OPA3875 Datasheet, PDF (15/22 Pages) Burr-Brown (TI) – Triple 2:1 High-Speed Video Multiplexer

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Evaluating the front-page schematic, using a
worst-case, +25°C offset voltage, bias current and
PSRR specifications and operating at ±6V, gives a
worst-case output equal to Equation 2:
±14mV
+
75W
x
±18mA
x
2
±
|5
-
6|
x
-
10
50
20
±
|-5
-
(-6)|
x
-
10
51
20
= ±22.7mV
DISTORTION PERFORMANCE
The OPA3875 provides good distortion performance
into a 100Ω load on ±5V supplies. Relative to
alternative solutions, it provides exceptional
performance into lighter loads. Generally, until the
fundamental signal reaches very high frequency or
power levels, the 2nd-harmonic dominates the
distortion with a negligible 3rd-harmonic component.
Focusing then on the 2nd-harmonic, increasing the
load impedance improves distortion directly. Also,
providing an additional supply decoupling capacitor
(0.01µF) between the supply pins (for bipolar
operation) improves the 2nd-order distortion slightly
(3dB to 6dB).
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 2nd-harmonic increases only by less than the
expected 6dB, whereas the 3rd-harmonic increases
by less than the expected 12dB. This also shows up
in the two-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 Characteristics 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 two tones centered at
20MHz, with 4dBm/tone into a matched 50Ω load
(that is, 1V_PP for each tone at the load, which
requires 4V_PP for the overall 2-tone envelope at the
output pin), the Typical Characteristics show a 82dBc
difference between the test-tone power and the
3rd-order intermodulation spurious levels.
OPA3875
SBOS341B – DECEMBER 2006 – REVISED DECEMBER 2006
NOISE PERFORMANCE
The OPA3875 offers an excellent balance between
voltage and current noise terms to achieve low
output noise. As long as the AC source impedance
looking out of the noninverting node is less than
100Ω, this current noise will not contribute
significantly to the total output noise. Figure 30
shows this device 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.
+5V
R_S
e_RS
4kTR_S
e_n
i_b
1/3 OPA3875
x1
e_o
x1
402W
402W
-5V
Channel
EN
Select
Figure 30. Noise 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 3 shows the
general form for the output noise voltage using the
terms shown in Figure 30.
e_o = 2 _en² _{+ (ibRS)}² _{+ 4kTRS}
Dividing this expression by the device gain (2V/V)
gives the equivalent input-referred spot noise voltage
at the noninverting input as shown in Equation 4.
e_n = _en² _{+ (ibRS)}² _{+ 4kTRS}
Evaluating these two equations for the OPA3875
circuit and component values shown in Figure 26
gives a total output spot noise voltage of 13.6nV/√Hz
and a total equivalent input spot noise voltage of
6.8nV/√Hz. This total input-referred spot noise
voltage is higher than the 6.7nV/√Hz specification for
the mux voltage noise alone. This number reflects
the noise added to the output by the bias current
noise times the source resistor.
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