AD547 Analog Devices, AD547 Datasheet - Page 8

• Manufacturer Part Number: AD547
• Description: High Performance/ BiFET Operational Amplifiers
• Manufacturer: 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
2. The effects of output loading should be carefully considered.
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.
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 10
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
contributor to self-heating and can be minimized by reducing
the power supplies to the lowest level allowed by the
application.
Greater power dissipation increases bias currents and de-
creases open loop gain.
Figure 29. Board Layout for Guarding Inputs
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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 k 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.
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).
Figure 31a. AD547 Used as DAC Output Amplifier
Figure 30. Input Protection
REV. B