LT1677 LINER [Linear Technology], LT1677 Datasheet - Page 11

• Manufacturer Part Number: LT1677
• Description: Low Noise, Rail-to-Rail Precision Op Amp
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Current noise is measured in the circuit shown in Figure 7
and calculated by the following formula:
The LT1677 achieves its low noise, in part, by operating
the input stage at 120 A versus the typical 10 A 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)
(resistor noise)
Three regions can be identified as a function of source
resistance:
(i) R
(ii) 400
*DEVICE UNDER TEST
NOTE: ALL CAPACITOR VALUES ARE FOR
NONPOLARIZED CAPACITORS ONLY
i
10
n
400
S
e
400 . Voltage noise dominates
VOLTAGE GAIN
= 50,000
no
–
+
LT1677
R
R
0.1 F
100k
2
*
S
S
2
]
1
1/2
Figure 6a. 0.1Hz to 10Hz Noise Test Circuit
M
50k at 1kHz
8k at 10Hz
130
U
nV
101
4.7 F
2k
U
101
2
}
2
+ (current noise • R
1 2
24.3k
Resistor noise
dominates
W
–
+
LT1001
100k
0.1 F
U
4.3k
S
)
2
+
22 F
2.2 F
(iii) R
Clearly the LT1677 should not be used in region (iii), where
total system noise is at least six times higher than the
110k
R
SCOPE
R
S
S
1677 F06a
IN
1
> 50k at 1kHz
> 8k at 10Hz
= 1M
Figure 8. Total Noise vs Source Resistance
1000
100
100
10
1
0.1
SOURCE RESISTANCE = 2R
R
R
100
90
80
70
60
50
40
30
500k
500k
Figure 6b. 0.1Hz to 10Hz Peak-to-Peak
Noise Tester Frequency Response
SOURCE RESISTANCE (k )
}
0.01
Figure 7
Current noise
dominates
1
–
+
100k
RESISTOR
NOISE ONLY
AT 10Hz
LT1677
0.1
AT 1kHz
FREQUENCY (Hz)
10
V
T
1
S
A
= 25 C
= 15V
1677 F08
1677 F07
100
e
LT1677
no
10
1677 F06b
11
100