AD215
REV. 0
–10–
When gain setting resistors are used, 0.325" channel centers can
still be achieved as shown in Figure 20b.
R
G
R
F
C
F
C2
C1
1
2
3
4
5
6
IN
IN COM
+V
ISO
–V
I
SO
C1, C2 ARE V
ISO
FILTER CAPACITORS.
R
F
, R
G
ARE FEEDBACK, GAIN RESISTORS.
C
F
IS A FEEDBACK BYPASS CAPACITOR.
R
G
R
F
C
F
C2
C1
1
2
3
4
5
6
IN
IN COM
+V
ISO
–V
I
SO
Figure 20b. PCB Layout for Gain Greater than Unity
APPLICATIONS EXAMPLES
Motor Control
Figure 21 shows an AD215 used in a dc motor control applica-
tion. Its excellent phase characteristics and wide bandwidth are
ideal for this type of application.
1
3
4
38
37
AD215
MOTOR
COMMAND
±10 VOLTS
2
COM
G = 1
ISOLATED
MOTOR
COMMAND
±10V
V
C
OUT LO
SHAFT
θ
I
MOTOR
MOTOR
OPTICAL
RESOLVER
OR
TACHOMETER
ENCODER
ENCODER FEEDBACK
MOTOR
CONTROL
UNIT
Figure 21. Motor Control Application
Multichannel Data Acquisition
The current drive capabilities of the AD215’s bipolar ±15 V dc
isolated power supply is more than adequate to meet the modest
±800 µA supply current requirements for the AD7502 multi-
plexer. Digital isolation techniques should be employed to iso-
late the Enable (EN), A0 and A1 logic control signals.
IN+
IN–
FB
IN COM
OUT HI
OUT LO
1
3
4
AD215
PWR
RTN
–15V
+15V
+V
ISO
–V
ISO
6.8µF
6
6.8µF
G = 1
42
38
(–15V)
GND
(+15V)
EN A1
A0
AD7502
37
2
5
2
COM
43
44
S1 – S4
S5 – S8
DTL/TTL TO CMOS LEVEL
TRANSLATOR
DECODER/DRIVER
Figure 22. Multichannel Data Acquisition Application
AC Transducer Applications
In applications such as vibration analysis, where the user must
acquire and process the spectral content of a sensor’s signal
rather than its “dc” level, the wideband characteristics of the
AD215 prove most useful. Key specifications for ac transducer
applications include bandwidth, slew rate and harmonic distor-
tion. Since the transducer may be mechanically bonded or
welded to the object under test, isolation is typically required to
eliminate ground loops as well as protect the electronics used in
the data acquisition system. Figure 23 shows an isolated strain
gage circuit employing the AD215 and a high speed operational
amplifier (AD744).
To alleviate the need for an instrumentation amplifier, the
bridge is powered by a bipolar excitation source. Under this ap-
proach the common-mode voltage is ±V
SPAN
which is typically
only a few millivolts, rather than the V
EXC
4 2 that would be
achieved with a unipolar excitation source and Wheatstone
bridge configuration.
Using two strain gages with a gage factor of 3 mV/V and a
±1.2 V excitation signal, a ±6.6 mV output signal will result. A
gain setting of 454 will scale this low level signal to ±3 V, which
can then be digitized by a high speed, 100 kHz sampling ADC
such as the AD7870.
The low voltage excitation is used to permit the front-end cir-
cuitry to be powered from the isolated power supplies of the
AD215, which can supply up to ±10 mA of isolated power at
±15 V. The bridge draws only 3.5 mA, leaving sufficient cur-
rent to power the micropower dual BiFET (400 µA quiescent
current) and the high speed AD744 BiFET amplifier (4 mA
quiescent current).
