Datasheet
OPA2683
17
SBOS244H
www.ti.com
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.
FIGURE 12. Frequency Response for 10MHz, 3rd-Order
Butterworth Low-Pass Filter.
1/2
OPA2683
1/2
OPA2683
C
L
0.1µF
800Ω
800Ω
+5V
R
S
R
S
200Ω
200Ω
500Ω (Optional)
ADC
10kΩ
10kΩ
50Ω
Source
15.3dB
Noise Figure
Gain = 8V/V
18.1dB
1:2
V
CM
V
CM
500Ω (Optional)
1/2
OPA2683
681Ω 953Ω
75Ω
78.7Ω
V
OUT
75Ω Line
CH0
+5V
+5V
1/2
OPA2683
681Ω 953Ω
75Ω
78.7Ω
V
OUT
75Ω Line
CH1
1/2
OPA2683
681Ω 953Ω
75Ω
78.7Ω
CH0
+5V
1/2
OPA2683
681Ω 953Ω
75Ω
78.7Ω
CH1
V
DIS
U1
U2
Power-Supply
Decoupling
Not Shown
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.
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.