OPA685 Datasheet, PDF (25/28 Pages) Burr-Brown (TI) – Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable TM

In most op amps, increasing the output voltage swing in-
creases harmonic distortion directly. The Typical Perfor-
mance Curves 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 OPA685 has extremely low 3rd-order harmonic distor-
tion. This also gives a high 2-tone, 3rd-order intermodulation
intercept, as shown in the Typical Performance Curves. This
intercept curve is defined at the 50Ω load when driven
through a 50Ω matching resistor to allow direct comparisons
to RF MMIC devices and is shown for both gains of ±8.
There is a slight improvement in intercept by operating the
OPA685 in the inverting mode. The output matching resistor
attenuates the voltage swing from the output pin to the load
by 6dB. If the OPA685 drives directly into the input of a
high impedance device, such as an ADC, this 6dB attenua-
tion is not taken. Under these conditions, the intercept will
increase by a minimum 6dBm.
The intercept is used to predict the intermodulation products
for two closely-spaced frequencies. If the two test frequen-
cies, f₁ and f₂, are specified in terms of average and delta
frequency, f_O = (f₁ + f₂)/2 and ∆f = | f₂ – f₁| /2, the two 3rd-
order, close-in spurious tones will appear at f_O ±3 • ∆f. The
difference between two equal test-tone power levels and
these intermodulation spurious power levels is given by
∆dBc = 2 • (IM3 – P_O), where IM3 is the intercept taken from
the Typical Performance Curve 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 –8, the OPA685
has an intercept of 42dBm at a matched 50Ω load. If the full
envelope of the two frequencies needs to be 2Vp-p, this
requires each tone to be 4dBm. The 3rd-order intermodulation
spurious tones will then be 2 • (42 – 4) = 76dBc below the
test-tone power level (–72dBm). If this same 2Vp-p 2-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 48dBm. With the same
signal and gain conditions, but now driving directly into a
light load, the 3rd-order spurious tones will then be at least
2 • (48 – 4) = 88dBc below the 4dBm test-tone power levels
centered on 50MHz. Tests have shown that, in reality, they
are much lower due to the lighter loading presented by most
ADCs.
NOISE PERFORMANCE
The OPA685 offers an excellent balance between voltage
and current noise terms to achieve low output noise. The
inverting current noise (19pA/√Hz) is lower than most other
current-feedback op amps while the input voltage noise
(1.7nV/√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 non-inverting input current
noise (13pA/√Hz). As long as the AC source impedance
looking out of the non-inverting 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 16
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
OPA685
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 16. Op Amp Noise Figure 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 9 shows the general form for the
output noise voltage using the terms shown in Figure 16.
(9)
( ) ^{( ) ( ) E}O ^{= E}NI2 ^{+ I}BN^RS ^{2 + 4kTR}S ^GN2 ^{+ I}BI^RF ^{2 + 4kTR}F^GN
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 10:
(10)
( ) E_N =
E_NI2 +
I _BN R _S
2
+
4kTR_S
+


I _BI R _F
NG


2
+
4 kTR _F
NG
Evaluating these two equations for the OPA685 circuit and
component values shown in Figure 1 will give a total output
spot-noise voltage of 18nV/√Hz and a total equivalent input
spot-noise voltage of 2.3nV/√Hz. This total input referred
spot-noise voltage is higher than the 1.7nV/√Hz specifica-
tion 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 10 will just
approach the 1.7nV/√Hz of the op amp itself. For example,
going to a gain of +20 (using R_F = 380Ω) will give a total
input referred noise of 2.0nV/√Hz.
®
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OPA685