OPA2684IDCNT Datasheet, PDF (23/34 Pages) Texas Instruments – Low-Power, Dual Current-Feedback OPERATIONAL AMPLIFIER

however, are somewhat higher and are unmatched. Whereas
bias current cancellation techniques are very effective with
most voltage-feedback op amps, they do not generally re-
duce 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 • V_OS) + (I_BN • R_S / 2 • NG) ± (I_BI • R_F)
where NG = noninverting signal gain
= ±(2 • 3.8mV) ± (11µA • 25Ω • 2) ± (800Ω • 17mA)
= ±7.6mV + 0.55mV ± 13.6mV
= ±21.75mV
While the last term, the inverting bias current error, is
dominant in this low-gain circuit, the input offset voltage will
become the dominant DC error term as the gain exceeds
5V/V. Where improved DC precision is required in a high-
speed amplifier, consider the OPA656 single and OPA2822
dual voltage-feedback amplifiers.
THERMAL ANALYSIS
The OPA2684 will not require external heatsinking for most
applications. Maximum desired junction temperature will set
the maximum allowed internal power dissipation as de-
scribed below. In no case should the maximum junction
temperature be allowed to exceed 175°C.
Operating junction temperature (T_J) is given by T_A + P_D • θ_JA.
The total internal power dissipation (P_D) is the sum of
quiescent power (P_DQ) and additional power dissipated in the
output stage (P_DL) to deliver load power. Quiescent power is
simply the specified no-load supply current times the total
supply voltage across the part. P_DL will depend on the
required output signal and load but would, for a grounded
resistive load, be at a maximum when the output is fixed at
a voltage equal to 1/2 of either supply voltage (for equal
bipolar supplies). Under this condition P_DL = V_S2/(4 • R_L)
where R_L includes feedback network loading.
Note that it is the power in the output stage and not into the
load that determines internal power dissipation.
As an absolute worst-case example, compute the maximum
T_J using an OPA2684IDCN (SOT23-8 package) in the circuit
of Figure 1 operating at the maximum specified ambient
temperature of +85°C with both outputs driving a grounded
100Ω load to 2.5V_DC.
P_D = 10V • 3.9mA + 2 • (5² /(4 • (100Ω || 1.6kΩ))) = 172mW
Maximum T_J = +85°C + (0.172W • 150°C/W) = 111°C
This maximum operating junction temperature is well below
most system level targets. Most applications will be lower
than this since an absolute worst-case output stage power in
both channels simultaneously was assumed in this calculation.
BOARD LAYOUT GUIDELINES
Achieving optimum performance with a high-frequency am-
plifier like the OPA2684 requires careful attention to board
layout parasitics and external component types. Recommen-
dations that will optimize performance include:
a) Minimize parasitic capacitance to any AC ground for
all of the signal I/O pins. Parasitic capacitance on the
output and inverting input pins can cause instability; on
the noninverting input, it can react with the source
impedance to cause unintentional bandlimiting. To re-
duce unwanted capacitance, a window around the sig-
nal I/O pins should be opened in all of the ground and
power planes around those pins. Otherwise, ground and
power planes should be unbroken elsewhere on the
board.
b) Minimize the distance (< 0.25") from the power-supply
pins to high-frequency 0.1µF decoupling capacitors. At
the device pins, the ground and power-plane layout
should not be in close proximity to the signal I/O pins.
Avoid narrow power and ground traces to minimize
inductance between the pins and the decoupling capaci-
tors. The power-supply connections should always be
decoupled with these capacitors. An optional supply de-
coupling capacitor (0.01µF) across the two power sup-
plies (for bipolar operation) will improve 2nd-harmonic
distortion performance. Larger (2.2µF to 6.8µF)
decoupling capacitors, effective at lower frequency,
should also be used on the main supply pins. These may
be placed somewhat farther from the device and may be
shared among several devices in the same area of the
PCB.
c) Careful selection and placement of external compo-
nents will preserve the high -frequency performance
of the OPA2684. Resistors should be a very low reac-
tance type. Surface-mount resistors work best and allow
a tighter overall layout. Metal film and carbon composi-
tion axially-leaded resistors can also provide good high-
frequency performance. Again, keep their leads and
PCB trace length as short as possible. Never use
wirewound type resistors in a high-frequency applica-
tion. Since the output pin and inverting input pin are the
most sensitive to parasitic capacitance, always position
the feedback and series output resistor, if any, as close
as possible to the output pin. Other network compo-
nents, such as noninverting input termination resistors,
should also be placed close to the package. The fre-
quency response is primarily determined by the feed-
back resistor value as described previously. Increasing
its value will reduce the peaking at higher gains, while
decreasing it will give a more peaked frequency re-
sponse at lower gains. The 800Ω feedback resistor used
in the Electrical Characteristics at a gain of +2 on ±5V
supplies is a good starting point for design. Note that an
800Ω feedback resistor, rather than a direct short, is
required for the unity-gain follower application. A cur-
rent-feedback op amp requires a feedback resistor even
in the unity-gain follower configuration to control stability.
OPA2684
23
SBOS239D
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