LT1678 LINER [Linear Technology], LT1678 Datasheet - Page 12

• Manufacturer Part Number: LT1678
• Description: Dual/Quad Low Noise, Rail-to-Rail, Precision Op Amps
• Manufacturer: LINER [Linear Technology]

LT1678/LT1679
APPLICATIO S I FOR ATIO
3. Sudden motion in the vicinity of the device can also
Current noise is measured in the circuit shown in Figure 5
and calculated by the following formula:
12
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:
*DEVICE UNDER TEST
NOTE: ALL CAPACITOR VALUES ARE FOR
NONPOLARIZED CAPACITORS ONLY
“feedthrough” to increase the observed noise.
i
10Ω
n
=
⎡
⎢
⎣
( )
e
VOLTAGE GAIN
= 50,000
no
–
+
LT1678
100Ω
0.1µF
100k
2
*
−
( )( )
1
(
Figure 3. 0.1Hz to 10Hz Noise Test Circuit
M
130
U
Ω
500k
500k
nV
101
4.7µF
Figure 5.
2k
•
U
101
–
+
100k
LT1678
)
2
⎤
⎥
⎦
1 2
24.3k
/
W
+
–
LT1001
100k
0.1µF
16789 F05
e
no
U
4.3k
22µF
2.2µF
Total Noise = [(op amp voltage noise)
+ (current noise R
Three regions can be identified as a function of source
resistance:
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.
110k
(i) R
(ii) 400Ω ≤ R
(iii) R
SCOPE
× 1
R
IN
16789 F03
= 1M
400Ω ≤ R
R
Figure 6. Total Noise vs Source Resistance
S
S
S
1000
≤ 400Ω. Voltage noise dominates
> 50k at 1kHz
> 8k at 10Hz
100
10
1
0.1
SOURCE RESISTANCE = 2R
100
S
90
80
70
60
50
40
30
S
S
)
R
R
0.01
Figure 4. 0.1Hz to 10Hz Peak-to-Peak
Noise Tester Frequency Response
2
≤ 50k at 1kHz
≤ 8k at 10Hz
]
SOURCE RESISTANCE (kΩ)
1/2
1
RESISTOR
NOISE ONLY
0.1
AT 10Hz
Current Noise
Dominates
FREQUENCY (Hz)
AT 1kHz
10
1
V
T
2
Resistor Noise
Dominates
A
S
+ (resistor noise)
= 25°C
= ±15V
16789 F06
10
100
sn16789 16789fs
16789 F04
100
2