Datasheet

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
WUUU
16789 F03
10Ω
0.1µF
4.7µF
VOLTAGE GAIN
= 50,000
24.3k
100k
–
+
–
+
*
LT1678
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
16789 F04
0.1
GAIN (dB)
16789 F05
100Ω
100k
–
+
LT1678
500k
500k
e
no
Figure 3. 0.1Hz to 10Hz Noise Test Circuit
Figure 4. 0.1Hz to 10Hz Peak-to-Peak
Noise Tester Frequency Response
Figure 5.
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
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
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 6. Total Noise vs Source Resistance
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