AMP01
HEATSINKING
External series resistors could 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.
To maintain high reliability, the die temperature of any IC
should be kept as low as practicable, preferably below 100°C.
Although most AMP01 application circuits will produce very
little internal heat — little more than the quiescent dissipation
of 90 mW—some circuits will raise that to several hundred
milliwatts (for example, the 4-20 mA current transmitter appli-
cation, Figure 37). Excessive dissipation will cause thermal
shutdown of the output stage thus protecting the device from
damage. A heatsink is recommended in power applications to
reduce the die temperature.
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
Zeners.
Several appropriate heatsinks are available; the Thermalloy
6010B is especially easy to use and is inexpensive. Intended for
dual-in-line packages, the heatsink may be attached with a
cyanoacrylate adhesive. This heatsink reduces the thermal resis-
tance between the junction and ambient environment to ap-
proximately 80°C/W. Junction (die) temperature can then be
calculated by using the relationship:
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 35).
Input components, whether multiplexers or resistors, should be
carefully selected to prevent the formation of thermocouple
junctions that would degrade the input signal.
TJ – TA
θJA
Pd =
where TJ and TA are the junction and ambient temperatures
respectively, θJA is the thermal resistance from junction to ambi-
ent, and Pd is the device’s internal dissipation.
OPTIONAL PROTECTION
RESISTORS, SEE TEXT.
LINEAR INPUT RANGE,
؎5V MAXIMUM
*
+15V
AMP01
–15V
DIFFERENTIAL PROTECTION
TO ؎30V
100⍀
1W*
OVERVOLTAGE PROTECTION
+IN
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 trans-
ducer, or removal of the amplifier power supply while the signal
source is connected may 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.
9.1V 1W
ZENERS
V
OUT
100⍀
1W*
–IN
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 over-
load protection is used so that noise performance would not be
significantly degraded.
Figure 35. Input Overvoltage Protection for Gains
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, not an uncommon
value, will produce a 10 µV input offset change. Consequently,
care should be taken in choosing a power unit that has a low
output noise level, good line and load regulation, and good
temperature stability.
AMP01 noise level approaches the theoretical noise floor of the
input stage which would be 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 transis-
tors contribute less than 1 nV/√Hz of noise. The measured noise
level is typically 5 nV/√Hz.
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 would degrade
the IB and IOS specifications. Decreasing the value of RG for
gains above 100 limits the maximum input overload protection
to ±10 V.
REV. D
–15–
ADI [ ADI ]
