AD797 datasheet(12/20 Pages) AD

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AD797

Rev. F | Page 12 of 20

from voltage noise (eN), current noise (iN), and resistor noise

(√4 kTRS).

]

)

(

[

kTR

total

(1)

where RS is the total input source resistance.

This equation is plotted for the AD797 in Figure 33. 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

0.1

100

1000

10000

SOURCE RESISTANCE (Ω)

TOTAL NOISE

RESISTOR

NOISE

ONLY

Figure 33. 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 3).

Table 3. Recommended Amplifiers for Different Source

Impedances

RS (kΩ)

Recommended Amplifier

0 to <1

AD797

1 to <10

AD743/AD745, OP27/OP37, OP07

10 to <100

AD743/AD745, OP07

>100

AD548, AD549, AD711, AD743/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 7 uses a slightly different technique: an

FFT-based instrument (Figure 34) 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.

HP 3465

DYNAMIC SIGNAL

ANALYZER

(10Hz)

1Ω

100kΩ

VOUT

+VS

–VS

1.5µF

AD797

*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.

Figure 34. 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.

BYPASSING CONSIDERATIONS

Taking full advantage of the very wide bandwidth and dynamic

range capabilities of the AD797 requires some precautions.

First, multiple bypassing is recommended in any precision

application. A 1.0 μF to 4.7 μF tantalum in parallel with 0.1 μF

ceramic bypass capacitors are sufficient in most applications.

When driving heavy loads, a larger demand is placed on the

supply bypassing. In this case, selective use of larger values of

tantalum capacitors and damping of their lead inductance with

small-value (1.1 Ω to 4.7 Ω) carbon resistors can achieve an

improvement. Figure 35 summarizes power supply bypassing

recommendations.

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