LMV821 datasheet(14/26 Pages) TI1

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Application Note (Continued)

3. Dual Amplifier Active Filters (DAAFs)

•

a. Low-Pass Filter (LPF)

•

b. High-Pass Filter (HPF)

4. Tri-level Voltage Detector

TELEPHONE-LINE TRANSCEIVER

The telephone-line transceiver of Figure 7 provides a full-

duplexed connection through a PCMCIA, miniature trans-

former. The differential configuration of receiver portion

(UR), cancels reception from the transmitter portion (UT).

Note that the input signals for the differential configuration of

UR, are the transmit voltage (V

T) and VT/2. This is because

match is chosen to match the coupled telephone-line imped-

ance; therefore dividing V

by two (assuming R1 >>

match). The differential configuration of UR has its resistors

chosen to cancel the V

T and VT/2 inputs according to the

following equation:

Note that Cr is included for canceling out the inadequacies of

the lossy, miniature transformer. Refer to application note

AN-397 for detailed explanation.

“SIMPLE” MIXER (AMPLITUDE MODULATOR)

The mixer of Figure 8 is simple and provides a unique form

of amplitude modulation. Vi is the modulation frequency

M), while a +3V square-wave at the gate of Q1, induces a

carrier frequency (F

C). Q1 switches (toggles) U1 between

inverting and non-inverting unity gain configurations. Offset-

ting a sine wave above ground at Vi results in the oscillo-

scope photo of Figure 9.

The simple mixer can be applied to applications that utilize

the Doppler Effect to measure the velocity of an object. The

difference frequency is one of its output frequency compo-

nents. This difference frequency magnitude (/F

M-FC/) is the

key factor for determining an object’s velocity per the Dop-

pler Effect. If a signal is transmitted to a moving object, the

reflected frequency will be a different frequency. This differ-

ence in transmit and receive frequency is directly propor-

tional to an object’s velocity.

DUAL AMPLIFIER ACTIVE FILTERS (DAAFs)

The LMV822/24 bring economy and performance to DAAFs.

The low-pass and the high-pass filters of Figure 10 and

Figure 11 (respectively), offer one key feature: excellent

sensitivity performance. Good sensitivity is when deviations

in component values cause relatively small deviations in a

filter’s parameter such as cutoff frequency (Fc). Single am-

plifier active filters like the Sallen-Key provide relatively poor

sensitivity performance that sometimes cause problems for

high production runs; their parameters are much more likely

to deviate out of specification than a DAAF would. The

DAAFs of Figure 10 and Figure 11 are well suited for high

volume production.

10012833

FIGURE 7. Telephone-line Transceiver for a PCMCIA

Modem Card

10012839

FIGURE 8. Amplitude Modulator Circuit

mod

carrier

10012840

FIGURE 9. Output signal per the Circuit of Figure 8

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