LM1812 datasheet(5/10 Pages) NSC

5 / 10 page

Application Hints (Continued)

Where additional power is desired a pulse amplifier or a

pulse stretcher can be used as shown in

Figure 7 The pulse

amplifier (

Figure 7a ) increases output current up to 5A The

pulse stretcher (

Figure 7b ) increases output current and

pulse width The wider pulse of

Figure 7b is especially useful

at lower frequencies where the relatively narrow 1 ms pulse

creates a large peak current demand for a given power lev-

el Pulse width as a function of R is plotted in

Figure 8

Pin 8 performs the function of switching the LM1812 into

either the transmit or receive mode When pin 8 is held high

the chip is in the transmit mode When held low it is in the

receive mode The input current at pin 8 should be designed

to operate withina1mA – 10mA range

RECEIVER

The receiver section (

Figure 9 ) contains two separate gain

stages

In some applications large voltages are applied across the

transducer during transmit Since the receiver input is cou-

TLH7892 – 7

FIGURE 7a Pulse Amplifier

pled to the transducer some protection is necessary to limit

the input current spikes to less than 50 mA Where the volt-

age across the transducer is less than 200 Vp-p a C4 reac-

tance of 5 kX at the operating frequency is adequate pro-

tection Above 200 Vp-p a5kX resistor should be inserted

in series with C4

Since the L1-C1 tank circuit is shared with the oscillator

both the transmitter and receiver are always tuned to the

same frequency The second stage voltage gain is given by

AV e

0L1C1

(4)

where Q e unloaded Q of L1-C1 tank

When the LM1812 is in the transmit mode the second gain

stage is turned OFF When switching back to the receive

mode the gain stage does not turn ON immediately but

instead turns ON after a slight delay as programmed by C9

This delay blanks the receiver (and therefore the detector)

momentarily giving the transducer time to stop ringing

TLH7892 – 8

FIGURE 7b Pulse Stretcher

TLH7892 – 9

FIGURE 8 Pulse Stretcher

Resistance vs Pulse Width

AV e 24 dB

TLH7892 – 10

FIGURE 9 Receiver Section

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