OPA4684M Datasheet, PDF (27/32 Pages) Texas Instruments – QUAD LOW-POWER CURRENT-FEEDBACK OPERATIONAL AMPLIFIER

OPA4684M
QUAD LOWĆPOWER CURRENTĆFEEDBACK
OPERATIONAL AMPLIFIER
SGLS145B − AUGUST 2003 − REVISED FEBRUARY 2004
noise performance
Wideband current-feedback op amps generally have a higher output noise than comparable voltage-feedback
op amps. The OPA4684 offers an excellent balance between voltage and current noise terms to achieve low
output noise in a low-power amplifier. The inverting current noise (17 pA/√Hz) is comparable to most other
current-feedback op amps while the input voltage noise (3.7 nV/√Hz) is lower than any unity-gain stable,
comparable slew rate, voltage-feedback op amp. This low input voltage noise was achieved at the price of
higher noninverting input current noise (9.4 pA/√Hz). As long as the AC source impedance looking out of the
noninverting node is less than 200 Ω, this current noise will not contribute significantly to the total output noise.
The op amp input voltage noise and the two input current noise terms combine to give low output noise under
a wide variety of operating conditions. Figure 55 shows the op amp noise analysis model with all the noise terms
included. In this model, all noise terms are taken to be noise voltage or current density terms in either nV/√Hz
or pA/√Hz.
Figure 55. Op Amp Noise Analysis Model
The total output spot noise voltage can be computed as the square root of the sum of all squared output noise
voltage contributors. Equation 3 shows the general form for the output noise voltage using the terms presented
in Figure 52.
Ǹǒ Ǔ EO +
ǒ Ǔ ǒ Ǔ ENI2 )
2
IBNRS ) 4kTRS
GN2 )
IBIRF
2
) 4kTRF GN
(3)
Dividing this expression by the noise gain (GN = (1+RF/RG)) will give the equivalent input referred spot noise
voltage at the noninverting input, as shown in Equation 4.
Ǹ ǒ Ǔ EO +
2
ǒ Ǔ ENI2 )
2
IBNRS ) 4kTRS )
IBIRF
GN
)
4kTRF
GN
(4)
Evaluating these two equations for the OPA4684 circuit and component values presented in Figure 41 will give
a total output spot noise voltage of 16.3 nV/√Hz and a total equivalent input spot noise voltage of 8.1 nV/√Hz.
This total input referred spot noise voltage is higher than the 3.7-nV/√Hz specification for the op amp voltage
noise alone. This reflects the noise added to the output by the inverting current noise times the feedback resistor.
As the gain is increased, this fixed output noise power term contributes less to the total output noise and the
total input referred voltage noise given by Equation 3 will approach just the 3.7 nV/√Hz of the op amp itself. For
example, going to a gain of +20 in the circuit of Figure 41, adjusting only the gain resistor to 42.1 Ω, will give
a total input referred noise of 3.9 nV/√Hz. A more complete description of op amp noise analysis can be found
in the Texas Instruments application note, AB−103, Noise Analysis for High Speed Op Amps (SBOA066),
located at www.ti.com.
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