AD713
A HIGH SPEED INSTRUMENTATION AMPLIFIER
CIRCUIT
A HIGH SPEED 4-OP-AMP CASCADED 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.
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.
+V
S
0.1µF
1µF
INPUT
3
2
4
1
5
6
20,000
1/4
CIRCUIT GAIN =
+ 1
7
10
9
R
AD713
G
1/4
AD713
1/4
8
12
13
1/4
AD713
14
11
3
2
*1.5pF TO 20pF
(TRIM FOR BEST SETTLING TIME)
2.15kΩ
AD713
–IN
1/4
AD713
OUTPUT
0.1µF
2.15kΩ
1
A1
2.15kΩ
1kΩ
1kΩ
2.15kΩ
1kΩ
10kΩ**
10kΩ
1kΩ
22MΩ
1µF
10kΩ**
+
SENSE
–V
9
S
–V
+V
S
7.5pF
7.5pF
S
4-OP-AMP CASCADED AMPLIFIER
GAIN = 100
BANDWIDTH (–3dB) = 632kHz
100kΩ
R
8
A3
G
10kΩ**
OPTIONAL V
OS
ADJUSTMENT
10
1/4
5pF
AD713
10kΩ**
Figure 35. High Speed 4-Op-Amp Cascaded Amplifier Circuit
10kΩ
6
5
TO SPECTRUM ANALYZER
7
13
A4
A2
ERROR SIGNAL
OUTPUT
TO BUFFERED
VOLTAGE
REFERENCE
OR REMOTE
GROUND SENSE
14
+IN
1/4
AD713
(ERROR/11)
NULL
12
1/4
AD713
1kΩ
ADJUST
100kΩ
10kΩ
10kΩ
AD713
PIN 4
+V
S
+
+V
S
0.1µF
0.1µF
1µF
*
VOLTRONICS SP20 TRIMMER CAPACITOR
OR EQUIVALENT
RATIO MATCHED 1% METAL FILM
RESISTORS
COM
+
1kΩ
+
**
1µF
0.1µF
1µF
4
AD713
PIN 11
LOW DISTORTION
SINEWAVE INPUT
1/4
–V
S
AD713
100pF
11
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.
1µF
0.1µF
+
–V
S
Figure 36. THD Test Circuit
Table 4. Performance Summary for the High Speed
Instrumentation Amplifier Circuit
HIGH SPEED OP AMP APPLICATIONS AND
TECHNIQUES
DAC Buffers (I-to-V Converters)
Gain
RG
Bandwidth
Settling Time (0.01%)
1
2
10
NC1
1.2 MHz
1.0 MHz
0.25 MHz
2 μs
2 μs
2 μs
20 kΩ
4.04 kΩ
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.
1 NC = no connect.
5V
100
90
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• • • •
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
10
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0%
2µs
Figure 34. Pulse Response of High Speed Instrumentation Amplifier,
Gain = 10
Rev. F | Page 12 of 20
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