ad521sd-883b Analog Devices, Inc., ad521sd-883b Datasheet - Page 4

• Manufacturer Part Number: ad521sd-883b
• Description: Integrated Circuit Precision Instrumentation Amplifier
• Manufacturer: Analog Devices, Inc.

These notes ensure the AD521 will achieve the high level of
performance necessary for many diversified IA applications.
APPLICATION NOTES FOR THE AD521
3. The resistors betWeen pins 10 and 13, (RSCALE) must equal
1. Gains below 1 are realized by adjusting the gain setting
2. Provide a return path to ground for input bias currents. The
is forced to flow in Rs because the collector currents of Q3
and <4 are constrained to be equal by their biasing (current
AD521
DESIGN PRINCIPLE
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
mirror). These conditions can only be satisfied if the differen-
tial voltage across Rs (and hence the output voltage of the
VOUT
AD521) is equal to ~I X Rs. The feedback amplifier, ApB
performs that function. Therefore, VOUT= V~ X Rs or
VIN
See Figure 6 for gains above 1000.
pins 10 and 13 should remain 100kn :1:15%,see application
stricted to :tl0V even though the gain may be less than 1.
vided, bias currents will cause the output to saturate. The
value of the resistor may be determined by dividing the
resistors as shown in Figure 2 (the resistor, as betWeen
note 3). For best results, the input voltage should be re-
or 3, as shown in Figure 3. If the return path is not pro-
maximum allowable common mode voltage for the appli-
cation by the bias current of the instrumentation
the output swing of the AD521 is reduced. At values below
80kU and above 120kU the stability of the AD521 may be
impaired.
AD521 is an instrumentation
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/
l00kn :t15% (Figure 2). If RSCALE is too low (below 85kn)
-
-
~
Rs
Figure 7. Simplified AD527 Schematic
'i
--
-
t
'x
CURRENT
- -
+V
V-
MIRROR
. - - ---
amplifier, not an isolation
IX
t
I-IrQ
--
VON
OR-VOUT.,,"
VOUT
"'RI.
v-.
V,N
1IQ
amplifier.
~~,
.'
SENSE
Ira
4. Do not exceed the allowable input signal range. The line-
I
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:
This allows stable operation for load capacitances up to
6. Signals having frequency components above the Instrumen-
5. Use the compensation pin (pin 9) and the applicable com-
3000pF, but limits the slew rate to approximately 0.16VIJ,Ls;
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.
arity of the ADS21 decreases if the inputs are drivenwithin
used at a gain less than
uate the input signal through a resistive divider networkand
will be transmitted
5 volts of the supply rails, particularly when the deviceis
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.
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.
tation Amplifier's output amplifier closed-loop bandwidth
attenuation.
supply line to the output common or to pin 11.1
4. Set Cx to 1000pF if no compensation was originally
1. Reduce 680n to 24n
2. Reduce BOn to 7.5n
3. Increase 1000pF to O.IJ,LF
+IN
-IN
used. Otherwise, do not alter the original value.
-
Figure 2. Operating Connections for AD527
--
V+
Therefore, it is advisable to decouple the V-
from
1. To avoidthis possibility, a tten-
V-
to the output
OUTPUT
OUTPUT
SIGNAL
COMMON
-
with little or no
--
GAIN VALUE OF RO
0.1
1
10
100
1000
REV. A
1I0Il1
1(JOkS1
101<!J
1k!1
100!J
-
j
-