AD712AH Analog Devices, AD712AH Datasheet - Page 12

• Manufacturer Part Number: AD712AH
• Description: Dual Precision/ Low Cost/ High Speed/ BiFET Op Amp
• Manufacturer: Analog Devices

AD712
DRIVING A LARGE CAPACITIVE LOAD
The circuit in Figure 36 employs a 100
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
capacitance, C
for this connection.
Figure 35. ADC Input Unity Gain Buffer Recovery Times
TYPICAL CAPACITANCE
LIMIT FOR VARIOUS
LOAD RESISTORS
2k
10k
20
Figure 36. Circuit for Driving a Large Capacitive Load
R
1
INPUT
1500pF
1500pF
1000pF
C
1
4.99k
UP TO
0%
100
10
0%
100
10
90
L
90
. Figure 37 shows a typical transient response
a. Source Current = 2 mA
b. Sink Current = 1 mA
500mV
500mV
30pF
AD712
–V
+V
1/2
–10V ADC IN
4.99k
IN
IN
–5V ADC IN
0.1 F
+
–
0.1 F
PD711 BUFF
PD711 BUFF
–
+
series resistor and the load
100
isolation resistor
C
200ns
1
200ns
R
1
OUTPUT
–12–
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 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 AD712 will
minimize both dc and ac errors in all active filter applications.
Figure 37. Transient Response R
0%
100
10
90
5V
L
= 2 k , C
1 s
L
= 500 pF
REV. B