AMP01AXSLASH883C datasheet(15/22 Pages) AD

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AMP01

–15–

REV. D

HEATSINKING

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.

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:

Pd =

T J – T A

where TJ and TA are the junction and ambient temperatures

respectively,

JA is the thermal resistance from junction to ambi-

ent, and Pd is the device’s internal dissipation.

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 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.

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.

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 R

G, 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

±10 V.

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.

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.

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.

VOUT

+15V

+IN

–IN

AMP01

9.1V 1W

ZENERS

100

OPTIONAL PROTECTION

RESISTORS, SEE TEXT.

LINEAR INPUT RANGE,

5V MAXIMUM

DIFFERENTIAL PROTECTION

30V

–15V

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.

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