AD797

Data Sheet

Rev. K | Page 12 of 19

NOISE AND SOURCE IMPEDANCE CONSIDERATIONS

The AD797 ultralow voltage noise of 0.9 nV/√Hz is achieved

with special input transistors running at nearly 1 mA of collector

current. Therefore, it is important to consider the total input-

referred noise (eNtotal), which includes contributions from voltage

noise (eN), current noise (iN), and resistor noise (√4 kTRS).

2

/

1

2

2

]

)

(

4

[

S

N

S

N

N

R

i

kTR

e

total

e









(1)

where RS is the total input source resistance.

This equation is plotted for the AD797 in Figure 34. Because

optimum dc performance is obtained with matched source

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

Equation 1 that eliminating the balancing source resistance

lowers the total noise by reducing the total RS by a factor of 2.

At very low source resistance (RS < 50 Ω), the voltage noise of the

amplifier dominates. As source resistance increases, the Johnson

noise of RS dominates until a higher resistance of RS > 2 kΩ is

achieved; the 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 34. Noise vs. Source Resistance

The AD797 is the optimum choice for low noise performance if

the source resistance is kept <1 kΩ. At higher values of source

resistance, optimum performance with respect to only noise is

obtained with other amplifiers from Analog Devices (Table 5).

For up to date information, see AN-940.

Table 5. Recommended Amplifiers for Different Source

Impedances

RS (kΩ)

Recommended Amplifier

0 to <1

AD8597/AD8599, AD797, ADA4004-1/

ADA4004-2/ADA4004-4, AD8671/AD8672/

AD8674

1 to <10

AD8675/AD8676, ADA4075-2, ADA4004-1/

ADA4004-2/ADA4004-4, OP1177, OP27/OP37,

OP184

10 to <100

AD8677, OP1177, OP2177, OP4177, OP471

>100

AD8610/AD8620, AD8605/AD8606/AD8608,

ADA4627-1, OP97, AD548, AD549, AD745

LOW FREQUENCY NOISE

Analog Devices specifies low frequency noise as a peak-to-peak

quantity in a 0.1 Hz to 10 Hz bandwidth. Several techniques can

be used to make this measurement. The usual technique involves

amplifying, filtering, and measuring the amplifier noise for a

predetermined test time. The noise bandwidth of the filter is

corrected for, and the test time is carefully controlled because

the measurement time acts as an additional low frequency roll-off.

The plot in Figure 6 uses a slightly different technique: an FFT-

based instrument (Figure 35) 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, which is also the instrument being

dc coupled.

Several precautions are necessary to attain optimum low

frequency noise performance:



Care must be used to account for the effects of RS. Even

a 10 Ω resistor has 0.4 nV/√Hz of noise (an error of 9%

when root sum squared with 0.9 nV/√Hz).



The test setup must be fully warmed up to prevent eOS drift

from erroneously contributing to input noise.



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

appears as 1/f noise and obscures the true device noise.



The results must be interpreted using valid statistical

techniques.

7

4

6

2

3

HP 3465

DYNAMIC SIGNAL

ANALYZER

(10Hz)

1Ω

100kΩ

*

*

VOUT

+VS

–VS

1.5µF

AD797

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 36.

Figure 35. Test Setup for Measuring 0.1 Hz to 10 Hz Noise

WIDEBAND NOISE

Due to its single-stage design, the noise of the AD797 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 performance in

wideband imaging applications. In sampled data systems, where

aliasing of out-of-band noise into the signal band is a problem,

the AD797 outperforms all previously available IC op amps.