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AD8553 Datasheet(PDF) 20 Page - Analog Devices

Part No. AD8553
Description  Wide Supply Range, Micropower,
Download  28 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD8553 Datasheet(HTML) 20 Page - Analog Devices

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AD8420
Data Sheet
Rev. 0 | Page 20 of 28
GAIN ACCURACY
Unlike most instrumentation amplifiers, the relative match of
the two gain setting resistors determines the gain accuracy of
the AD8420 rather than a single resistor. For example, if two
resistors have exactly the same absolute error, there is no error
in gain. Conversely, two 1% resistors can cause approximately 2%
maximum gain error at high gains. Temperature coefficient
mismatch of the gain setting resistors increases the gain drift
of the instrumentation amplifier circuit. Because these external
resistors do not have to match any on-chip resistors, resistors
with good TC tracking can achieve excellent gain drift.
When the differential voltage at the inputs approaches the
differential input limit, the diodes start to conduct, limiting
the voltage seen by the inputs. This can look like increased gain
error at large differential inputs. Performance of the AD8420 is
specified for ±1 V differential from −40°C to +85°C. However,
at higher temperatures, the reduced forward voltage of the diodes
limits the differential input to a smaller voltage. Figure 60 tracks
1% error across the operating temperature range to show the
effect of temperature on the input limit.
–40 –25 –10
5
20
35
50
65
80
95
110 125
TEMPERATURE (°C)
0
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
POSITIVE VOLTAGE
NEGATIVE VOLTAGE
VS = ±15V
Figure 60. Differential Input Limit vs. Temperature
INPUT VOLTAGE RANGE
The allowed input range of the AD8420 is much simpler than
traditional architectures. For the transfer function of the AD8420
to be valid, the input voltage should follow two rules:
Keep the differential input voltage within ±1 V.
Keep the voltage on the +IN, −IN, REF, and FB pins in the
specified input voltage range.
Because the output swing is completely independent of the
input common-mode voltage, there are no hexagonal figures
or complicated formulas to follow, and no limitation for the
output swing the amplifier has for input signals with changing
common mode.
INPUT PROTECTION
The current into the AD8420 inputs is limited internally. This
ensures that the diodes that limit the differential voltage seen by
the internal amplifier do not draw excessive current when they
turn on. The part can handle large differential input voltages,
regardless of the amount of gain applied, without damage. As a
result, the AD8420 inputs are protected from voltages beyond
the positive rail. If voltages beyond the negative rail are expected,
external protection must be used.
Keep all of the AD8420 terminals within the voltage range specified
in the Absolute Maximum Ratings section. All terminals of the
AD8420 are protected against ESD.
Input Voltages Beyond the Rails
For applications that require protection beyond the negative rail,
one option is to use an external resistor in series with each input
to limit current during overload conditions. In this case, size the
resistors to limit the current into the AD8420 to 6 mA.
RPROTECT ≥ (Negative Supply − VIN)/6 mA
Although the AD8420 inputs must still be kept within the −VS +
40 V limitation, the I × R drop across the protection resistor
increases the protection on the positive side to approximately
(40 V + Negative Supply) + 300 μA × RPROTECT
An alternate protection method is to place diodes at the AD8420
inputs to limit voltage and resistors in series with the inputs to
limit the current into these diodes. To keep input bias current at
a minimum for normal operation, use low leakage diode clamps,
such as the BAV199. The AD8420 also combines well with TVS
diodes, such as the PTVSxS1UR.
SIMPLE METHOD
ALTERNATE METHOD
AD8420
RPROTECT
RPROTECT
RPROTECT
RPROTECT
VIN+
+
VIN–
VIN+
VIN–
+
+VS
AD8420
–VS
+VS
–VS
+VS
–VS
+VS
–VS
I
+
+
Figure 61. Protection for Voltages Beyond the Rails
Large Differential Input Voltage
The AD8420 is able to handle large differential input voltage
without damage to the part. Refer to Figure 9, Figure 10,
Figure 11, and Figure 12 for overvoltage performance. The
AD8420 differential voltage is internally limited with diodes to
±1 V. If this limit is exceeded, the diodes start to conduct and
draw current, as shown in Figure 22. This current is limited
internally to a value that is safe for the AD8420, but if the input
current cannot be tolerated in the system, place resistors in
series with each input with the following value:
MAX
DIFF
PROTECT
I
V
R
V
1
2
1


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