AD624
GROUNDING
“inside the loop” of an instrumentation amplifier to provide the
required current without significantly degrading overall perfor-
mance. The effects of nonlinearities, offset and gain inaccuracies
of the buffer are reduced by the loop gain of the IA output
amplifier. Offset drift of the buffer is similarly reduced.
Many data-acquisition components have two or more ground
pins which are not connected together within the device. These
grounds must be tied together at one point, usually at the sys-
tem power supply ground. Ideally, a single solid ground would
be desirable. However, since current flows through the ground
wires and etch stripes of the circuit cards, and since these paths
have resistance and inductance, hundreds of millivolts can be
generated between the system ground point and the data acqui-
sition components. Separate ground returns should be provided
to minimize the current flow in the path from the most 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 measure-
ment errors (see Figure 34).
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, from ground, to
be shared between signal and reference offset.
+V
S
SENSE
V
+
IN
IN
ANALOG P.S.
+15V –15V
DIGITAL P.S.
+5V
C
C
AD624
LOAD
REF
V
–
0.1 0.1
F F
0.1 0.1
F F
1F
1F
1F
–V
S
V
OFFSET
AD711
DIG
COM
+
DIGITAL
DATA
OUTPUT
Figure 36. Use of Reference Terminal to Provide Output
Offset
AD583
SAMPLE
AND HOLD
AD624
AD574A
ANALOG
GROUND*
SIGNAL
GROUND
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, including those caused by
PC layouts or other connection techniques, which appears
between the reference pin and ground will increase the gain of
the noninverting signal path, thereby upsetting the common-
mode rejection of the IA. Inadvertent thermocouple connections
created in the sense and reference lines should also be avoided
as they will directly affect the output offset voltage and output
offset voltage drift.
OUTPUT
REFERENCE
*IF INDEPENDENT, OTHERWISE RETURN AMPLIFIER REFERENCE
TO MECCA AT ANALOG P.S. COMMON
Figure 34. Basic Grounding Practice
Since the output voltage is developed with respect to the poten-
tial on the reference terminal an instrumentation amplifier can
solve many grounding problems.
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.
In the AD624 a reference source resistance will unbalance the
CMR trim by the ratio of 10 kΩ/RREF. For example, if the refer-
ence source impedance is 1 Ω, CMR will be reduced to 80 dB
(10 kΩ/1 Ω = 80 dB). An operational amplifier may be used to
provide that low impedance reference point as shown in Figure
36. The input offset voltage characteristics of that amplifier will
add directly to the output offset voltage performance of the
instrumentation amplifier.
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 37.
V+
(SENSE)
OUTPUT
CURRENT
BOOSTER
V
+
IN
IN
SENSE
X1
AD624
+INPUT
R
1
X
R
L
+V –
V
–
I
L
(REF)
AD624
V–
AD711
A2
–INPUT
REF
Figure 35. AD624 Instrumentation Amplifier with Output
Current Booster
V
V
LOAD
IN
40.000
R
G
X
1 +
I
=
=
L
Typically, IC instrumentation amplifiers are rated for a full
±10 volt output swing into 2 kΩ. In some applications, how-
ever, the need exists to drive more current into heavier loads.
Figure 35 shows how a current booster may be connected
R
R
1
1
Figure 37. Voltage-to-Current Converter
REV. C
–10–
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