AD711
voltage. If the A/D conversion speed is not excessive and the
bandwidth of the amplifier is sufficient, the amplifier’s output
will return to the nominal value before the converter makes its
comparison. However, many amplifiers have relatively narrow
bandwidth yielding slow recovery from output transients. The
AD711 is ideally suited to drive high speed A/D converters since
it offers both wide bandwidth and high open-loop gain.
large value input resistors, bias currents flowing through these
resistors will also generate an offset voltage.
In addition, at higher frequencies, an op amp’s dynamics must
be carefully considered. Here, slew rate, bandwidth, and open-loop
gain play a major role in op amp selection. The slew rate must
be fast as well as symmetrical to minimize distortion. The amplifier’s
bandwidth in conjunction with the filter’s gain will dictate the
frequency response of the filter.
The use of a high performance amplifier such a s the AD711
will minimize both dc and ac errors in all active filter applica-
tions.
SECOND ORDER LOW PASS FILTER
a. Source Current = 2 mA
b. Sink Current = 1 mA
Figure 15 depicts the AD711 configured as a second order
Butterworth low pass filter. With the values as shown, the corner
frequency will be 20 kHz; however, the wide bandwidth of the
AD711 permits a corner frequency as high as several hundred
kilohertz. Equations for component selection are shown below.
R1
=
R2
=
user selected
(typical
values:
10
kW
– 100
kW)
C1=
Where:
C1
and
C2
are in farads.
C1
560pF
+15V
0.1 F
R1
20k
V
IN
Figure 12. ADC Input Unity Gain Buffer Recovery Times
(4)
(5)
The circuit in Figure 13 employs a 100
W
isolation resistor which
enables the amplifier to drive capacitive loads exceeding 1500 pF;
the resistor effectively isolates the high frequency feedback from
the load and stabilizes the circuit. Low frequency feedback is
returned to the amplifier summing junction via the low pass
filter formed by the 100
W
series resistor and the load capaci-
tance, C
L
. Figure 14 shows a typical transient response for
this connection.
4.99k
30pF
+V
S
INPUT
4.99k
TYPICAL CAPACITANCE
LIMIT FOR VARIOUS
LOAD RESISTORS
R
L
2k
10k
20k
C
L
UP TO
1500pF
1500pF
1000pF
0.1 F
100
C
L
R
L
DRIVING A LARGE CAPACITIVE LOAD
1.414
0.707
,
C2
=
(2
p)(
f
cutoff
)(R1)
(2
p)(
f
cutoff
)(R1)
R2
20k
C2
280pF
AD711
0.1 F
V
OUT
AD711
0.1 F
–V
S
OUTPUT
–15V
Figure 15. Second Order Low Pass Filter
Figure 13. Circuit for Driving a Large Capacitive Load
An important property of filters is their out-of-band rejection.
The simple 20 kHz low pass filter shown in Figure 15, might be
used to condition a signal contaminated with clock pulses or
sampling glitches which have considerable energy content at
high frequencies.
The low output impedance and high bandwidth of the AD711
minimize high frequency feedthrough as shown in Figure 16.
The upper trace is that of another low-cost BiFET op amp
showing 17 dB more feedthrough at 5 MHz.
Figure 14. Transient Response R
L
= 2 k
W
, C
L
= 500 pF
ACTIVE FILTER APPLICATIONS
In active filter applications using op amps, the dc accuracy of the
amplifier is critical to optimal filter performance. The amplifier’s
offset voltage and bias current contribute to output error. Offset
voltage will be passed by the filter and may be amplified to produce
excessive output offset. For low frequency applications requiring
REV. E
–11–
Figure 16.
AD [ ANALOG DEVICES ]
