OPA2695 Datasheet, PDF (29/40 Pages) Texas Instruments – Dual, Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER with Disable

OPA2695
www.ti.com..................................................................................................................................................................................................... SBOS354 – APRIL 2008
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 difference
between it and the 2nd harmonic decreases less than
the expected 6dB, while the difference between it and
the 3rd decreases by less than the expected 12dB.
The OPA2695 has extremely low third-order
harmonic distortion. This also gives a high two-tone,
third-order intermodulation intercept, as shown in the
Typical Characteristics. This intercept curve is
defined at the 50Ω load when driven through a 50Ω
matching resistor to allow direct comparisons to R_F
MMIC devices and is shown for both gains of ±8V/V.
There is a slight improvement in third-order intercept
by operating the OPA2695 in the inverting mode. The
output matching resistor attenuates the voltage swing
from the output pin to the load by 6dB. If the
OPA2695 drives directly into the input of a high
impedance device, such as an ADC, this 6dB
attenuation is not taken. Under these conditions, the
intercept increases by a minimum of 6dBm.
The third-order intercept is used to predict the
intermodulation products for two closely-spaced
frequencies. If the two test frequencies, F₁ and F₂,
are specified in terms of average and delta
frequency, F_O = (F₁ + F₂)/2 and ΔF = |F₂ – F₁|/2, the
two third-order, close-in spurious tones appear at
F_O ±3 × ΔF. The difference between two equal
test-tone power levels and these intermodulation
spurious power levels is given by ΔdBc = 2 × (OP₃ –
P_O), where OP₃ is the intercept taken from the
Typical Characteristic curves (see Figure 14,
Figure 44, Figure 56, and Figure 60) and P_O is the
power level in dBm at the 50Ω load for one of the two
closely-spaced test frequencies. For example, at
50MHz, gain of –8V/V, the OPA2695 has an intercept
of 32dBm at a matched 50Ω load. If the full envelope
of the two frequencies must be 2V_PP, each tone must
be 4dBm. The third-order intermodulation spurious
tones are then 2 × (32 – 4) = 56dBc below the
test-tone power level (–52dBm). If this same 2V_PP
two-tone envelope were delivered directly into the
input of an ADC without the matching loss or the
loading of the 50Ω network, the intercept would
increase to at least 38dBm. With the same signal and
gain conditions, but now driving directly into a light
load, the third-order spurious tones are then at least 2
× (38 – 4) = 68dBc below the 4dBm test-tone power
levels centered on 50MHz. Tests have shown that, in
reality, the third-order spurious levels are much lower
as a result of the lighter loading presented by most
ADCs.
NOISE PERFORMANCE
The OPA2695 offers an excellent balance between
voltage and current noise terms to achieve low output
noise. The inverting current noise (22pA/√Hz) is lower
than most other current-feedback op amps while the
input voltage noise (1.8nV/√Hz) is lower than any
unity-gain stable, wideband, voltage-feedback op
amp. This low-input voltage noise was achieved at
the price of a higher noninverting input current noise
(18pA/√Hz). As long as the ac source impedance
looking out of the noninverting node is less than 50Ω,
this current noise does 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 78 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
OPA2695
R_S
I_BN
E_O
E_RS
Ö4kTR_S
4kT
R_G
R_F
R_G
I_BI
Ö4kTR_F
4kT = 1.6E - 20J
at 290°K
Figure 78. 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 8 shows the
general form for the output noise voltage using the
terms shown in Figure 78.
Ö( ) EO = ENI2 + (IBNRS)2 + 4kTRS GN2 + (IBIRF)2 + 4kTRFGN
(8)
Dividing this expression by the noise gain (NG = (1 +
R_F/R_G)) gives the equivalent input-referred spot-noise
voltage at the noninverting input as shown in
Equation 9:
Ö ^{( ) EO =}
E_NI2 + (I_BNR_S)² + 4kTR_S +
I_BIR_F
NG
2
+
4kTR_F
NG
(9)
Evaluating these two equations for the OPA2695
circuit and component values shown in Figure 68
gives a total output spot-noise voltage of 18.7nV/√Hz
and a total equivalent input spot-noise voltage of
2.3nV/√Hz. This total input-referred spot-noise
Copyright © 2008, Texas Instruments Incorporated
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