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AMP01 Datasheet PDF : 30 Pages
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AMP01
HEATSINKING
To maintain high reliability, the die temperature of any IC must
be kept as low as practicable, preferably below 100°C. Although
most AMP01 application circuits produce very little internal
heat—little more than the quiescent dissipation of 90 mW—
some circuits raise that to several hundred milliwatts (for
example, the 4 mA to 20 mA current transmitter application;
see Figure 40). Excessive dissipation causes thermal shutdown
of the output stage, thus protecting the device from damage. A
heatsink is recommended in power applications to reduce the
die temperature.
Several appropriate heatsinks are available; the Thermalloy
6010B is especially easy to use and is inexpensive. Intended for
dual-in-line packages, the heatsink can be attached with a
cyanoacrylate adhesive. This heatsink reduces the thermal
resistance between the junction and ambient environment to
approximately 80°C/W. Junction (die) temperature can then be
calculated by using the following relationship:
Pd
TJ TA
θ JA
where:
Pd is the internal dissipation of the device.
TJ is the junction temperature.
TA is the ambient temperature.
θJA is the thermal resistance from junction to ambient.
OVERVOLTAGE PROTECTION
Instrumentation amplifiers invariably sit at the front end of
instrumentation systems where there is a high probability of
exposure to overloads. Voltage transients, failure of a
transducer, or removal of the amplifier power supply while the
signal source is connected can destroy or degrade the
performance of an unprotected amplifier. Although it is
impractical to protect an IC internally against connection to
power lines, it is relatively easy to provide protection against
typical system overloads.
The AMP01 is internally protected against overloads for gains
of up to 100. At higher gains, the protection is reduced and
some external measures may be required. Limited internal
overload protection is used so that noise performance is not
significantly degraded.
AMP01 noise level approaches the theoretical noise floor of the
input stage, which is 4 nV/√Hz at 1 kHz when the gain is set at
1000. Noise is the result of shot noise in the input devices and
Johnson noise in the resistors. Resistor noise is calculated from
the values of RG (200 Ω at a gain of 1000) and the input protection
resistors (250 Ω). Active loads for the input transistors contribute
less than 1 nV/√Hz of noise. The measured noise level is typically
5 nV/√Hz.
Data Sheet
Diodes across the input transistor’s base-emitter junctions,
combined with 250 Ω input resistors and RG, protect against
differential inputs of up to ±20 V for gains of up to 100. The
diodes also prevent avalanche breakdown that degrade the IB
and IOS specifications. Decreasing the value of RG for gains above
100 limits the maximum input overload protection to ±10 V.
External series resistors can be added to guard against higher
voltage levels at the input, but resistors alone increase the input
noise and degrade the signal-to-noise ratio, especially at high
gains.
Protection can also be achieved by connecting back to back
9.1 V Zener diodes across the differential inputs. This technique
does not affect the input noise level and can be used down to a
gain of 2 with minimal increase in input current. Although
voltage-clamping elements look like short circuits at the
limiting voltage, the majority of signal sources provide less than
50 mA, producing power levels that are easily handled by low
power Zener diodes.
Simultaneous connection of the differential inputs to a low
impedance signal above 10 V during normal circuit operation is
unlikely. However, additional protection involves adding 100 Ω
current-limiting resistors in each signal path prior to the voltage
clamp, the resistors increase the input noise level to just
5.4 nV/√Hz (refer to Figure 38).
Input components, whether multiplexers or resistors, should be
carefully selected to prevent the formation of thermocouple
junctions that would degrade the input signal.
100Ω
1W*
+IN
+15V
LINEAR INPUT RANGE,
±5V MAXIMUM
DIFFERENTIAL PROTECTION
TO ±30V
100Ω
1W*
–IN
9.1V 1W
ZENERS
AMP01
VOUT
*OPTIONAL PROTECTION
RESISTORS, SEE TEXT.
–15V
Figure 38. Input Overvoltage Protection for Gains of 2 to 10,000
POWER SUPPLY CONSIDERATIONS
Achieving the rated performance of precision amplifiers in a
practical circuit requires careful attention to external influences.
For example, supply noise and changes in the nominal voltage
directly affect the input offset voltage. A PSR of 80 dB means
that a change of 100 mV on the supply produces a 10 μV input
offset change. Consequently, care must be taken in choosing a
power source with low output noise, good line and load
regulation, and good temperature stability.
Rev. E | Page 22 of 29
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