OP184_06 Datasheet, PDF (16/24 Pages) Analog Devices – Precision Rail-to-Rail Input and Output Operational Amplifiers

OP184/OP284/OP484
R
e_NR
"NOISELESS"
e_NOA
i_NOA
R
e_NR
"NOISELESS"
i_NOA
IDEAL
NOISELESS
OP AMP
R_S = 2R
Figure 48. Op Amp Noise Circuit Model Used to Determine Total Circuit
Equivalent Input Noise Voltage and Noise Figure
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.
100
FREQUENCY = 1kHz
T_A = 25°C
OP284 TOTAL
EQUIVALENT NOISE
10
RESISTOR THERMAL
NOISE ONLY
1
100
1k
10k
100k
TOTAL SOURCE RESISTANCE, R_S (Ω)
Figure 49. OP284 Total Noise vs. 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. 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
( ) _NF
(dB
)
=
10
log
⎡
⎢1
⎢⎣
+
⎜⎜⎝⎛
(e_nOA
)² + i_nOA
( ) e_nRS ²
R_S
2
⎟⎟⎠⎞⎥⎥⎦⎤
where:
NF (dB) is the noise figure of the circuit, expressed in dB.
RS is the effective, or equivalent, source resistance presented to
the amplifier.
(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 =
(4kTRS ).
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 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.
10
FREQUENCY = 1kHz
9
T_A = 25°C
8
7
6
5
4
3
2
1
0
100
1k
10k
100k
TOTAL SOURCE RESISTANCE, R_S (Ω)
Figure 50. OP284 Noise Figure 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.
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.
R1
10kΩ
R2
10kΩ
+5V
V_IN
10V STEP
2
8
R3
9kΩ
1/2
OP284
1
3
4
–5V
V_OUT
Figure 51. Output Overload Recovery Test Circuit
Rev. D | Page 16 of 24