OPA698_12 Datasheet, PDF (23/32 Pages) Texas Instruments – Unity-Gain Stable, Wideband Voltage Limiting Amplifier

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50
HD2
HD3
V_O = 2V_PP
f₁ = 5MHz
100
Load Resistance (Ω)
1000
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 shown in Figure 25.
(3)
( ) ( ) E_O = ^E_NI2 + I_BNR_S ² + 4kTR_S ^NG² + I_BIR_F ² + 4kTR_FNG
Dividing this expression by the noise gain (NG = (1+R_F/R_G))
will give the equivalent input-referred spot noise voltage at
the noninverting input, as shown in Equation 4.
(4)
( ) E_N =
E_NI2 +
I_BNR_S
2
+ 4kTR_S
+

I_BIR_F
NG

2
+
4kTR_F
NG
FIGURE 23. 5MHz Harmonic Distortion vs Load Resistance.
NOISE PERFORMANCE
High slew rate, unity-gain stable, voltage feedback op amps
usually achieve their slew rate at the expense of a higher
input noise voltage. The 5.6nV/√Hz input voltage noise for
the OPA698, however, is much lower than comparable
amplifiers. The input-referred voltage noise, and the two
input-referred current noise terms, combine to give low
output noise under a wide variety of operating conditions.
Figure 24 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.
E_NI
OPA698
E_O
R_S
I_BN
E_RS
√4kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6E –20J
at 290°K
FIGURE 24. Op Amp Noise Analysis Model.
Evaluating these two equations for the OPA698 circuit and
component values (see Figure 1) will give a total output spot
noise voltage of 11.9nV/√Hz and a total equivalent input spot
noise voltage of 6nV/√Hz. This total input-referred spot noise
voltage is only slightly higher than the 5.6nV/√Hz specifica-
tion for the op amp voltage noise alone. This will be the case
as long as the impedances appearing at each op amp input
are limited to a maximum value of 300Ω. Keeping both
(R_F || R_G) and the noninverting input source impedance less
than 300Ω will satisfy both noise and frequency response
flatness considerations. Since the resistor-induced noise is
relatively negligible, additional capacitive decoupling across
the bias current cancellation resistor (R_T) for the inverting op
amp configuration of Figure 3 is not required, but is still
desirable.
DC ACCURACY AND OFFSET CONTROL
The balanced input stage of a wideband voltage feedback op
amp allows good output DC accuracy in a large variety of
applications. The power-supply current trim for the OPA698
gives even tighter control than comparable products. Al-
though the high-speed input stage does require relatively
high input bias current (typically ±8µA at each input terminal),
the close matching between them may be used to reduce the
output DC error caused by this current. The total output offset
voltage may be considerably reduced by matching the DC
source resistances appearing at the two inputs. This reduces
the output DC error due to the input bias currents to the offset
current times the feedback resistor. Evaluating the configura-
tion of Figure 1, using worst-case +25°C input offset voltage
and current specifications, gives a worst-case output offset
voltage equal to: –(NG = noninverting signal gain)
±(NG • V_OS(MAX)) ± (R_F • I_OS(MAX))
= ±(2 • 5mV) ± (402Ω • 1.4µA)
= ±10.6mV
OPA698
23
SBOS258D
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