AD713
loop gain. At high frequencies, where the loop gain is low, the
amplifier output impedance can approach its open-loop value.
1mV
AD713 BUFF
100
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V
STS
LOGIC
12/8 (MSB) DB11
3
CS
DB10
DB9
DB8
DB7
DB6
DB5
DB4
DB3
DB2
DB1
HIGH
BITS
4
A
O
AD574A
TOP VIEW
(Not to Scale)
5
R/C
CE
6
V
7
CC
MIDDLE
BITS
8
REF OUT
AC
GAIN ADJUST
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REF IN
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+15V
0.1µF
R2 100Ω
R1 100Ω
V
EE
LOW
BITS
BIP OFF
10V
500mV
–5V ADC IN
200ns
IN
IN
(LSB) DB0
DC
4
OFFSET ADJUST
20V
±10V
ANALOG
INPUT
Figure 39. Buffer Recovery Time Sink Current = 1 mA
11
0.1µF
ANALOG COM
Driving A Large Capacitive Load
1/4
AD713
–15V
The circuit of Figure 40 uses a 100 Ω isolation resistor that
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 Ω series resistor and the load
capacitance, CL. Figure 41 shows a typical transient response for
this connection.
Figure 37. AD713 as an ADC Buffer
Most IC amplifiers exhibit a minimum open-loop output imped-
ance of 25 Ω, due to current limiting resistors. A few hundred
microamps reflected from the change in converter loading can
introduce errors in instantaneous input voltage. If the analog-
to-digital conversion speed is not excessive and the bandwidth
of the amplifier is sufficient, the amplifier output returns to
the nominal value before the converter makes its comparison.
However, many amplifiers have relatively narrow bandwidths,
yielding slow recovery from output transients. The AD713 is
ideally suited as a driver for ADCs because it offers both a wide
bandwidth and a high open-loop gain.
4.99kΩ
30pF
+V
S
0.1µF
4.99kΩ
4
INPUT
OUTPUT
100Ω
1/4
TYPICAL CAPACITANCE
LIMIT FOR VARIOUS
LOAD RESISTORS
AD713
1mV
AD713 BUFF
C
R
L
L
11
0.1µF
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R
C UP TO
L
L
2kΩ 1500pF
10kΩ 1500pF
20kΩ 1000pF
–V
S
Figure 40. Circuit for Driving a Large Capacitance Load
5V
1µs
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500mV
10V ADC IN
200ns
Figure 38. Buffer Recovery Time Source Current = 2 mA
10
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Figure 41. Transient Response, RL = 2 kΩ, CL = 500 pF
Rev. F | Page 13 of 20
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