AMP01EX datasheet(23/30 Pages) AD

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AMP01

Data Sheet

Rev. E | Page 22 of 29

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:





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.

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.

VOUT

+15V

+IN

–IN

AMP01

9.1V 1W

ZENERS

100Ω

RESISTORS, SEE TEXT.

*OPTIONAL PROTECTION

LINEAR INPUT RANGE,

±5V MAXIMUM

DIFFERENTIAL PROTECTION

TO ±30V

–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.

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