LT1007, LT1007A, LT1037, LT1037A

LOW-NOISE, HIGH-SPEED, PRECISION OPERATIONAL AMPLIFIERS

SLOS017C – D3195, FEBRUARY 1989 – REVISED JANUARY 1993

POST OFFICE BOX 655303

19

APPLICATION INFORMATION

Special test precautions are required to measure the typical 60-nV peak-to-peak noise performance of the

LT1007 and LT1037:

1.

The device should be warmed up for at least five minutes. As the operational amplifier warms up, the offset

voltage typically changes 3

µV, due to the chip temperature increasing 10°C to 20°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.

2.

The device must be well shielded from air currents to eliminate thermoelectric effects. In excess of a few

nanovolts, thermoelectric effects would invalidate the measurements.

3.

Sudden motion in the vicinity of the device can produce a feedthrough effect that increases observed noise.

When measuring noise on a large number of units, a noise-voltage density test is recommended. A 10-Hz

noise-voltage density measurement will correlate well with a 0.1-Hz to 10-Hz peak-to-peak noise reading since

both results are determined by the white noise and the location of the 1/f corner frequency.

Figure 40 shows a circuit that measures noise current and presents the formula for calculating noise current.

eno

500 k

Ω

10 k

Ω

500 k

Ω

100

Ω

+

1M

W x 100

Figure 40. Noise Test Circuit

The LT1007 and LT1037 achieve low noise, in part, by operating the input stage at 120

µA versus the typical

10

µA for most other operational amplifiers. Voltage noise is directly proportional to the square root of the stage

current; therefore, the LT1007 and LT1037 noise current is relatively high. At low frequencies, the low 1/f

current-noise corner frequency (

≈120 Hz) minimizes noise current to some extent.

In most practical applications, however, noise current will not limit system performance; this is illustrated in

Figure 27, where:

Three regions can be identified as a function of source resistance:

(i)

Voltage noise dominates in region (i)

(ii)

Resistor noise dominates in region (ii)

(iii)

Current noise dominates in region (iii)

The LT1007 and LT1037 should not be used in region (iii) where total system noise is at least six times higher

than the noise voltage of the operational amplifier (i.e., the low-voltage noise specification is completely wasted).