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AD713JNZ

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21
AD713
Rev. F | Page 12 of 20
A HIGH SPEED INSTRUMENTATION AMPLIFIER
CIRCUIT
The instrumentation amplifier circuit shown in Figure 33 can
provide a range of gains from unity up to 1000 and higher using
only a single AD713. The circuit bandwidth is 1.2 MHz at a gain
of 1 and 250 kHz at a gain of 10; settling time for the entire
circuit is less than 5 µs to within 0.01% for a 10 V step, (G = 10).
Other uses for Amplifier A4 include an active data guard and an
active sense input.
10kΩ
+IN
–IN
SENSE
TO BUFFERED
VOLTAGE
REFERENCE
OR REMOTE
GROUND SENSE
10kΩ**
10kΩ**
10kΩ**
R
G
7.5pF
*1.5pF TO 20pF
(TRIM FOR BEST SETTLING TIME)
5pF
A1
1
3
2
A3
8
9
10
A4
14
13
12
A2
7
6
5
7.5pF
10kΩ
10kΩ**
1/4
AD713
1/4
AD713
1/4
AD713
1/4
AD713
CIRCUIT GAIN = + 1
20,000
R
G
VOLTRONICS SP20 TRIMMER CAPACITOR
OR EQUIVALENT
RATIO MATCHED 1% METAL FILM
RESISTORS
*
**
+
1µF0.1µF
+
1µF0.1µF
+V
S
COM
–V
S
AD713
PIN 4
AD713
PIN 11
00824-033
Figure 33. High Speed Instrumentation Amplifier Circuit
Table 4 provides a performance summary for this circuit. Figure 34
shows the pulse response of this circuit for a gain of 10.
Table 4. Performance Summary for the High Speed
Instrumentation Amplifier Circuit
Gain R
G
Bandwidth Settling Time (0.01%)
1 NC
1
1.2 MHz 2 μs
2 20 kΩ 1.0 MHz 2 μs
10 4.04 kΩ 0.25 MHz 2 μs
1
NC = no connect.
00824-034
•••••••• •••• ••• • •••• •••• •••• •••• •••• ••••
•••••••• •••• ••• • •••• •••• •••• •••• •••• ••••
100
90
10
0%
5V
2µs
Figure 34. Pulse Response of High Speed Instrumentation Amplifier,
Gain = 10
A HIGH SPEED 4-OP-AMP CASCADED AMPLIFIER
CIRCUIT
Figure 35 shows how the four amplifiers of the AD713 can be
connected in cascade to form a high gain, high bandwidth
amplifier. This gain of 100 amplifier has a −3 dB bandwidth
greater than 600 kHz.
1/4
AD713
–V
S
+V
S
2.15kΩ
INPUT
+
1µF
0.1µF
1µF
0.1µF
OUTPUT
4-OP-AMP CASCADED AMPLIFIER
GAIN = 100
BANDWIDTH (–3dB) = 632kHz
1/4
AD713
1kΩ
1kΩ
1kΩ
1kΩ
1/4
AD713
2.15kΩ
1/4
AD713
2.15kΩ
2.15kΩ
OPTIONAL V
OS
ADJUSTMENT
+V
S
–V
S
22MΩ
100kΩ
00824-035
3
4
1
2
12
11
14
13
5
7
6
10
8
9
Figure 35. High Speed 4-Op-Amp Cascaded Amplifier Circuit
00824-036
+V
S
10kΩ
100kΩ
1kΩ
LOW DISTORTION
SINEWAVE INPUT
ERROR SIGNAL
OUTPUT
(ERROR/11)
1kΩ
NULL
ADJUST 10kΩ
–V
S
1/4
AD713
4
11
+
1µF 0.1µF
+
1µF
100pF
0.1µF
TO SPECTRUM ANALYZER
Figure 36. THD Test Circuit
HIGH SPEED OP AMP APPLICATIONS AND
TECHNIQUES
DAC Buffers (I-to-V Converters)
The wide input dynamic range of JFET amplifiers makes them
ideal for use in both waveform reconstruction and digital audio
DAC applications. The AD713, in conjunction with a 16-bit
DAC, can achieve 0.0016% THD without requiring the use of a
deglitcher in digital audio applications.
Driving the Analog Input of an Analog-to-Digital
Converter
An op amp driving the analog input of an analog-to-digital
converter (ADC), such as that shown in Figure 37, must be
capable of maintaining a constant output voltage under dynami-
cally changing load conditions. In successive approximation
converters, the input current is compared to a series of switched
trial currents. The comparison point is diode clamped but may
vary by several hundred millivolts, resulting in high frequency
modulation of the analog-to-digital input current. The output
impedance of a feedback amplifier is made artificially low by its
AD713
Rev. F | Page 13 of 20
loop gain. At high frequencies, where the loop gain is low, the
amplifier output impedance can approach its open-loop value.
STS
1
(MSB) DB11
HIGH
BITS
MIDDLE
BITS
LOW
BITS
(LSB) DB0
2
DB10
3
DB9
4
28
27
A
O
26
25
DB8
5
DB7
6
DB6
7
CE
24
REF OUT
23
AC
22
DB5
8
REF IN
21
DB4
9
V
EE
20
DB3
10
BIP OFF
19
DB2
11
10V
IN
18
DB1
12
V
LOGIC
17
13
V
CC
16
DC
14
20V
IN
15
AD574A
TOP VIEW
(Not to Scale)
12/8
CS
R/C
GAIN ADJUST
±10V
ANALOG
INPUT
R2 100Ω
R1 100Ω
OFFSET ADJUST
ANALOG COM
00824-039
1/4
AD713
+15V
0.1µF
4
–15V
0.1µF
11
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.
00824-040
•••••••• •••• •••• •••• •••• •••• •••• •••• ••••
•••••••• •••• •••• •••• •••• •••• •••• •••• ••••
100
90
10
0%
1mV
AD713 BUFF
200ns
500mV 10V ADC IN
Figure 38. Buffer Recovery Time Source Current = 2 mA
00824-041
•••••••• •••• ••• • •••• •••• •••• •••• •••• ••••
•••••••• •••• ••• • •••• •••• •••• •••• •••• ••••
100
90
10
0%
1mV
AD713 BUFF
200ns
500mV –5V ADC IN
Figure 39. Buffer Recovery Time Sink Current = 1 mA
Driving A Large Capacitive Load
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, C
L
. Figure 41 shows a typical transient response for
this connection.
+V
S
–V
S
1/4
AD713
4
11
0.1µF
30pF
4.99kΩ
4.99kΩ
C
L
R
L
100Ω
0.1µF
OUTPUT
INPUT
TYPICAL CAPACITANCE
LIMIT FOR VARIOUS
LOAD RESISTORS
R
L
2kΩ
10kΩ
20kΩ
C
L
UP TO
1500pF
1500pF
1000pF
00824-042
Figure 40. Circuit for Driving a Large Capacitance Load
00824-043
•••••••• •••• •••• •••• •••• •••• •••• •••• ••••
•••••••• •••• •••• •••• •••• •••• •••• •••• ••••
100
90
10
0%
5V 1µs
Figure 41. Transient Response, R
L
= 2 kΩ, C
L
= 500 pF
AD713
Rev. F | Page 14 of 20
00824-044
19
OUT1
AGND
3
DGND
2018
V
REF
AD7545
R1*
*REFER TO TABLE 5.
R2*
DB11 TO DB0
GAIN
ADJUST
C1
33pF
V
DD
IN
V
OUT
V
DD
R
FB
ANALOG
COMMON
+15V
–15V
0.1µF
0.1µF
1/4
AD713
4
11
1
2
CMOS DAC APPLICATIONS
The AD713 is an excellent output amplifier for CMOS DACs. It
can be used to perform both two- and four-quadrant operation.
The output impedance of a DAC using an inverted R-2R ladder
approaches R for codes containing many 1s, 3R for codes
containing a single 1, and infinity for codes containing all 0s.
For example, the output resistance of the AD7545 modulates
between 11 kΩ and 33 kΩ. Therefore, with the DAC’s internal
feedback resistance of 11 kΩ, the noise gain varies from 2 to
4/3. This changing noise gain modulates the effect of the input
offset voltage of the amplifier, resulting in nonlinear DAC
amplifier performance. The AD713, with its guaranteed 1.5 mV
input offset voltage, minimizes this effect, achieving 12-bit
performance.
Figure 42. Unipolar Binary Operation
FILTER APPLICATIONS
A Programmable State Variable Filter
For the state variable or universal filter configuration of Figure 44
to function properly, DAC A1 and DAC B1 must control the
gain and Q of the filter characteristic, and DAC A2 and DAC B2
must accurately track for the simple expression of f
C
to be true.
This is readily accomplished using two AD7528 DACs and one
AD713 quad op amp. Capacitor C3 compensates for the effects
of op amp gain bandwidth limitations.
Figure 42 and Figure 43 show the AD713 and a 12-bit CMOS
DAC, the AD7545, configured for either a unipolar binary (two-
quadrant multiplication) or bipolar (four-quadrant multiplication)
operation. Capacitor C1 provides phase compensation, which
reduces overshoot and ringing.
Table 5. Recommended Trim Resistor Values vs. Grades for
AD7545 for V
D
= 5 V
This filter provides low-pass, high-pass, and band-pass outputs
and is ideally suited for applications where microprocessor
control of filter parameters is required. The programmable
range for component values shown is f
C
= 0 kHz to 15 kHz and
Q = 0.3 to 4.5.
Trim Resistor JN/AQ KN/BQ LN/CQ GLN/GCQ
R1 500 Ω 200 Ω 100 Ω 20 Ω
R2 150 Ω 68 Ω 33 Ω 6.8 Ω
1/4
AD713
1/4
AD713
00824-045
19
OUT1
AGND
2018
V
REF
AD7545
R1*
R2*
R4
20kΩ
1%
R3
10kΩ
1%
R5
20kΩ
1%
12
3
DGND
C1
33pF
V
DD
V
IN
V
OUT
V
DD
R
FB
ANALOG
COMMON
1
2
+15V
0.1µF
4
–15V
0.1µF
11
GAIN
ADJUST
DATA INPUT
DB11 TO DB0
*REFER TO TABLE 5.
Figure 43. Bipolar Operation
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AD713JNZ

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