AD8037ACHIPS datasheet(21/24 Pages) AD

21 / 24 page

REV. B

–20–

AD8036/AD8037

Figure 14. Full-Wave Rectifier Scope

Thus for either positive or negative input signals, the output is

unity times the absolute value of the input signal. The circuit

can be easily configured to produce the negative absolute value

of the input by applying the input to VH instead of VL.

The circuit can get to within about 40 mV of ground during the

time when the input crosses zero. This voltage is fixed over a

wide frequency range and is a result of the switching between

the conventional op amp input and the clamp input. But because

there are no diodes to rapidly switch from forward to reverse bias,

the performance far exceeds that of diode based full wave rectifiers.

The 40 mV offset mentioned can be removed by adding an off-

set to the circuit. A 27.4 k

Ω input resistor to the inverting input

will have a gain of 0.01, while changing the gain of the circuit

by only 1%. A plus or minus 4 V dc level (depending on the

polarity of the rectifier) into this resistor will compensate for

the offset.

Full wave rectifiers are useful in many applications including

AM signal detection, high frequency ac voltmeters and various

arithmetic operations.

Amplitude Modulator

In addition to being able to be configured as an amplitude

demodulator (AM detector), the AD8037 can also be config-

ured as an amplitude modulator as shown in Figure 15.

CARRIER IN

AM OUT

MODULATION IN

+5V

274

–5V

100

0.1 F10 F

AD8037

0.1 F

10 F

274

Figure 15. Amplitude Modulator

The positive input of the AD8037 is driven with a square wave

of sufficient amplitude to produce clamping action at both the

high and low levels. This is the higher frequency carrier signal.

The modulation signal is applied to both the input of a unity

gain inverting amplifier and to VL, the lower clamping input.

VH is biased at 0.5 V dc.

To understand the circuit operation, it is helpful to first con-

sider a simpler circuit. If both VL and VH were dc biased at

–0.5 V and the carrier and modulation inputs driven as above,

the output would be a 2 V p-p square wave at the carrier fre-

quency riding on a waveform at the modulating frequency. The

inverting input (modulation signal) is creating a varying offset to

the 2 V p-p square wave at the output. Both the high and low

levels clamp at twice the input levels on the clamps because the

noise gain of the circuit is two.

When VL is driven by the modulation signal instead of being held

at a dc level, a more complicated situation results. The resulting

waveform is composed of an upper envelope and a lower enve-

lope with the carrier square wave in between. The upper and

lower envelope waveforms are 180

° out of phase as in a typical

AM waveform.

The upper envelope is produced by the upper clamp level being

offset by the waveform applied to the inverting input. This offset

is the opposite polarity of the input waveform because of the

inverting configuration.

The lower envelope is produced by the sum of two effects. First,

it is offset by the waveform applied to the inverting input as in

the case of the simplified circuit above. The polarity of this off-

set is in the same direction as the upper envelope. Second, the

output is driven in the opposite direction of the offset at twice

the offset voltage by the modulation signal being applied to VL.

This results from the noise gain being equal to two, and since

there is no inversion in this connection, it is opposite polarity

from the offset.

The result at the output for the lower envelope is the sum of

these two effects, which produces the lower envelope of an

amplitude modulated waveform. See Figure 16.

Figure 16. AM Waveform

The depth of modulation can be modified in this circuit by

changing the amplitude of the modulation signal. This changes

the amplitude of the upper and lower envelope waveforms.

The modulation depth can also be changed by changing the dc

bias applied to VH. In this case the amplitudes of the upper and

lower envelope waveforms stay constant, but the spacing between

them changes. This alters the ratio of the envelope amplitude to

the amplitude of the overall waveform.

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