AD524
REV. E –11–
acquisition components. Separate ground returns should be
provided to minimize the current flow in the path from the sensi-
tive points to the system ground point. In this way supply currents
and logic-gate return currents are not summed into the same
return path as analog signals where they would cause measure-
ment errors.
Since the output voltage is developed with respect to the poten-
tial on the reference terminal, an instrumentation amplifier can
solve many grounding problems.
0.1
mF
0.1
mF
DIGITAL P.S.
+5V
C–15V
ANALOG P.S.
1mF
DIG
COM
AD574A
C+15V
6
OUTPUT
REFERENCE
*ANALOG
GROUND
AD524
DIGITAL
DATA
OUTPUT
SIGNAL
GROUND
*IF INDEPENDENT; OTHERWISE RETURN AMPLIFIER REFERENCE
TO MECCA AT ANALOG P.S. COMMON
1mF1mF
0.1
mF
0.1
mF
AD583
SAMPLE
AND HOLD
Figure 37. Basic Grounding Practice
SENSE TERMINAL
The sense terminal is the feedback point for the instrument
amplifier’s output amplifier. Normally it is connected to the
instrument amplifier output. If heavy load currents are to be
drawn through long leads, voltage drops due to current flowing
through lead resistance can cause errors. The sense terminal can
be wired to the instrument amplifier at the load, thus putting
the IxR drops “inside the loop” and virtually eliminating this
error source.
V–
V+
X1
AD524
OUTPUT
CURRENT
BOOSTER
(REF)
(SENSE)
R
L
V
IN
+
V
IN
–
Figure 38. AD524 Instrumentation Amplifier with Output
Current Booster
Typically, IC instrumentation amplifiers are rated for a full ±10
volt output swing into 2 kΩ. In some applications, however, the
need exists to drive more current into heavier loads. Figure 38
shows how a high-current booster may be connected “inside the
loop” of an instrumentation amplifier to provide the required
current boost without significantly degrading overall perfor-
mance. Nonlinearities, offset and gain inaccuracies of the buffer
are minimized by the loop gain of the IA output amplifier. Off-
set drift of the buffer is similarly reduced.
REFERENCE TERMINAL
The reference terminal may be used to offset the output by up
to ±10 V. This is useful when the load is “floating” or does not
share a ground with the rest of the system. It also provides a
direct means of injecting a precise offset. It must be remem-
bered that the total output swing is ±10 volts to be shared be-
tween signal and reference offset.
When the IA is of the three-amplifier configuration it is neces-
sary that nearly zero impedance be presented to the reference
terminal.
Any significant resistance from the reference terminal to ground
increases the gain of the noninverting signal path, thereby upset-
ting the common-mode rejection of the IA.
In the AD524 a reference source resistance will unbalance the
CMR trim by the ratio of 20 kΩ/R
REF
. For example, if the refer-
ence source impedance is 1 Ω, CMR will be reduced to 86 dB
(20 kΩ/1 Ω = 86 dB). An operational amplifier may be used to
provide that low impedance reference point as shown in Figure
39. The input offset voltage characteristics of that amplifier will
add directly to the output offset voltage performance of the
instrumentation amplifier.
–V
S
+V
S
AD524
REF
SENSE
LOAD
V
IN
+
V
IN
–
V
OFFSET
AD711
Figure 39. Use of Reference Terminal to Provide Output
Offset
An instrumentation amplifier can be turned into a voltage-to-
current converter by taking advantage of the sense and reference
terminals as shown in Figure 40.
AD524
REF
SENSE
LOAD
AD711
+INPUT
–INPUT
R1
V
X
I
L
V
X
R1
I
L
= =
= (1 +
V
IN
R1
)
40,000
R
G
A2
Figure 40. Voltage-to-Current Converter
By establishing a reference at the “low” side of a current setting
resistor, an output current may be defined as a function of input
voltage, gain and the value of that resistor. Since only a small
current is demanded at the input of the buffer amplifier A
2
, the
forced current I
L
will largely flow through the load. Offset and
drift specifications of A
2
must be added to the output offset and
drift specifications of the IA.
