Datasheet

OPA3684
15
SBOS241C
www.ti.com
WIDEBAND PGA FOR ADC DRIVING
Using the 3 channels of the OPA3684, and the shutdown
feature, can give an easy to use PGA function—which can
be applied to driving an ADC. Since the bandwidth does not
vary with gain for the CFB
PLUS
OPA3684, each channel can
be set up to a desired gain setting, with each of the
noninverting inputs driven with the same input signal. Select-
ing one of the 3 channels passes on the input with the gain
setting provided by the selected channel. Figure 8 shows an
example where the channels are set to gains of 2, 5, and 10.
Again, the output signal will be divided down back to the
inverting inputs of the inactive channels. 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 0.7Vp-p. As the
signal across the buffers of the inactive channels exceeds
412Ω
243Ω
82.5Ω
806Ω
100pF
56pF220pF
+5V
402Ω
237Ω
97.6Ω
806Ω
1/3
OPA3684
100pF
56pF220pF
+5V
75.5Ω
1/3
OPA3684
806Ω
100µF
953Ω
+5V
1/3
OPA3684
953Ω
806Ω
806Ω
120pF
100µF
V
O
Video
In
+5V
0.7Vp-p, diodes across the inputs begin to turn on causing a
nonlinear load to the active channel. This will degrade signal
fidelity under those conditions.
VIDEO DAC RECONSTRUCTION FILTER
Wideband current-feedback op amps make ideal elements
for implementing high-speed active filters where the amplifier
is used as a fixed gain block inside a passive RC circuit
network. The triple channel OPA3684 can be used as a very
effective video Digital-to-Analog Converter (DAC) recon-
struction filter and line driver. Figure 9 shows an example of
this where the delay-equalized filter compensates for the
DAC’s sin(x)/x response, and minimizes aliasing artifacts. It
is shown here as a single +5V design expecting a 13.5MSPS
DAC sampling rate, and giving a 5.5MHz cutoff frequency.
FIGURE 8. Wideband PGA for ADC Driving.
1/3
OPA3684
20Ω
806Ω 806Ω
G = +2
1/3
OPA3684
200Ω
200Ω 806Ω
G = +5
1/3
OPA3684
20Ω
90.9Ω 806Ω
G = +10
74HC238
–5V
+5V
Power-supply
decoupling not shown.
V
IN
+5V
D
1
D
2
Y
0
Y
1
Y
2
50Ω
100Ω
100Ω
100Ω
ADS826
10-Bit
60MSPS
0.1µF
0.1µF
4.99kΩ
0.1µF
4.99kΩ
0.1µF
100pF
REFT
+3.5V
REFB
+1.5V
+In
–In
CM
FIGURE 9. Composite Video Filter.