AD542 데이터 시트보기 (PDF) - Analog Devices

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AD542

High Performance/ BiFET Operational Amplifiers

Analog Devices 

AD542/AD544/AD547

BiFET Application Hints

APPLICATION NOTES

The BiFET series was designed for high performance op amp

applications that require true dc precision. To capitalize on all

of the performance available from the BiFETs there are some

practical error sources that should be considered.

The bias currents of JFET input amplifiers double with every

10°C increase in chip temperature. Therefore, minimizing the

junction temperature of the chip will result in extending the

performance limits of the device.

1. Heat dissipation due to power consumption is the main

contributor to self-heating and can be minimized by reducing

the power supplies to the lowest level allowed by the

application.

2. The effects of output loading should be carefully considered.

Greater power dissipation increases bias currents and de-

creases open loop gain.

current-to-voltage converting amplifier. This possibility necessi-

tates some form of input protection. Many electrometer type

devices, especially CMOS designs, can require elaborate Zener

protection schemes which often compromise overall perfor-

mance. The BiFET series requires input protection only if the

source is not current-limited, and as such is similar to many

JFET-input designs. The failure mode would be overheating

from excess current rather than voltage breakdown. If the

source is not current-limited, all that is required is a resistor in

series with the affected input terminal so that the maximum

overload current is 1.0 mA (for example, 100 kΩ for a 100 volt

overload). This simple scheme will cause no significant reduc-

tion in performance and give complete overload protection. Fig-

ure 30 shows proper connections.

GUARDING

The low input bias current (25 pA) and low noise characteristics

of the high performance BiFET op amp make it suitable for

electrometer applications such as photo diode preamplifiers and

picoampere current-to-voltage converters. The use of guarding

techniques in printed circuit board layout and construction is

critical for achieving the ultimate in low leakage performance

available from these amplifiers. The input guarding scheme

shown in Figure 29 will minimize leakage as much as possible;

the guard ring is connected to a low impedance potential at the

same level as the inputs. High impedance signal lines should not

be extended for any unnecessary length on a printed circuit.

Figure 29. Board Layout for Guarding Inputs

Figure 30. Input Protection

D/A CONVERTER APPLICATIONS

The BiFET series of operational amplifiers can be used with

CMOS DACs to perform both 2-quadrant and 4-quadrant

operation. The output impedance of a CMOS DAC varies with

the digital word, thus changing the noise gain of the amplifier

circuit. The effect will cause a nonlinearity the magnitude of

which is dependent on the offset voltage of the amplifier. The

BiFET series with trimmed offset will minimize this effect. Ad-

ditionally, the Schottky protection diodes recommended for use

with many older CMOS DACs are not required when using one

of the BiFET series amplifiers.

Figure 31a shows the AD547 and AD7541 configured for uni-

polar binary (2-quadrant multiplication) operation. With a dc

reference voltage or current (positive or negative polarity) ap-

plied at pin 17, the circuit operates as a unipolar converter.

With an ac reference voltage or current, the circuit provides

2-quadrant multiplication (digitally controlled attenuation).

INPUT PROTECTION

The BiFET series is guaranteed for a maximum safe input

potential equal to the power supply potential. The input stage

design also allows differential input voltages of up to ± 1 volt

while maintaining the full differential input resistance of 1012 Ω.

This makes the BiFET series suitable for comparator situations

employing a direct connection to high impedance source.

Many instrumentation situations, such as flame detectors in gas

chromatographs, involve measurement of low level currents

from high-voltage sources. In such applications, a sensor fault

condition may apply a very high potential to the input of the

Figure 31a. AD547 Used as DAC Output Amplifier

–8–

REV. B

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