Application Note (Continued)
10012854
FIGURE 5. Pulse Response per Figure 4
10012861
INPUT BIAS CURRENT CONSIDERATION
Input bias current (IB) can develop a somewhat significant
offset voltage. This offset is primarily due to IB flowing
through the negative feedback resistor, RF. For example, if IB
FIGURE 2. Unity-Gain Frequency vs Common Mode
Voltage for Various Loads
@
is 90 nA (max room) and RF is 100 kΩ, then an offset of 9
UNITY GAIN PULSE RESPONSE CONSIDERATION
mV will be developed (VOS=IBx RF).Using a compensation
resistor (RC), as shown in Figure 6, cancels out this affect.
But the input offset current (IOS) will still contribute to an
offset voltage in the same manner - typically 0.05 mV at
room temp.
A pull-up resistor is well suited for increasing unity-gain,
pulse response stability. For example, a 600 Ω pull-up resis-
tor reduces the overshoot voltage by about 50%, when
driving a 220 pF load. Figure 3 shows how to implement the
pull-up resistor for more pulse response stability.
10012841
FIGURE 3. Using a Pull-up Resistor at the Output for
Stabilizing Capacitive Loads
Higher capacitances can be driven by decreasing the value
of the pull-up resistor, but its value shouldn’t be reduced
beyond the sinking capability of the part. An alternate ap-
proach is to use an isolation resistor as illustrated in Figure 4
.
Figure 5 shows the resulting pulse response from a LMV824,
while driving a 10,000 pF load through a 20Ω isolation
resistor.
10012859
FIGURE 6. Canceling the Voltage Offset Effect of Input
Bias Current
APPLICATION CIRCUITS
10012843
This section covers the following application circuits:
1. Telephone-Line Transceiver
FIGURE 4. Using an Isolation Resistor to Drive Heavy
Capacitive Loads
2. “Simple” Mixer (Amplitude Modulator)
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