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LT1028/LT1128

APPLICATI

I FOR ATIO

0.1Hz to 10Hz Noise Test Circuit

0.1Hz to 10Hz Peak-to-Peak Noise

Tester Frequency Response

FREQUENCY (Hz)

100

0.01

1.0

100

LT1028/1128 • AI03

0.1

–

VOLTAGE GAIN = 50,000

* DEVICE UNDER TEST

NOTE ALL CAPACITOR VALUES ARE FOR

NONPOLARIZED CAPACITORS ONLY

100k

Ω

–

4.7

µF

0.1

µF

100k

24.3k

µF

2.2

µF

4.3k

110k

SCOPE

× 1

RIN = 1M

0.1

µF

1028/1128 AI02

LT1001

– OISE

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 60

Ω

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 10

Ω source and 300Ω feedback

resistor. Even a 10

Ω 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 perfor-

mance of the LT1028/LT1128 requires special test pre-

cautions:

(a) The device should be warmed up for at least five

minutes. As the op amp warms up, its offset voltage

changes typically 10

µV due to its chip temperature

increasing 30

°C to 40°C from the moment the power

supplies are turned on. In the 10 second measure-

ment 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 thermo-

electric effects in excess of a few nanovolts, which

would invalidate the measurements.

“feedthrough” to increase the observed noise.

A noise-voltage density test is recommended when mea-

suring 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.

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