AD797

REV. C

–9–

NOISE AND SOURCE IMPEDANCE CONSIDERATIONS

The AD797’s ultralow voltage noise of 0.9 nV/

√Hz is achieved

with special input transistors running at nearly 1 mA of collector

current. It is important then to consider the total input referred

Equation 1

This equation is plotted for the AD797 in Figure 30. Since opti-

mum dc performance is obtained with matched source resis-

tances, this case is considered even though it is clear from

Equation 1 that eliminating the balancing source resistance will

Ω), the amplifiers’ voltage

noise dominates. As source resistance increases the Johnson

current noise component is larger than the resistor noise.

100

1

0.1

10

10

100

1000

10000

SOURCE RESISTANCE –

Ω

TOTAL NOISE

RESISTOR

NOISE

ONLY

Figure 30. Noise vs. Source Resistance

The AD797 is the optimum choice for low noise performance

provided the source resistance is kept <1 k

Ω. At higher values of

source resistance, optimum performance with respect to noise

alone is obtained with other amplifiers from Analog Devices (see

Table I).

Table I. Recommended Amplifiers for Different Source

Impedances

Recommended Amplifier

0 to <1 k

AD797

1 k to <10 k

AD707, AD743/AD745, OP27/OP37, OP07

10 k to <100 k

AD705, AD743/AD745, OP07

>100 k

AD548, AD549, AD645, AD711, AD743/

AD745

LOW FREQUENCY NOISE

Analog Devices specifies low frequency noise as a peak to peak

(p-p) quantity in a 0.1 Hz to 10 Hz bandwidth. Several tech-

niques can be used to make this measurement. The usual tech-

nique involves amplifying, filtering, and measuring the amplifiers

noise for a predetermined test time. The noise bandwidth of the

filter is corrected for and the test time is carefully controlled

since the measurement time acts as an additional low frequency

roll-off.

The plot in Figure 4 was made using a slightly different tech-

nique. Here an FFT based instrument (Figure 31) is used to

generate a 10 Hz “brickwall” filter. A low frequency pole at

0.1 Hz is generated with an external ac coupling capacitor, the

instrument being dc coupled.

Several precautions are necessary to get optimum low frequency

noise performance:

10

Ω resistor has 0.4 nV/√Hz of noise (an error of 9% when

root sum squared with 0.9 nV/

√Hz).

from erroneously contributing to input noise.

3. Circuitry must be shielded from air currents. Heat flow out

of the package through its leads creates the opportunity for a

thermoelectric potential at every junction of different metals.

Selective heating and cooling of these by random air currents

will appear as 1/f noise and obscure the true device noise.

4. The results must be interpreted using valid statistical

techniques.

100k

Ω

1.5

µF

1

Ω

HP 3465

DYNAMIC SIGNAL

ANALYZER

(10Hz)

** USE POWER SUPPLY BYPASSING SHOWN IN FIGURE 32.

AD797

**

2

7

3

4

6

**

Figure 31. Test Setup for Measuring 0.1 Hz to 10 Hz Noise

WIDEBAND NOISE

The AD797, due to its single stage design, has the property that

its noise is flat over frequencies from less than 10 Hz to beyond

1 MHz. This is not true of most dc precision amplifiers where

second stage noise contributes to input referred noise beyond

the audio frequency range. The AD797 offers new levels of per-

formance in wideband imaging applications. In sampled data

systems, where aliasing of out of band noise into the signal band

is a problem, the AD797 will out perform all previously avail-

able IC op amps.