LMV821 datasheet(12/24 Pages) NSC

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1.3 Input Bias Current Consideration

Input bias current (I

B) can develop a somewhat significant

offset voltage. This offset is primarily due to I

flowing

through the negative feedback resistor, R

F. For example, if IB

is 90nA (max room) and R

F is 100 kΩ, then an offset of 9 mV

will be developed (V

OS=IBxRF).Using a compensation resis-

tor (R

C), as shown in Figure 6, cancels out this affect. But the

input offset current (I

OS) will still contribute to an offset volt-

age in the same manner - typically 0.05 mV at room temp.

2.0 APPLICATION CIRCUITS

This section covers the following application circuits:

1. Telephone-Line Transceiver

2. “Simple” Mixer (Amplitude Modulator)

3. Dual Amplifier Active Filters (DAAFs)

•

a. Low-Pass Filter (LPF)

•

b. High-Pass Filter (HPF)

5. Tri-level Voltage Detector

2.1 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 (Vt) and Vt/2. This is because

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

ance; therefore dividing Vt by two (assuming R1 >> R

match).

The differential configuration of UR has its resistors chosen

to cancel the Vt 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.

2.2“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 in-

verting 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.

DS100128-54

FIGURE 5. Pulse Response per

Figure 4

DS100128-59

FIGURE 6. Canceling the Voltage Offset Effect of Input

Bias Current

DS100128-33

FIGURE 7. Telephone-line Transceiver for a PCMCIA

Modem Card

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