Datasheet

OPA695
26
SBOS293G
www.ti.com
FIGURE 17. Op Amp Noise Figure Analysis Model.
4kT
R
G
R
G
R
F
R
S
OPA695
I
BI
E
O
I
BN
4kT = 1.6E –20J
at 290°K
E
RS
E
NI
4kTR
S
√
4kTR
F
√
The OPA695 has extremely low 3rd-order harmonic distor-
tion. This also gives a high 2-tone, 3rd-order intermodulation
intercept, as shown in the Typical Characteristic 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 OPA695
in the inverting mode. The output matching resistor attenu-
ates the voltage swing from the output pin to the load by 6dB.
If the OPA695 drives directly into the input of a high imped-
ance device, such as an ADC, this 6dB attenuation 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, F
1
and F
2
, are specified in terms of average and delta
frequency, F
O
= (F
1
+ F
2
)/2 and ∆F = |F
2
– F
1
|/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 • (OP
3
– P
O
), where OP
3
is the intercept taken from
the Typical Characteristic 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 OPA695
has an intercept of 42dBm at a matched 50Ω load. If the full
envelope of the two frequencies needs to be 2V
PP
, 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 2V
PP
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, the
3rd-order spurious levels are much lower due to the lighter
loading presented by most ADCs.
NOISE PERFORMANCE
The OPA695 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 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 17
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 .
The total output spot-noise voltage can be computed as the
square root of the sum of all squared output noise voltage
contributors. Equation 12 shows the general form for the
output noise voltage using the terms shown in Figure 13.
(12)
E E I R kTR G I R kTR G
O
NI BN
SS
NBIF FN
=+
( )
+




+
( )
+
2
2
2
2
44
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 13:
(13)
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 OPA695 circuit and
component values shown in Figure 1 will give 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 voltage is higher than the 1.8nV/
√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 13 will just ap-
proach the 1.8nV/
√Hz of the op amp itself. For example,
going to a gain of +20 (using R
F
= 200Ω) will give a total input
referred noise of 2.0nV/
√Hz .
For a more complete discussion of op amp noise calculation,
see TI Application Note, SBOA066, Noise Analysis for High
Speed Op Amps
, available through the TI web site.
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp like the OPA695 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