LPV321 Datasheet, PDF (10/21 Pages) National Semiconductor (TI) – General Purpose, Low Voltage, Low Power, Rail-to-Rail Output Operational Amplifiers

Application Notes (Continued)
Low Supply Voltage. National provides guaranteed perfor-
mance at 2.7V and 5V. These guarantees ensure operation
throughout the battery lifetime.
Rail-to-Rail Output. Rail-to-rail output swing provides maxi-
mum possible dynamic range at the output. This is particu-
larly important when operating on low supply voltages.
Input Includes Ground. Allows direct sensing near GND in
single supply operation.
The differential input voltage may be larger than V ⁺ without
damaging the device. Protection should be provided to pre-
vent the input voltages from going negative more than −0.3V
(at 25˚C). An input clamp diode with a resistor to the IC input
terminal can be used.
2.0 Capacitive Load Tolerance
The LPV321/358/324 can directly drive 200 pF in unity-gain
without oscillation. The unity-gain follower is the most sensi-
tive configuration to capacitive loading. Direct capacitive
loading reduces the phase margin of amplifiers. The combi-
nation of the amplifier’s output impedance and the capacitive
load induces phase lag. This results in either an under-
damped pulse response or oscillation. To drive a heavier ca-
pacitive load, circuit in Figure 1 can be used.
ing the value of R _F due to the input bias current of the
LPV321/358/324. C _F and R_ISO serve to counteract the loss
of phase margin by feeding the high frequency component of
the output signal back to the amplifier’s inverting input,
thereby preserving phase margin in the overall feedback
loop. Increased capacitive drive is possible by increasing the
value of C_F . This in turn will slow down the pulse response.
DS100920-5
FIGURE 3. Indirectly Driving A Capacitive Load with
DC Accuracy
3.0 Input Bias Current Cancellation
The LPV321/358/324 family has a bipolar input stage. The
typical input bias current of LPV321/358/324 is 1.5nA with
5V supply. Thus a 100kΩ input resistor will cause 0.15mV of
error voltage. By balancing the resistor values at both invert-
ing and non-inverting inputs, the error caused by the ampli-
fier’s input bias current will be reduced. The circuit in Figure
4 shows how to cancel the error caused by input bias
current.
DS100920-4
FIGURE 1. Indirectly Driving A Capacitive Load Using
Resistive Isolation
In Figure 1, the isolation resistor R_ISO and the load capacitor
C_L form a pole to increase stability by adding more phase
margin to the overall system. The desired performance de-
pends on the value of R_ISO. The bigger the R_ISO resistor
value, the more stable V_OUT will be. Figure 2 is an output
waveform of Figure 1 using 100kΩ for R_ISO and 1000pF for
C_L.
DS100920-6
FIGURE 4. Cancelling the Error Caused by Input Bias
Current
4.0 Typical Single-Supply Application Circuits
4.1 Difference Amplifier
The difference amplifier allows the subtraction of two volt-
ages or, as a special case, the cancellation of a signal com-
mon to two inputs. It is useful as a computational amplifier, in
making a differential to single-ended conversion or in reject-
ing a common mode signal.
DS100920-75
FIGURE 2. Pulse Response of the LPV324 Circuit in
Figure 1
The circuit in Figure 3 is an improvement to the one in Figure
1 because it provides DC accuracy as well as AC stability. If
there were a load resistor in Figure 1, the output would be
voltage divided by R_ISO and the load resistor. Instead, in Fig-
ure 3, R_F provides the DC accuracy by using feed-forward
techniques to connect V_IN to R_L. Caution is needed in choos-
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