Datasheet

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, R
S
(Ω)
100
1
EQUIVALENT THERMAL NOISE (nV/ Hz)
10
10k
OP284 TOTAL
EQUIVALENT NOISE
RESISTOR THERMAL
NOISE ONLY
00293-049
100 1k 100k
FREQUENCY = 1kHz
T
A
= 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
1log10dB
nRS
SnOAnOA
e
Rie
NF
where:
NF (dB) is the noise figure of the circuit, expressed in decibels.
(e
nOA
)
2
is the OP284 noise voltage spectral power (1 Hz bandwidth).
(i
nOA
)
2
is the OP284 noise current spectral power (1 Hz bandwidth).
(e
nRS
)
2
is the source resistance thermal noise voltage power =
(4kTR
S
).
R
S
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 n
oise figure of the OP284 is expressed as a function of
the s
ource 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, R
S
(Ω)
10
100
NOISE FIGURE (dB)
5
10k 100k1k
0
9
8
7
6
4
3
2
1
00293-050
FREQUENCY = 1kHz
T
A
= 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
10kΩ
R3
9kΩ
R2
10kΩ
V
IN
10V STEP
–5V
V
OUT
1/2
OP284
00293-051
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.