AD640 DATASHEET PDF

Low power operation typically mW at 65 V Low cost plastic packages also available Product Details The AD is a complete monolithic logarithmic amplifier. Two ADs in cascade can provide up to 95 dB of dynamic range at reduced bandwidth. The AD uses a successive detection scheme to provide an output current proportional to the logarithm of the input voltage. Each stage has an associated full-wave detector, whose output current depends on the absolute value of its input voltage. On chip resistors can be used to convert this output current to a voltage with several convenient slope options. Scaling is also guaranteed for sinusoidal inputs.

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Figure This is what is actually achieved by interposing the on-chip attenuator, which has the necessary temperature dependence to cause the input to the first stage to vary in proportion to abso- lute temperature. The end limits of the dynamic range are now totally independent of temperature. Consequently, this is the preferred method of intercept stabilization for applications where the input signal is sufficiently large. Unless corrected, the whole output function would drift up or down by this amount with changes in temperature.

This effectively maintains a constant intercept VXO. This correction is active in the default state Pin 8 open circuited. When using the attenuator, Pin 8 should be grounded, which disables the compensation current. The drift term needs to be compensated only once; when the outputs of two ADs are summed, Pin 8 should be grounded on at least one of the two devices both if the attenuator is used.

Conversion Range Practical logarithmic converters have an upper and lower limit on the input, beyond which errors increase rapidly. The upper limit occurs when the first stage in the chain is driven into limit- ing. Above this, no further increase in the output can occur and the transfer function flattens off.

The lower limit arises because a finite number of stages provide finite gain, and therefore at low signal levels the system becomes a simple linear amplifier.

Note that this lower limit is not determined by the intercept voltage, VX; it can occur either above or below VX, depending on the design. When using two ADs in cascade, input offset voltage and wideband noise are the major limitations to low level accuracy. Offset can be eliminated in various ways. Noise can only be reduced by lowering the system bandwidth, using a filter between the two devices. Thus, the logarithmic output in response to an amplitude-symmetric square wave is a steady value.

For a sinusoidal input the fluctuating output current will usually be low-pass filtered to extract the baseband signal. The unfiltered output is at twice the carrier frequency, simplifying the design of this filter when the video bandwidth must be maxi- mized. The averaged output depends on waveform in a roughly analogous way to waveform dependence of rms value.

The effect is to change the apparent intercept voltage. The intercept volt- age appears to be doubled for a sinusoidal input, that is, the averaged output in response to a sine wave of amplitude not rms value of 20 mV would be the same as for a dc or square wave input of 10 mV. Other waveforms will result in different inter- cept factors. An amplitude-symmetric-rectangular waveform has the same intercept as a dc input, while the average of a baseband unipolar pulse can be determined by multiplying the response to a dc input of the same amplitude by the duty cycle.

It is important to understand that in responding to pulsed RF signals it is the waveform of the carrier usually sinusoidal not the modulation envelope, that determines the effective intercept voltage.

Table I shows the effective intercept and resulting deci- bel offset for commonly occurring waveforms. The input wave- form does not affect the slope of the transfer function. Figure 22 shows the absolute deviation from the ideal response of cascaded ADs for three common waveforms at input levels from —80 dBV to —10 dBV. The measured sine wave and triwave responses are 6 dB and 8.

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