Datasheet

AD8132
Rev. I | Page 29 of 32
FULL-WAVE RECTIFIER
If there is not enough forward bias (V
OUT, cm
too low), the lower
sharp cusps of the full-wave rectified output waveform are rounded
off. In addition, as the frequency increases, there tends to be some
rounding of the lower cusps. The forward bias can be increased
to yield sharper cusps at higher frequencies.
The balanced outputs of the AD8132, along with a couple of
Schottky diodes, can create a very high speed, full-wave rectifier.
Such circuits are useful for measuring ac voltages and other
computational tasks.
There is not a reliable, entirely quantifiable, means to measure
the performance of a full-wave rectifier. Because the ideal
waveform has periodic sharp discontinuities, it has (mostly
even) harmonics that have no upper bound on the frequency.
However, for a practical circuit, as the frequency increases, the
higher harmonics become attenuated and the sharp cusps that
are present at low frequencies become significantly rounded.
Figure 82 shows the configuration of such a circuit. Each of the
AD8132 outputs drives the anode of an HP2835 Schottky diode.
These Schottky diodes were chosen for their high speed operation.
At lower frequencies (approximately lower than 10 MHz), a silicon
signal diode, such as a 1N4148, can be used. The cathodes of the
two diodes are connected together, and this output node is
connected to ground by a 100 Ω resistor.
R
G1
348Ω
R
F1
348Ω
R
F2
348Ω
R
G2
348Ω
+5
V
–5V
R
L
100Ω
R
T2
24.9Ω
R
T1
49.9Ω
V
IN
HP2835
V
OUT
+5V
CR1
10kΩ
01035-080
When running the circuit at a frequency up to 300 MHz, though it
stays functional, the major harmonic that remains in the output
is the second. This looks like a sine wave at 600 MHz. Figure 83 is
an oscilloscope plot of the output when driven by a 100 MHz,
2.5 V p-p input.
Sometimes a second harmonic generator is useful for creating a
clock to oversample a DAC by a factor of two. If the output of
this circuit is run through a low-pass filter, it can be used as a
second harmonic generator.
Figure 82. Full-Wave Rectifier
100mV 2ns
1V
01035-081
Operate the diodes such that they are slightly forward-biased
when the differential output voltage is zero. For the Schottky
diodes, this is approximately 400 mV. The forward biasing is
conveniently adjusted by CR1, which, in this circuit, raises and
lowers V
OUT, cm
without creating a differential output voltage.
One advantage of this circuit is that the feedback loop is never
momentarily opened while the diodes reverse their polarity within
the loop. This scheme is sometimes used for full-wave rectifiers
that use conventional op amps. These conventional circuits do
not work well at frequencies above approximately 1 MHz.
Figure 83. Full-Wave Rectifier Response with 100 MHz Input
AUTOMOTIVE PRODUCTS
The AD8132W is qualified per the AEC-Q100 for use in
automotive applications. Custom variants of this product may
be available to meet stringent automotive performance and
quality requirements.