OP484FS datasheet(16/24 Pages) AD

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OP184/OP284/OP484

Rev. D | Page 16 of 24

eNR

eNOA

iNOA

"NOISELESS"

IDEAL

NOISELESS

OP AMP

RS = 2R

Circuit noise figure is straightforward to calculate because the

signal level in the application is not required to determine it.

However, many designers using NF calculations as the basis for

achieving optimum SNR believe that low noise figure is equal to

low total noise. In fact, the opposite is true, as shown in Figure 50.

Here, the noise figure of the OP284 is expressed as a function of

the source resistance level. Note that the lowest noise figure for

the OP284 occurs at a source resistance level of 10 kΩ. However,

Figure 48. Op Amp Noise Circuit Model Used to Determine Total Circuit

Equivalent Input Noise Voltage and Noise Figure

Figure 49 shows that this source resistance level and the OP284

generate approximately 14 nV/√Hz of total equivalent circuit

noise. Signal levels in the application invariably increase to

maximize circuit SNR, which is not an option in low voltage,

single-supply applications.

As a design aid, Figure 49 shows the total equivalent input noise

of the OP284 and the total thermal noise of a resistor for com-

parison. Note that for source resistance less than 1 kΩ, the

equivalent input noise voltage of the OP284 is dominant.

TOTAL SOURCE RESISTANCE, RS (Ω)

100

10k

100k

FREQUENCY = 1kHz

TA = 25°C

TOTAL SOURCE RESISTANCE, RS (Ω)

100

10k

OP284 TOTAL

EQUIVALENT NOISE

RESISTOR THERMAL

NOISE ONLY

100

100k

FREQUENCY = 1kHz

TA = 25°C

Figure 50. OP284 Noise Figure vs. Source Resistance

Figure 49. OP284 Total Noise vs. Source Resistance

In single-supply applications, therefore, it is recommended for

optimum circuit SNR to choose an operational amplifier with

the lowest equivalent input noise voltage and to choose source

resistance levels consistent in maintaining low total circuit

noise.

Because circuit SNR is the critical parameter in the final

analysis, the noise behavior of a circuit is often expressed in

terms of its noise figure, NF. Noise figure is defined as the ratio

of a circuit’s output signal-to-noise to its input signal-to-noise.

An expression of a circuit NF in dB, and in terms of the

operational amplifier voltage and current noise parameters

defined previously, is given by

OVERDRIVE RECOVERY

The overdrive recovery time of an operational amplifier is the

time required for the output voltage to recover to its linear

region from a saturated condition. The recovery time is

important in applications where the amplifier must recover

quickly after a large transient event. The circuit shown in

()

(

)

()

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

log

nRS

nOA

where:

Figure 51 was used to evaluate the OP284 overload recovery

time. The OP284 takes approximately 2 μs to recover from

positive saturation and approximately 1 μs to recover from

negative saturation.

NF (dB) is the noise figure of the circuit, expressed in dB.

RS is the effective, or equivalent, source resistance presented to

the amplifier.

+5V

10kΩ

9kΩ

10kΩ

VIN

10V STEP

–5V

VOUT

1/2

OP284

(enOA)2 is the OP284 noise voltage spectral power (1 Hz BW).

(inOA)2 is the OP284 noise current spectral power (1 Hz BW).

(enRS)2 is the source resistance thermal noise voltage power =

(4kTR ).

Figure 51. Output Overload Recovery Test Circuit

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