AD524
Rev. F | Page 18 of 28
GROUNDING
Many data acquisition components have two or more ground
pins that are not connected together within the device. These
grounds must be tied together at one point, usually at the system
power-supply ground. Ideally, a single solid ground would be
desirable. However, because current flows through the ground
wires and etch stripes of the circuit cards, and because these
paths have resistance and inductance, hundreds of millivolts can
be generated between the system ground point and the data
acquisition components. Separate ground returns should be
provided to minimize the current flow in the path from the
sensitive 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
measurement errors.
Because the output voltage is developed with respect to the
potential on the reference terminal, an instrumentation
amplifier can solve many grounding problems.
DIGITAL P.S.
+5V
C–15V
ANALOG P.S.
AD574A
C+15V
6
AD524
AD583
SAMPLE
AND HOLD
DIG
COM
DIGITAL
DATA
OUTPUT
SIGNAL
GROUND
ANALOG
GROUND*
OUTPUT
REFERENCE
*IF INDEPENDENT; OTHERWISE, RETURN AMPLIFIER REFERENCE
TO MECCA AT ANALOG P.S. COMMON.
1µF1µF 1µF
0.1
µF
0.1
µF
0.1
µF
0.1
µF
2
1
8
7
10
9
7 9 11 15
1
00500-043
Figure 43. 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–
+
X1
AD524
(REF)
(SENSE)
OUTPUT
CURRENT
BOOSTER
R
L
V
IN
+
V
IN
–
2
3
12
1
7
6
9
10
8
00500-044
Figure 44. 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 44 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
performance. Nonlinearities and offset and gain inaccuracies of
the buffer are minimized by the loop gain of the AD524 output
amplifier. Offset drift of the buffer is similarly reduced.
REFERENCE TERMINAL
The reference terminal can 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 remembered that
the total output swing is ±10 V to be shared between signal and
reference offset.
When the AD524 is of the 3-amplifier configuration it
is necessary 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 upsetting the common-mode rejection of the AD524.
In the AD524, a reference source resistance unbalances the CMR
trim by the ratio of 20 kΩ/R
REF
. For example, if the reference
source impedance is 1 Ω, CMR is reduced to 86 dB (20 kΩ/1 Ω
= 86 dB). An operational amplifier can be used to provide that
low impedance reference point, as shown in Figure 45. The
input offset voltage characteristics of that amplifier adds directly
to the output offset voltage performance of the instrumentation
amplifier.
