OP184/OP284/OP484

Rev. J | Page 16 of 24

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

1

10

10k

OP284 TOTAL

EQUIVALENT NOISE

RESISTOR THERMAL

NOISE ONLY

100

1k

100k

FREQUENCY = 1kHz

TA = 25°C

Figure 49. OP284 Equivalent Thermal Noise vs. Total Source Resistance

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. The 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

( )

( )

(

)

( )

































×

+

+

=

2

2

2

1

log

10

dB

nRS

S

nOA

nOA

e

R

i

e

NF

where:

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

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

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

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

(4kTRS).

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

the amplifier.

Calculation of the circuit noise figure is straightforward 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 a low noise figure is equal to

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

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 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.

TOTAL SOURCE RESISTANCE, RS (Ω)

10

100

5

10k

100k

1k

0

9

8

7

6

4

3

2

1

FREQUENCY = 1kHz

TA = 25°C

Figure 50. OP284 Noise Figure vs. Source Resistance

Therefore, to achieve optimum circuit SNR in single-supply

applications, it is recommended that an operational amplifier

with the lowest equivalent input noise voltage be chosen, along

with source resistance levels that are consistent with maintaining

low total circuit noise.

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 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.

2

3

1

+5V

8

4

R1

10k

Ω

R3

9k

Ω

R2

10k

Ω

VIN

10V STEP

–5V

VOUT

1/2

OP284

Figure 51. Output Overload Recovery Test Circuit

SINGLE-SUPPLY, 3 V INSTRUMENTATION

AMPLIFIER

The low noise, wide bandwidth, and rail-to-rail input/output

operation of the OP284 make it ideal for low supply voltage

applications such as in the two op amp instrumentation amplifier

shown in Figure 52. The circuit uses the classic two op amp

instrumentation amplifier topology with four resistors to set the

gain. The transfer equation of the circuit is identical to that of a

noninverting amplifier. Resistor R2 and Resistor R3 should be

closely matched to each other, as well as to Resistors (R1 + P1)

and Resistor R4 to ensure good common-mode rejection

performance.