Datasheet

OPA693
17
SBOS285A
www.ti.com
WIDEBAND, DC-COUPLED,
SINGLE-TO-DIFFERENTIAL CONVERSION
The frequency response shown in Figure 7 for the improved
gain of +1V/V buffer closely matches the inverting gain of
–1V/V frequency response. Combining two OPA693s to give
a +1 and –1 response will give a very broadband, DC-
coupled, single-ended input to differential output conversion.
Figure 9 shows this implementation where the input match is
now set by R
M
in parallel with the R
G
resistor of the inverting
stage. This circuit is essentially providing a DC to 700MHz
1:1 transformer operation. A 50Ω input match is shown, but
this may be increased by increasing R
M
. For instance,
targeting a 200Ω input impedance requires an R
M
= 600Ω to
get the parallel combination of R
M
and R
G
= 200Ω.
Figure 9. DC → 700MHz, Single-to-Differential Conversion.
OPA693
+5V
DIS
DIS
+V
I
R
F
300Ω
R
G
300Ω
R
M
60.4Ω
−5V
V
I
OPA693
+5V
−V
I
2V
I
R
F
300Ω
R
G
300Ω
−5V
HIGH-FREQUENCY ACTIVE FILTERS
The extremely wide bandwidth of the OPA693 allows a wide
range of active filter topologies to be implemented with
minimal amplifier bandwidth interaction in the filter shape.
Sallen-Key filters, using either a gain of 1 or gain of 2
amplifier, may be easily implemented with no external gain
setting elements. In general, given a desired filter W
O
, the
amplifier should have at least 20X that W
O
to minimize filter
interaction with the amplifier frequency response. Figure 10
Figure 10. Line Driver with 40 MHz Low-Pass Active Filter.
OPA693
+5V
−5V
50Ω
50Ω
22pF
226Ω100Ω
0Ω
Source
R
G
300Ω
R
F
300Ω
22pF
V
I
V
O
This type of filter depends on a low output impedance from
the amplifier through very high frequencies to continue to
provide an increasing attenuation with frequency. As the
amplifier output impedance rises with frequency, any input
signal or noise starts to feed directly through to the output via
the feedback capacitor. Since the OPA693 used in Figure 10
has a 700MHz bandwidth, the active filter will continue to roll
off through frequencies exceeding 200MHz. Figure 11 shows
the frequency response for the filter of Figure 10, where the
desired 40MHz cutoff is achieved and a 40dB/dec rolloff is
held through very high frequencies.
Figure 11. 40MHz Low-Pass Active Filter Response.
3
0
−3
−6
−9
−12
−15
−18
−21
−24
−27
−30
Gain (dB)
1 10 100 1000
Frequency (MHz)
shows an example gain of +2 line driver using the OPA693
that incorporates a 40MHz low-pass Butterworth response
with just a few external components. The filter resistor values
have been adjusted slightly here from an ideal filter analysis
to account for parasitic effects.