Datasheet

OPA691
18
SBOS226D
www.ti.com
dynamic range does not decrease significantly. For two
tones centered at 20MHz, with 10dBm/tone into a matched
50Ω load (i.e., 2V
PP
for each tone at the load, which requires
8V
PP
for the overall 2-tone envelope at the output pin), the
Typical Characteristics show 48dBc difference between the
test-tone power and the 3rd-order intermodulation spurious
levels. This exceptional performance improves further when
operating at lower frequencies.
NOISE PERFORMANCE
Wideband current feedback op amps generally have a higher
output noise than comparable voltage feedback op amps. The
OPA691 offers an excellent balance between voltage and
current noise terms to achieve low output noise. The inverting
current noise (15pA/
√Hz ) is significantly lower than earlier
solutions while the input voltage noise (1.7nV/
√Hz ) is lower
than most unity-gain stable, wideband, voltage feedback op
amps. This low input voltage noise was achieved at the price
of higher noninverting input current noise (12pA/
√Hz ). As long
as the AC source impedance looking out of the noninverting
node is less than 100Ω, 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 10 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 .
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 6.
(6)
E E I R kTR
IR
NG
kTR
NG
NNIBN
SS
BI F F
=+
( )
++






+
2
2
2
4
4
Evaluating these two equations for the OPA691 circuit and
component values (see Figure 1) will give a total output spot
noise voltage of 8.0nV/
√Hz and a total equivalent input spot
noise voltage of 4.0nV/
√Hz . This total input-referred spot
noise voltage is higher than the 1.7nV/
√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. If the feedback resistor is reduced in high
gain configurations (as suggested previously), the total input-
referred voltage noise given by Equation 5 will approach just
the 1.7nV/
√Hz of the op amp itself. For example, going to a
gain of +10 using R
F
= 180Ω will give a total input-referred
noise of 2.1nV/
√Hz .
DC ACCURACY AND OFFSET CONTROL
A current feedback op amp like the OPA691 provides excep-
tional bandwidth in high gains, giving fast pulse settling but
only moderate DC accuracy. The Typical Specifications show
an input offset voltage comparable to high-speed voltage
feedback amplifiers. However, the two input bias currents are
somewhat higher and are unmatched. Whereas bias current
cancellation techniques are very effective with most voltage
feedback op amps, they do not generally reduce the output DC
offset for wideband current feedback op amps. Since the two
input bias currents are unrelated in both magnitude and
polarity, matching the source impedance looking out of each
input to reduce their error contribution to the output is ineffec-
tive. Evaluating the configuration of Figure 1, using worst-case
+25°C input offset voltage and the two input bias currents,
gives a worst-case output offset range equal to:
± (NG • V
OS(MAX)
) + (I
BN
• R
S
/2 • NG) ± (I
BI
• R
F
)
where NG = noninverting signal gain
= ± (2 • 2.5mV) + (35µA • 25Ω • 2) ± (402Ω • 25µA)
= ±5mV + 1.75mV ± 10.05mV
= –13.3mV → +16.8mV
A fine-scale, output offset null, or DC operating point adjust-
ment, is sometimes required. Numerous techniques are
available for introducing DC offset control into an op amp
circuit. Most simple adjustment techniques do not correct for
temperature drift. It is possible to combine a lower speed,
precision op amp with the OPA691 to get the DC accuracy
of the precision op amp along with the signal bandwidth of
the OPA691. See Figure 11 for a noninverting G = +10 circuit
that holds an output offset voltage less than ±7.5mV over-
temperature with > 150MHz signal bandwidth.
This DC-coupled circuit provides very high signal bandwidth
using the OPA691. At lower frequencies, the output voltage
is attenuated by the signal gain and compared to the original
FIGURE 10. Op Amp Noise Analysis Model.
4kT
R
G
R
G
R
F
R
S
OPA691
I
BI
E
O
I
BN
4kT = 1.6E –20J
at 290°K
E
RS
E
NI
√4kTR
F
√4kTR
S
The total output spot noise voltage can be computed as the
square root of the sum of all squared output noise voltage
contributors. Equation 5 shows the general form for the
output noise voltage using the terms shown in Figure 10.
(5)
E E I R kTR NG I R kTR NG
O
NI BN
SS
BI F F
=+
( )
+




+
( )
+
2
2
2
2
44