Datasheet

OPA2694
SBOS320D − SEPTEMBER 2004 − REVISED APRIL 2013
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18
a zero at a higher frequency. The additional zero acts to
cancel the phase lag from the capacitive load pole, thus
increasing the phase margin and improving stability.
The Typical Characteristics show the recommended R
S
vs
Capacitive Load and the resulting frequency response at
the load. Parasitic capacitive loads greater than 2pF can
begin to degrade the performance of the OPA2694. Long
PCB traces, unmatched cables, and connections to
multiple devices can easily cause this value to be
exceeded. Always consider this effect carefully, and add
the recommended series resistor as close as possible to
the OPA2694 output pin (see the Board Layout Guidelines
section).
DISTORTION PERFORMANCE
The OPA2694 provides good distortion performance into
a 100Ω load on ±5V supplies. Generally, until the
fundamental signal reaches very high frequency or power
levels, the 2nd-harmonic will dominate the distortion with
a negligible 3rd-harmonic component. Focusing then on
the 2nd-harmonic, increasing the load impedance
improves distortion directly. Remember that the total load
includes the feedback network—in the noninverting
configuration (see Figure 1), this is the 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
increases harmonic distortion directly. The Typical
Characteristics show the 2nd-harmonic increasing at a
little less than the expected 2x rate, while the 3rd-harmonic
increases at a little less than the expected 3x rate. Where
the test power doubles, the 2nd-harmonic increases by
less than the expected 6dB, while the 3rd-harmonic
increases by less than the expected 12dB. This also
shows up in the 2-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 Characteristics
show, the spurious intermodulation powers do not
increase as predicted by a traditional intercept model. As
the fundamental power level increases, the dynamic range
does not decrease significantly.
NOISE PERFORMANCE
Wideband, current-feedback op amps generally have a
higher output noise than comparable voltage-feedback op
amps. The OPA2694 offers an excellent balance between
voltage and current noise terms to achieve low output
noise. The inverting current noise (24pA/√Hz
) is
significantly lower than earlier solutions, while the input
voltage noise (2.1nV/√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 (22pA/√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 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
.
4kT
R
G
R
G
R
F
R
S
1/2
OPA2694
I
BI
E
O
I
BN
4kT = 1.6
×
10
−
20
J
at 290K
E
RS
E
NI
√
4kTR
F
√
4kTR
S
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 (5) shows the general form for the
output noise voltage using the terms shown in Figure 12.
E
O
+
ǒ
E
NI
2
)
ǒ
I
BN
R
S
Ǔ
2
) 4kTR
S
Ǔ
NG
2
)
ǒ
I
BI
R
F
Ǔ
2
) 4kTR
F
NG
Ǹ
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.
E
N
+ E
NI
2
)
ǒ
I
BN
R
S
Ǔ
2
) 4kTR
S
)
ǒ
I
BI
R
F
NG
Ǔ
2
)
4kTR
F
NG
Ǹ
Evaluating these two equations for the OPA2694 circuit
and component values (see Figure 1) gives a total output
spot noise voltage of 11.2nV/√Hz
and a total equivalent
input spot noise voltage of 5.6nV/√Hz
. This total
input-referred spot noise voltage is higher than the
2.1nV/√Hz
specification for the op amp voltage noise
(5)
(6)