OPA2684IDCNT Datasheet, PDF (15/34 Pages) Texas Instruments – Low-Power, Dual Current-Feedback OPERATIONAL AMPLIFIER

into this midpoint voltage bias. The input voltage can swing
to within 1.25V of either supply pin, giving a 2.5Vp-p input
signal range centered between the supply pins. The input
impedance of Figure 3 is set to give a 50Ω input match. If the
source does not require a 50Ω match, remove this and drive
directly into the blocking capacitor. The source will then see
the 5kΩ load of the biasing network. The gain resistor (R_G)
is AC-coupled, giving the circuit a DC gain of +1, which puts
the noninverting input DC bias voltage (2.5V) on the output
as well. The feedback resistor value has been adjusted from
the bipolar ±5V supply condition to re-optimize for a flat
frequency response in +5V only, gain of +2, operation. On a
single +5V supply, the output voltage can swing to within
1.0V of either supply pin while delivering more than 70mA
output current giving 3V output swing into 100Ω (8dBm
maximum at a matched 50Ω load). The circuit of Figure 3
shows a blocking capacitor driving into a 50Ω output resistor
then into a 50Ω load. Alternatively, the blocking capacitor
could be removed if the load is tied to a supply midpoint or
to ground if the DC current required by the load is acceptable.
of a current-feedback amplifier, wideband operation is re-
tained even under this condition.
The circuits of Figure 3 and 4 show single-supply operation
at +5V. These same circuits may be used up to single
supplies of +12V with minimal change in the performance of
the OPA2684.
+5V
10kΩ
0.1µF
+
6.8µF
0.1µF
1/2
10kΩ _OPA2684
0.1µF _50Ω
50Ω Load
50Ω Source
0.1µF
R_G
1kΩ
R_F
1kΩ
V_I
R_M
52.3Ω
+5V
50Ω Source _0.1µF
V_I
R_M
50Ω
10kΩ
0.1µF
+
6.8µF
1/2
10kΩ _OPA2684
0.1µF _50Ω
50Ω Load
R_F
1kΩ
R_G
1kΩ
0.1µF
FIGURE 4. AC-Coupled, G = –1V/V, Single-Supply Specifi-
cations and Test Circuit.
DIFFERENTIAL INTERFACE APPLICATIONS
Dual op amps are particularly suitable to differential input to
differential output applications. Typically, these fall into either
Analog-to-Digital Converter (ADC) input interface or line
driver applications. Two basic approaches to differential I/O
are noninverting or inverting configurations. Since the output
is differential, the signal polarity is somewhat meaningless—
the noninverting and inverting terminology applies here to
where the input is brought into the OPA2684. Each has its
advantages and disadvantages. Figure 5 shows a basic
starting point for noninverting differential I/O applications.
FIGURE 3. AC-Coupled, G = +2V/V, Single-Supply Specifi-
+V_CC
cations and Test Circuit.
Figure 4 shows the AC-coupled, single +5V supply, gain of
–1V/V circuit configuration used as a basis for the +5V
Typical Characteristics for each channel. In this case, the
midpoint DC bias on the noninverting input is also decoupled
with an additional 0.1µF decoupling capacitor. This reduces
the source impedance at higher frequencies for the
noninverting input bias current noise. This 2.5V bias on the
noninverting input pin appears on the inverting input pin and,
since R_G is DC blocked by the input capacitor, will also
appear at the output pin. One advantage to inverting opera-
tion is that since there is no signal swing across the input
stage, higher slew rates and operation to even lower supply
voltages is possible. To retain a 1Vp-p output capability,
operation down to a 3V supply is allowed. At a +3V supply,
the input stage is saturated, but for the inverting configuration
1/2
OPA2684
R_F
800Ω
V_I
R_G
R_F
800Ω
V_O
1/2
OPA2684
–V_CC
FIGURE 5. Noninverting Differential I/O Amplifier.
OPA2684
15
SBOS239D
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