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