^{1/ 2}
 ^e_no ^{− 130nV • 101} 
i_n = ^

(^1MΩ)(¹⁰¹)
The LT1677 achieves its low noise, in part, by operating
the input stage at 120µA versus the typical 10µA of most
other op amps. Voltage noise is inversely proportional
while current noise is directly proportional to the square
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 = [(voltage noise)² + (current noise • R_S)² +
(resistor noise)²]^1/2
Three regions can be identified as a function of source
resistance:
(i) R_S ≤ 400Ω. Voltage noise dominates
} (ii) 400Ω ≤ RS ≤ 50k at 1kHz Resistor noise
400Ω ≤ R_S ≤ 8k at 10Hz ^dominates
100k
100Ω
500k _–
LT1677
500k ₊
e_no
1677 F07
Figure 7
1000
R
R
V_S = ±15V
T_A = 25°C
100 ^{SOURCE RESISTANCE = 2R}
AT 1kHz
AT 10Hz
10
RESISTOR
NOISE ONLY
1
0.1
1
10
SOURCE RESISTANCE (kΩ)
100
1677 F08
Figure 8. Total Noise vs Source Resistance
} (iii) RS > 50k at 1kHz Current noise
R_S > 8k at 10Hz ^dominates
Clearly the LT1677 should not be used in region (iii), where
total system noise is at least six times higher than the
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