LT1028_15 Datasheet, PDF (12/28 Pages) Linear Technology – Ultralow Noise Precision High Speed Op Amps

LT1028/LT1128
Applications Information – Noise
Noise Testing – Voltage Noise
The LT1028/LT1128’s RMS voltage noise density can be
accurately measured using the Quan Tech Noise Analyzer,
Model 5173 or an equivalent noise tester. Care should be
taken, however, to subtract the noise of the source resistor
used. Prefabricated test cards for the Model 5173 set the
device under test in a closed-loop gain of 31 with a 60Ω
source resistor and a 1.8k feedback resistor. The noise
of this resistor combination is 0.13√58 = 1.0nV/√Hz. An
LT1028/LT1128 with 0.85nV/√Hz noise will read (0.85² +
1.0²)^1/2 = 1.31nV/√Hz. For better resolution, the resistors
should be replaced with a 10Ω source and 300Ω feedback
resistor. Even a 10Ω resistor will show an apparent noise
which is 8% to 10% too high.
The 0.1Hz to 10Hz peak-to-peak noise of the LT1028/
LT1128 is measured in the test circuit shown. The fre-
quency response of this noise tester indicates that the
0.1Hz corner is defined by only one zero. The test time
to measure 0.1Hz to 10Hz noise should not exceed 10
seconds, as this time limit acts as an additional zero to
eliminate noise contributions from the frequency band
below 0.1Hz.
Measuring the typical 35nV peak-to-peak noise per-
formance of the LT1028/LT1128 requires special test
precautions:
(a) The device should be warmed up for at least five
minutes. As the op amp warms up, its offset voltage
changes typically 10µV due to its chip temperature
increasing 30°C to 40°C from the moment the power
supplies are turned on. In the 10 second measurement
interval these temperature-induced effects can easily
exceed tens of nanovolts.
(b) For similar reasons, the device must be well shielded
from air current to eliminate the possibility of ther-
moelectric effects in excess of a few nanovolts, which
would invalidate the measurements.
(c) Sudden motion in the vicinity of the device can also
feedthrough to increase the observed noise.
A noise-voltage density test is recommended when measur-
ing noise on a large number of units. A 10Hz noise-voltage
density measurement will correlate well with a 0.1Hz to
10Hz peak-to-peak noise reading since both results are
determined by the white noise and the location of the 1/f
corner frequency.
0.1µF
100k
–
10Ω
*
+
2k
4.7µF
VOLTAGE GAIN = 50,000
* DEVICE UNDER TEST
NOTE ALL CAPACITOR VALUES ARE FOR
NONPOLARIZED CAPACITORS ONLY
+
LT1001
–
100k
24.3k
0.1µF
4.3k
2.2µF
22µF
SCOPE
×1
R_IN = 1M
110k
1028 F02
Figure 2. 0.1Hz to 10Hz Noise Test Circuit
100
90
80
70
60
50
40
30
0.01
0.1
1.0
10
FREQUENCY (Hz)
100
1028 F03
Figure 3. 0.1Hz to 10Hz Peak-to-Peak
Noise Tester Frequency Response
1028fc
12
For more information www.linear.com/LT1028