OPA2683 Datasheet, PDF (17/33 Pages) Texas Instruments – Very Low-Power, Dual, Current-Feedback Operational Amplifier

SINGLE-SUPPLY, HIGH GAIN DIFFERENTIAL
ADC DRIVER
Where a very low-power differential I/O interface to a moder-
ate performance ADC is required, the circuit of Figure 11 may
be considered. The circuit builds on the inverting differential
I/O configuration of Figure 6 by adding the input transformer
and the output low-pass filter. The input transformer provides
a single-to-differential conversion where the input signal is
still very low power—it also provides a gain of 2 and removes
any common-mode signal from the inputs. This single +5V
design sets a midpoint bias from the supply at each of the
noninverting inputs.
+5V
10kΩ
V_CM
0.1µF
10kΩ
1/2
OPA2683
50Ω
Source
200Ω
1:2
200Ω
800Ω
800Ω
15.3dB
Noise Figure
Gain = 8V/V
18.1dB
1/2
OPA2683
V_CM
ADC
500Ω (Optional)
R_S
C_L
R_S
500Ω (Optional)
CH0
+5V
V_DIS
+5V
U1
1/2
75Ω
OPA2683
681Ω
953Ω
Power-Supply
Decoupling
Not Shown
78.7Ω
V_OUT
75Ω Line
CH1
75Ω
1/2
OPA2683
681Ω
953Ω
78.7Ω
V_OUT
75Ω Line
CH0
75Ω
+5V
U2
1/2
OPA2683
681Ω
953Ω
78.7Ω
FIGURE 11. Single-Supply Differential ADC Driver.
This circuit also includes optional 500Ω pull-down resistors at
the output. With a 2.5V DC common-mode operating point
(set by V_CM), this will add 5mA to ground in the output stage.
This essentially powers up the NPN side of the output stage
significantly reducing distortion. It is important for good 2nd-
order distortion to connect the grounds of these two resistors
at the same point to minimize ground plane current for the
differential output signal.
LOW-POWER MUX/LINE DRIVER
Using the shutdown feature, two OPA2683s can provide an
easy low-power way to select one of two possible sources for
moderate-resolution monitors. Figure 12 shows a recom-
mended circuit where each of the outputs are combined in a
way that provides a net gain of 1 to the matched 75Ω load
with a 75Ω output impedance. This brings the two outputs for
each color together through a 78.7Ω resistor with a slightly
> 2 gain provided by the amplifiers. When one channel is
shutdown, the feedback network is still present, slightly
attenuating the signal and combining in parallel with the
78.7Ω to give a 75Ω source impedance.
CH1
75Ω
1/2
OPA2683
681Ω
953Ω
78.7Ω
FIGURE 12. Frequency Response for 10MHz, 3rd-Order
Butterworth Low-Pass Filter.
Since the OPA2683 does not disable quickly, this approach
is not suitable for pixel-by-pixel multiplexing—however, it
does provide an easy way to switch between two possible
RGB sources. The output swing provided by the active
channel will divide back through the inactive channel feed-
back to appear at the inverting input of the OFF channel. To
retain good pulse fidelity, or low distortion, this divided down
output signal at the inverting inputs of the OFF channels, plus
the OFF channel input signals, should not exceed 0.7V_PP. As
the signal across the buffers of the inactive channels ex-
ceeds 0.7V_PP, diodes across the inputs may begin to turn on
causing a nonlinear load to the active channel. This will
degrade signal linearity under those conditions.
OPA2683
17
SBOS244H
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