AD8132ARZ-R7 datasheet(28/32 Pages) AD

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AD8132

Rev. F | Page 28 of 32

FULL-WAVE RECTIFIER

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.

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 con-

nected to ground by a 100 Ω resistor.

RG1

348Ω

RF1

348Ω

RF2

348Ω

RG2

348Ω

+5V

–5V

100Ω

RT2

24.9Ω

RT1

49.9Ω

VIN

HP2835

VOUT

+5V

CR1

10kΩ

Figure 82. Full-Wave Rectifier

Operate the diodes such that they are slightly forward-biased

when the differential output voltage is 0. For the Schottky diodes,

this is approximately 400 mV. The forward biasing is conveniently

adjusted by CR1, which, in this circuit, raises and lowers VOUT, 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.

If there is not enough forward bias (VOUT, 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.

There is not a reliable, entirely quantifiable means to measure

the performance of a full-wave rectifier. Since the ideal wave-

form has periodic sharp discontinuities, it has (mostly even)

harmonics that have no upper bound on the frequency. How-

ever, 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.

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.

100mV

2ns

Figure 83. Full-Wave Rectifier Response with 100 MHz Input

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