OPA2681 Datasheet, PDF (18/21 Pages) Burr-Brown (TI) – Dual Wideband, Current Feedback OPERATIONAL AMPLIFIER With Disable

harmonic decreases less than the expected 6dB while the
difference between it and the 3rd decreases 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 Performance Curves 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.
For 2 tones centered at 20MHz, with 10dBm/tone into a
matched 50Ω load (i.e. 2Vp-p for each tone at the load,
which requires 8Vp-p for the overall 2-tone envelope at the
output pin), the Typical Performance Curves show 62dBc
difference between the test tone power and the 3rd order
intermodulation spurious levels. This exceptional perfor-
mance improves further when operating at lower frequen-
cies.
NOISE PERFORMANCE
Wideband current feedback op amps generally have a higher
output noise than comparable voltage feedback op amps.
The OPA2681 offers an excellent balance between voltage
and current noise terms to achieve low output noise. The
inverting current noise (15pA/√Hz) is significantly lower
than earlier solutions while the input voltage noise
(2.2nV/√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 non-inverting input current
noise (12pA/√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 10
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 4 shows the general form for the
output noise voltage using the terms shown in Figure 10.
( ) ^{( ) ( ) E}O ⁼
E
2
NI
+
I _BN R _S
2 _{+ 4kTRS}
NG² +
I _BI R _F
2 _{+ 4kTRFNG}
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 5.
( ) E_N =
E_NI2 +
I _BN R _S
2
+
4 kTR _S
+


I _BI R _F
NG


2
+
4 kTR _F
NG
Evaluating these two equations for the OPA2681 circuit and
component values shown in Figure 1 will give a total output
spot noise voltage of 8.4nV/√Hz and a total equivalent input
spot noise voltage of 4.2nV/√Hz. This total input referred
spot noise voltage is higher than the 2.2nV/√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 5 will ap-
proach just the 2.2nV/√Hz of the op amp itself. For example,
going to a gain of +10 using R_F = 180Ω will give a total
input referred noise of 2.4nV/√Hz .
E_NI
1/2
R_S
I_BN
OPA2681
E_O
E_RS
÷4kTR_S
4kT
R_G
R_F
÷4kTR_F
R_G
I_BI
4kT = 1.6E –20J
at 290°K
FIGURE 10. Op Amp Noise Analysis Model.
DC ACCURACY AND OFFSET CONTROL
A current feedback op amp like the OPA2681 provides
exceptional bandwidth in high gains, giving fast pulse set-
tling but only moderate DC accuracy. The Typical Specifi-
cations show an input offset voltage comparable to high
speed voltage feedback amplifiers. However, the two input
bias currents are somewhat higher and are unmatched.
Whereas bias current cancellation techniques are very effec-
tive with most voltage feedback op amps, they do not
generally reduce the output DC offset for wideband current
feedback op amps. Since the two input bias currents are
unrelated in both magnitude and polarity, matching the
source impedance looking out of each input to reduce their
error contribution to the output is ineffective. Evaluating the
configuration of Figure 1, using worst case +25°C input
offset voltage and the two input bias currents, gives a worst
case output offset range equal to:
± (NG x V_OS(MAX)) + (I_BN x R_S/2 x NG) ± (I_BI x R_F)
where NG = non-inverting signal gain
= ± (2 x 5.0mV) + (55uA x 25Ω x 2) ± (402Ω x 40µA)
= ±10mV + 2.75mV ± 16mV
= –23.25mV → +28.25mV
®
OPA2681
18