AD711
loading can introduce errors in instantaneous input 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.
The circuit in Figure 36 employs a 100
Ω
isolation resistor
which enables the amplifier to drive capacitive loads exceeding
1500 pF; the resistor effectively isolates the high frequency feed-
back 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
Ω
series resistor and the load
capacitance, C
L
. Figure 37 shows a typical transient response
for this connection.
DRIVING A LARGE CAPACITIVE LOAD
SECOND ORDER LOW PASS FILTER
Figure 38 depicts the AD711 configured as a second order
Butterworth low pass filter. With the values as shown, the cor-
ner frequency will be 20 kHz; however, the wide bandwidth of
the AD711 permits a corner frequency as high as several hun-
dred kilohertz. Equations for component selection are shown
below.
R1
=
R2
=
user selected
(typical
values:
10
kΩ
– 100
kΩ)
C1=
1.414
0.707
,
C2
=
(2
π)(
f
cutoff
)(R1)
(2
π)(
f
cutoff
)(R1)
Where C1 and C2 are in farads.
Figure 38. Second Order Low Pass Filter
An important property of filters is their out-of-band rejection.
The simple 20 kHz low pass filter shown in Figure 38, might be
used to condition a signal contaminated with clock pulses or
sampling glitches which have considerable energy content at
high frequencies.
Figure 36. Circuit for Driving a Large Capacitive Load
The low output impedance and high bandwidth of the AD711
minimize high frequency feedthrough as shown in Figure 39.
The upper trace is that of another low-cost BiFET op amp
showing 17 dB more feedthrough at 5 MHz.
Figure 37. Transient Response R
L
= 2 k
Ω
, 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 am-
plified to produce excessive output offset. For low frequency
applications requiring 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 as the AD711 will
minimize both dc and ac errors in all active filter applications.
Figure 39.
9-POLE CHEBYCHEV FILTER
Figure 40 shows the AD711 and its dual counterpart, the
AD712, as a 9-pole Chebychev filter using active frequency de-
pendent negative resistors (FDNR). With a cutoff frequency of
50 kHz and better than 90 dB rejection, it may be used as an
anti-aliasing filter for a 12-bit Data Acquisition System with
100 kHz throughput.
As shown in Figure 40, the filter is comprised of four FDNRs
(A, B, C, D) having values of 4.9395 10
–15
and 5.9276
10
–15
farad-seconds. Each FDNR active network provides a
two-pole response; for a total of 8 poles. The 9th pole consists
of a 0.001
µF
capacitor and a 124 kΩ resistor at Pin 3 of ampli-
fier A2. Figure 41 depicts the circuits for each FDNR with the
REV. A
–11–
AD [ ANALOG DEVICES ]
