Datasheet

NOISE PERFORMANCE
4kT
R
G
R
G
R
F
R
S
1/2
OPA2695
I
BI
E
O
I
BN
4kT=1.6E 20J-
at290 K°
E
RS
E
NI
4kTR
S
Ö
4kTR
F
Ö
E =
O
E +(I R ) +4kTR
NI BN S S
2 2
Ö
( )
G +(I R ) +4kTR G
N BI F F N
2
2
E =
O
E +(I R ) +4kTR
NI BN S S
+
2 2
I R
BI F
NG
4kTR
F
NG
+
Ö
)(
2
(9)
OPA2695
www.ti.com ...................................................................................................................................................... SBOS354A – APRIL 2008 – REVISED AUGUST 2008
In most op amps, increasing the output voltage swing
increases harmonic distortion directly. The Typical
The OPA2695 offers an excellent balance between
Characteristics show the 2nd harmonic increasing at
voltage and current noise terms to achieve low output
a little less than the expected 2x rate, while the 3rd
noise. The inverting current noise (22pA/ √ Hz) is lower
harmonic increases at a little less than the expected
than most other current-feedback op amps while the
3x rate. Where the test power doubles, the difference
input voltage noise (1.8nV/ √ Hz) is lower than any
between it and the 2nd harmonic decreases less than
unity-gain stable, wideband, voltage-feedback op
the expected 6dB, while the difference between it and
amp. This low-input voltage noise was achieved at
the 3rd decreases by less than the expected 12dB.
the price of a higher noninverting input current noise
The OPA2695 has extremely low third-order (18pA/ √ Hz). As long as the ac source impedance
harmonic distortion. This also gives a high two-tone, looking out of the noninverting node is less than 50 Ω ,
third-order intermodulation intercept, as shown in the this current noise does not contribute significantly to
Typical Characteristics . This intercept curve is the total output noise. The op amp input voltage noise
defined at the 50 Ω load when driven through a 50 Ω and the two input current noise terms combine to give
matching resistor to allow direct comparisons to R
F
low output noise under a wide variety of operating
MMIC devices and is shown for both gains of ± 8V/V. conditions. Figure 78 shows the op amp noise
There is a slight improvement in third-order intercept analysis model with all the noise terms included. In
by operating the OPA2695 in the inverting mode. The this model, all noise terms are taken to be noise
output matching resistor attenuates the voltage swing voltage or current density terms in either nV/ √ Hz or
from the output pin to the load by 6dB. If the pA/ √ Hz.
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, F
1
and F
2
,
are specified in terms of average and delta
frequency, F
O
= (F
1
+ F
2
)/2 and Δ F = |F
2
– F
1
|/2, the
two third-order, close-in spurious tones appear at
F
O
± 3 × Δ F. The difference between two equal
test-tone power levels and these intermodulation
spurious power levels is given by Δ dBc = 2 × (OP
3
–
P
O
), where OP
3
is the intercept taken from the
Typical Characteristic curves (see Figure 14 ,
Figure 78. Op Amp Noise Analysis Model
Figure 44 , Figure 56 , and Figure 60 ) and P
O
is the
power level in dBm at the 50 Ω load for one of the two
The total output spot-noise voltage can be computed
closely-spaced test frequencies. For example, at
as the square root of the sum of all squared output
50MHz, gain of – 8V/V, the OPA2695 has an intercept
noise voltage contributors. Equation 8 shows the
of 32dBm at a matched 50 Ω load. If the full envelope
general form for the output noise voltage using the
of the two frequencies must be 2V
PP
, each tone must
terms shown in Figure 78 .
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
(8)
two-tone envelope were delivered directly into the
input of an ADC without the matching loss or the
Dividing this expression by the noise gain (NG = (1 +
loading of the 50 Ω network, the intercept would
R
F
/R
G
)) gives the equivalent input-referred spot-noise
increase to at least 38dBm. With the same signal and
voltage at the noninverting input as shown in
gain conditions, but now driving directly into a light
Equation 9 :
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
Evaluating these two equations for the OPA2695
as a result of the lighter loading presented by most
circuit and component values shown in Figure 68
ADCs.
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
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