SINGLE-SUPPLY, HIGH GAIN DIFFERENTIAL
ADC DRIVER
+5V
VDIS
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.
Power-Supply
Decoupling
Not Shown
+5V
U1
CH0
78.7Ω
1/2
OPA2683
75Ω
VOUT
75Ω Line
681Ω
953Ω
CH1
+5V
78.7Ω
1/2
OPA2683
75Ω
VOUT
10kΩ
75Ω Line
VCM
681Ω
953Ω
1/2
OPA2683
0.1µF
10kΩ
ADC
500Ω (Optional)
+5V
200Ω
200Ω
800Ω
RS
1:2
50Ω
Source
U2
CL
800Ω
RS
CH0
78.7Ω
1/2
OPA2683
75Ω
15.3dB
Noise Figure
1/2
OPA2683
Gain = 8V/V
18.1dB
681Ω
953Ω
VCM
500Ω (Optional)
CH1
FIGURE 11. Single-Supply Differential ADC Driver.
78.7Ω
1/2
OPA2683
75Ω
This circuit also includes optional 500Ω pull-down resistors at
the output. With a 2.5V DC common-mode operating point
(set by VCM), 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.
681Ω
953Ω
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 0.7VPP. As
the signal across the buffers of the inactive channels ex-
ceeds 0.7VPP, 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.
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.
OPA2683
SBOS244H
17
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