SINGLE SUPPLY DIFFERENTIAL A/D
CONVERTER DRIVER
Figure 3 shows a gain of +10 Diff. In/Diff. Out single supply
ADC driver. Using a dual amplifier like the OPA2681 helps
reducing the necessary board space, as it also reduces the
amount of required supply bypassing components. From a
signal point of view, dual amplifiers provide excellent per-
formance matching, e.g., gain and phase matching. The
differential ADC driver circuit shown in Figure 3 takes
advantage of this fact. A transformer converts the single-
ended input signal into a low level differential signal which
is applied to the high impedance non-inverting inputs of
each of the two amplifiers in the OPA2681. Resistor R
G
between the inverting inputs controls the ac-gain of this
circuit according to equation G = 1 + 2R
F
/R
G
. With the
resistor values shown the AC-gain is set to 10. Adding a
capacitor (0.1µF) in series with R
G
blocks the dc-path giving
a DC gain of +1 for the common-mode voltage. This allows,
in a very simple way, to apply the required DC bias voltage
of +2.5V to the inputs of the amplifiers, which will also
appear at their outputs. Like the OPA2681 the A/D converter
ADS823 operates on a single +5V supply. Its internal
common-mode voltage is typically +2.5V which equals the
required bias voltage for the OPA2681. Connecting two
resistors between the top-reference (REFT = +3.5V) and
bottom reference (REFB = +1.5V) develop a +2.5V voltage
level at their midpoint. Applying that to the center tap of the
transformer biases amplifiers appropriately. Sufficient by-
passing at the center tap must be provided to keep this point
at a solid AC ground. Resistors R
S
isolate the op amp output
from the capacitive input of the converter, as well as forming
a first order low-pass filter with capacitor C
I
to attenuate
some of the wideband noise. This interface will provide
> 150MHz full scale input bandwidth to the ADS823.
WIDEBAND VIDEO MULTIPLEXING
One common application for video speed amplifiers which
include a disable pin is to wire multiple amplifier outputs
together, then select which one of several possible video
inputs to source onto a single line. This simple “Wired-OR
Video Multiplexer” can be easily implemented using the
OPA2681N as shown in Figure 4.
Typically, channel switching is performed either on sync or
retrace time in the video signal. The two inputs are approxi-
mately equal at this time. The “make-before-break” disable
characteristic of the OPA2681 ensures that there is always
one amplifier controlling the line when using a wired-OR
circuit like that shown in Figure 4. Since both inputs may be
on for a short period during the transition between channels,
the outputs are combined through the output impedance
matching resistors (82.5Ω in this case). When one channel is
disabled, its feedback network forms part of the output
impedance and slightly attenuates the signal in getting out
onto the cable. The gain and output matching resistor have
been slightly increased to get a signal gain of +1 at the
matched load and provide a 75Ω output impedance to the
cable. The video multiplexer connection (Figure 4) also
insures that the maximum differential voltage across the
inputs of the unselected channel do not exceed the rated
±1.2V
maximum for standard video signal levels.
The section on Disable Operation shows the turn-on and
turn-off switching glitches using a grounded input for a
single channel is typically less than
±50mV.
Where two
outputs are switched (as shown in Figure 4), the output line
is always under the control of one amplifier or the other due
to the “make-before-break” disable timing. In this case, the
switching glitches for two 0V inputs drop to < 20mV.
+5V
+5V
R
S
24.9Ω
IN
C
1
10pF
+V
S
1:1
V
I
1/2
OPA2681
R
F
200Ω
50Ω
0.1µF
+
4.7µF
R
G
44Ω
0.1µF
R
F
200Ω
ADS823
10-Bit
60MSPS
R
S
24.9Ω
C
1
10pF
IN
REFT
(+3.5V)
2kΩ
REFB
(+1.5V)
1/2
OPA2681
GND
2 R
F
G=1+
= 10
R
G
V
BIAS
= +2.5V
2kΩ
0.1µF
0.1µF
FIGURE 3. Wideband, Single Supply, Differential ADC Driver.
®
OPA2681
12
BURR-BROWN [ BURR-BROWN CORPORATION ]
