Datasheet

AD521
DESIGNPRINCIPLE
Figure 1 is a simplified schematic of the AD521. A differential
input voltage, VIN,appears across RG causing an imbalance in
the currents through Ql and <l2,~I=VIN/RG' That imbalance
is forced to flow in Rs because the collector currents of Q3
and <4 are constrained to be equal by their biasing (current
mirror). These conditions can only be satisfied if the differen-
tial voltage across Rs (and hence the output voltage of the
AD521) is equal to ~I X Rs. The feedback amplifier, ApB
performs that function. Therefore, VOUT= V~ X Rs or
VOUT - Rs
VIN - ~
+V
VON
I-IrQ
VOUT V,N
"'RI. 1IQ
OR-VOUT.,,"
v-. Ira
SENSE
'i
t'x
~~,
.'
IX t
CURRENT MIRROR
V-
Figure 7.Simplified AD527 Schematic
APPLICATION NOTES FOR THE AD521
These notes ensure the AD521 will achieve the high level of
performance necessary for many diversified IA applications.
1. Gains below 1 are realized by adjusting the gain setting
resistors as shown in Figure 2 (the resistor, as betWeen
pins 10 and 13 should remain 100kn :1:15%,see application
note 3). For best results, the input voltage should be re-
stricted to :tl0V even though the gain may be less than 1.
See Figure 6 for gains above 1000.
2. Provide a return path to ground for input bias currents. The
AD521 is an instrumentation amplifier, not an isolation
amplifier. When using a thermocouple or other "floating"
source, this return path may be provided directly to ground
or indirectly through a resistor to ground from pins 1 and/
or 3, as shown in Figure 3. If the return path is not pro-
vided, bias currents will cause the output to saturate. The
value of the resistor may be determined by dividing the
maximum allowable common mode voltage for the appli-
cation by the bias current of the instrumentation amplifier.
3. The resistors betWeen pins 10 and 13, (RSCALE) must equal
l00kn :t15% (Figure 2). If RSCALE is too low (below 85kn)
the output swing of the AD521 is reduced. At values below
80kU and above 120kU the stability of the AD521 may be
impaired.
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4. Do not exceed the allowable input signalrange. The line-
arity of the ADS21 decreases if the inputs are drivenwithin
5 volts of the supply rails, particularly when the deviceis
used at a gain less than 1.To avoidthispossibility,atten-
uate the input signalthrough a resistive divider networkand
use the ADS21 as a buffer, as shown in Figure 4. The resis-
tor R/2 matches the impedance seen by both AD521 in-
puts so that the voltage offset caused by bias currents will
be minimized.
5. Use the compensation pin (pin 9) and the applicable com-
pensation circuit when the amplifier is required to drive a
capacitive load. It is worth mentioning that coaxial cables
can '~invisibly" provide such capacitance since many popu-
lar coaxial cables display capacitance in the vicinity of 3OpF
per foot.
This compensation (bandwidth control) feature permits the
user to fit the response of the AD521 to the particular appli-
cation as illustrated by Figure S. In cases of extremely high
load capacitance the compensation circuit may be changed
as follows:
1. Reduce 680n to 24n
2. Reduce BOn to 7.5n
3. Increase 1000pF to O.IJ,LF
4. Set Cx to 1000pF if no compensation was originally
used. Otherwise, do not alter the original value.
This allows stable operation for load capacitances up to
3000pF, but limits the slew rate to approximately 0.16VIJ,Ls;
6. Signals having frequency components above the Instrumen-
tation Amplifier's output amplifier closed-loop bandwidth
will be transmitted from V- to the output with little or no
attenuation. Therefore, it is advisable to decouple the V-
supply line to the output common or to pin 11.1
V+
+IN
GAIN VALUE OF RO
0.1 1I0Il1
1 1(JOkS1
10 101<!J
100 1k!1
1000 100!J
OUTPUT
-IN
OUTPUT
SIGNAL
COMMON
Figure 2. Operating Connections for AD527
I For further details, refer to "An I.C. User's Guide to Decoupling,
Grounding, and Making Things Go Right for a Change," by A. 'ceS
Paul Brokaw. This application note is available from Analog Devi
without charge upon request.
REV. A
j
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Not Recommended for new Designs