OPA2691ID Datasheet, PDF (12/30 Pages) Texas Instruments – Dual Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable

SINGLE-SUPPLY DIFFERENTIAL ADC DRIVER
Figure 3 shows a gain of +10 Single-Ended In/Diff. Out single-
supply ADC driver. Using a dual amplifier like the OPA2691
helps reduce 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 perfor-
mance matching (for example, 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 noninverting inputs of each of the two amplifiers in
the OPA2691. 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 gives
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 OPA2691, the ADC ADS823 operates on
a single +5V supply. Its internal common-mode voltage is
typically +2.5V which equals the required bias voltage for the
OPA2691.
Connecting two resistors between the top reference
(REFT = +3.5V) and bottom reference (REFB = +1.5V)
develops a +2.5V voltage level at their midpoint. Applying
that to the center tap of the transformer biases the ampli-
fiers appropriately. Sufficient bypassing 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 1st-order, low-
pass filter with capacitor C₁ 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
OPA2691I-14D, see 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 OPA2691 ensures that there is always
one amplifier controlling the line when using a wired-OR
circuit like that presented 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
resistors 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 (see 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 (see 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.
1:1
V_IN
50Ω
+
0.1µF
4.7µF
+5V
1/2
OPA2691
R_F
200Ω
R_G
44Ω
0.1µF
R_F
200Ω
G = 1 + ^{2 RF} = 10
R_G
1/2
OPA2691
V_BIAS = +2.5V
+5V
R_S
+V_S
24.9Ω
IN
C₁
10pF
R_S
24.9Ω
C₁
10pF
2kΩ
2kΩ
ADS823
10-Bit
60MSPS
IN
REFT
(+3.5V)
REFB
(+1.5V)
GND
0.1µF
0.1µF
FIGURE 3. Wideband, Single-Supply, Differential ADC Driver.
12
OPA2691
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