OPA2680 Datasheet, PDF (18/21 Pages) Burr-Brown (TI) – Dual Wideband, Voltage Feedback OPERATIONAL AMPLIFIER With Disable

proves distortion directly. Remember that the total load
includes the feedback network; in the non-inverting configu-
ration (Figure 1) this is sum of R_F + R_G, while in the
inverting configuration, it is just R_F. Also, providing an
additional supply decoupling capacitor (0.1µF) between the
supply pins (for bipolar operation) improves the 2nd-order
distortion slightly (3dB to 6dB).
In most op amps, increasing the output voltage swing in-
creases harmonic distortion directly. The new output stage
used in the OPA2680 actually holds the difference between
fundamental power and the 2nd and 3rd harmonic powers
relatively constant with increasing output power until very
large output swings are required (>4Vp-p). This also shows
up in the two-tone, 3rd-order intermodulation spurious (IM3)
response curves. The 3rd-order spurious levels are extremely
low at low output power levels. The output stage continues
to hold them low even as the fundamental power reaches
very high levels. As the Typical Performance Curves show,
the spurious intermodulation powers do not increase as
predicted by a traditional intercept model. As the fundamen-
tal power level increases, the dynamic range does not de-
crease significantly. For 2 tones centered at 20MHz, with
10dBm/tone into a matched 50Ω load (i.e., 2Vp-p for each
tone at the load, which requires 8Vp-p for the overall two-
tone envelope at the output pin), the Typical Performance
Curves show 57dBc difference between the test tone powers
and the 3rd-order intermodulation spurious powers. This
exceptional performance improves further when operating at
lower frequencies.
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 4.8nV/√Hz input voltage noise for
the OPA2680 is, however, 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 12 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
1/2
R_S
I_BN
OPA2680
E_O
E_RS
√4kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6E –20J
at 290°K
FIGURE 12. 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 shown in Figure 12.
Equation 3:
( ) ^{( ) ( ) E}O ^{= E}NI^{2 + I}BN^RS ^{2 + 4kTR}S ^{NG2 + I}BI^RF ^{2 + 4kTR}F^NG
Dividing this expression by the noise gain (NG = (1+R_F/R_G))
will give the equivalent input-referred spot noise voltage at
the non-inverting input, as shown in Equation 4.
Equation 4:
( ) E_N =
E_NI² +
I _BN R _S
2
+
4 kTR _S
+


I _BI R _F
NG


2
+
4 kTR _F
NG
Evaluating these two equations for the OPA2680 circuit and
component values shown in Figure 1 will give a total output
spot noise voltage of 11nV/√Hz and a total equivalent input
spot noise voltage of 5.5nV/√Hz. This is including the noise
added by the bias current cancellation resistor (175Ω) on the
non-inverting input. This total input-referred spot noise
voltage is only slightly higher than the 4.8nV/√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 the previously recommend maximum value of
300Ω. Keeping both (R_F || R_G) and the non-inverting input
source impedance less than 300Ω will satisfy both noise and
frequency response flatness considerations. Since the resis-
tor-induced noise is relatively negligible, additional capaci-
tive decoupling across the bias current cancellation resistor
(R_B) for the inverting op amp configuration of Figure 9 is not
required.
DC ACCURACY AND OFFSET CONTROL
The balanced input stage of a wideband voltage feedback op
amp allows good output DC accuracy in a wide variety of
applications. The power supply current trim for the OPA2680
gives even tighter control than comparable amplifiers. Al-
though the high speed input stage does require relatively
high input bias current (typically 14µA out of 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 match-
ing 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 configuration of Figure 1, using worst-case
+25°C input offset voltage and current specifications, gives
a worst-case output offset voltage equal to:
±(NG • V_OS(MAX)) ± (R_F • I_OS(MAX))
= ±(2 • 4.5mV) ± (402Ω • 0.7µA)
= ±9.3mV
– (NG = non-inverting signal gain)
®
OPA2680
18