AD713
A HIGH SPEED INSTRUMENTATION AMPLIFIER
CIRCUIT
A HIGH SPEED FOUR OP AMP CASCADED AMPLIFIER
CIRCUIT
The instrumentation amplifier circuit shown in Figure 5 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 7 shows how the four amplifiers of the AD713 may be
connected in cascade to form a high gain, high bandwidth am-
plifier. This gain of 100 amplifier has a –3 dB bandwidth greater
than 600 kHz.
Figure 7. A High Speed Four Op Amp Cascaded
Amplifier Circuit
Figure 5. A High Speed Instrumentation Amplifier Circuit
Table I provides a performance summary for this circuit. The
photo of Figure 6 shows the pulse response of this circuit for a
gain of 10.
Table I. Performance Summary for the High Speed
Instrumentation Amplifier Circuit
Gain
RG
Bandwidth
T Settle (0.01%)
1
2
10
NC
20 kΩ
4.04 kΩ
1.2 MHz
1.0 MHz
0.25 MHz
2 µs
2 µs
5 µs
Figure 8. 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 the AD1860
DAC, can achieve 0.0016% THD (here at a 4fs or a 176.4 kHz
update rate) without requiring the use of a deglitcher. Just such
a circuit is shown in Figure 9. The 470 pF feedback capacitor
used with IC2a, along with op amp IC2b and its associated
components, together form a 3-pole low-pass filter. Each or all
of these poles can be tailored for the desired attenuation and
phase characteristics required for a particular application. In this
application, one half of an AD713 serves each channel in a two-
channel stereo system.
Figure 6. The Pulse Response of the High Speed
Instrumentation Amplifier. Gain = 10
–8–
REV. C
ADI [ ADI ]
