OPA4872-EP Datasheet, PDF (16/24 Pages) Texas Instruments – 4:1 HIGH-SPEED MULTIPLEXER

OPA4872-EP
SBOS444 – DECEMBER 2008........................................................................................................................................................................................... www.ti.com
DRIVING CAPACITIVE LOADS
One of the most demanding, yet very common load
conditions, is capacitive loading. Often, the capacitive
load is the input of an analog-to-digital converter
(ADC)—including additional external capacitance that
may be recommended to improve ADC linearity. A
high-speed device such as the OPA4872 can be very
susceptible to decreased stability and closed-loop
response peaking when a capacitive load is placed
directly on the output pin. When the device open-loop
output resistance is considered, this capacitive load
introduces an additional pole in the signal path that
can decrease the phase margin. Several external
solutions to this problem have been suggested. When
the primary considerations are frequency response
flatness, pulse response fidelity, and/or distortion, the
simplest and most effective solution is to isolate the
capacitive load from the feedback loop by inserting a
series isolation resistor between the amplifier output
and the capacitive load. This isolation resistor does
not eliminate the pole from the loop response, but
rather shifts it and adds a zero at a higher frequency.
The additional zero acts to cancel the phase lag from
the capacitive load pole, thus increasing the phase
margin and improving stability.
The Typical Characteristics show the recommended
R_S versus capacitive load and the resulting frequency
response at the load; see Figure 5. Parasitic
capacitive loads greater than 2 pF can begin to
degrade the performance of the OPA4872. Long PCB
traces, unmatched cables, and connections to
multiple devices can easily cause this value to be
exceeded. Always consider this effect carefully, and
add the recommended series resistor as close as
possible to the OPA4872 output pin (see the Board
Layout Guidelines section).
DC ACCURACY
The OPA4872 offers excellent dc signal accuracy.
Parameters that influence the output dc offset voltage
are:
• Output offset voltage
• Input bias current
• Gain error
• Power-supply rejection ratio
• Temperature
Leaving both temperature and gain error parameters
aside, the output offset voltage envelope can be
described as shown in Equation 7:
V_{OSO_envelope} = V_OS ´ G ± I_bi x R_F ± (R_S ´ I_b) ´ G
±
½5
-
(V_S+)½
´
10^-
PSRR+
20
±½-5
-
(V_S-)½
´
10^-
PSRR-
20
(7)
Where:
R_S: Input resistance seen by R0, R1, G0, G1, B0,
or B1.
I_b: Noninverting input bias current
I_bi: Inverting input bias current
G: Gain
V_S+: Positive supply voltage
V_S–: Negative supply voltage
PSRR+: Positive supply PSRR
PSRR–: Negative supply PSRR
V_OS: Input Offset Voltage
Evaluating the front-page schematic, using a
worst-case, +25°C offset voltage, bias current and
PSRR specifications and operating at ±6 V, gives a
worst-case output equal to Equation 8:
±10mV + 75W ´ ±14mA ´ 2
+523W
´
±18mA
±½5
-
6½
´
10^-
50
20
±½-5
-
(-6)½
´
10^-
51
20
= ±29.2mV
(8)
DISTORTION PERFORMANCE
The OPA4872 provides good distortion performance
into a 150-Ω load on ±5-V 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 directly improves distortion. Also,
providing an additional supply decoupling capacitor
(0.01 µF) between the supply pins (for bipolar
operation) improves the 2nd-order distortion slightly
(3 dB to 6 dB).
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 6
dB, whereas the 3rd harmonic increases by less than
the expected 12 dB.
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