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AD648 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer AD648
Beschreibung Low Power BiFET Op Amp
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 13 Seiten
AD648 Datasheet, Funktion
a
Dual Precision,
Low Power BiFET Op Amp
FEATURES
DC Performance
400 A max Quiescent Current
10 pA max Bias Current, Warmed Up (AD648B)
1 V max Offset Voltage (AD648B)
10 V/؇C max Drift (AD648B)
2 V p-p Noise, 0.1 Hz to 10 Hz
AC Performance
1.8 V/s Slew Rate
1 MHz Unity Gain Bandwidth
Available in Plastic Mini-DIP, CERDIP, and Plastic SOIC
Packages
MIL-STD-883B Parts Available
Surface Mount (SOIC) Package Available in Tape and
Reel in Accordance with EIA-481A Standard
Single Version: AD548
AD648
CONNECTION DIAGRAM
Plastic Mini-Dip (N) Package,
Plastic SOIC (R) Package
and
CERDIP (Q) Package
PRODUCT DESCRIPTION
The AD648 is a matched pair of low power, precision mono-
lithic operational amplifiers. It offers both low bias current
(10 pA max, warmed up) and low quiescent current (400 µA
max) and is fabricated with ion-implanted FET and laser wafer
trimming technologies. Input bias current is guaranteed over the
AD648’s entire common-mode voltage range.
The economical J grade has a maximum guaranteed offset
voltage of less than 2 mV and an offset voltage drift of less than
20 µV/°C. This level of dc precision is achieved using Analog’s
laser wafer drift trimming process. The combination of low
quiescent current and low offset voltage drift minimizes changes
in input offset voltage due to self-heating effects. Five grades are
offered over the commercial, industrial and military temperature
ranges.
The AD648 is recommended for any dual supply op amp
application requiring low power and excellent dc and ac per-
formance. In applications such as battery-powered, precision
instrument front ends and CMOS DAC buffers, the AD648’s
excellent combination of low input offset voltage and drift, low
bias current, and low 1/f noise reduces output errors. High
common-mode rejection (82 dB, min on the “B” grade) and
high open-loop gain ensures better than 12-bit linearity in high
impedance, buffer applications.
The AD648 is pinned out in a standard dual op amp configura-
tion and is available in seven performance grades. The AD648J
and AD648K are rated over the commercial temperature range
of 0°C to 70°C. The AD648 and AD648B are rated over the
industrial temperature range of –40°C to +85°C. The AD648S
and AD648T are rated over the military temperature range of
–55°C to +125°C and the AD648T* grade is available pro-
cessed to MIL-STD-883B, Rev. C.
The AD648 is available in an 8-lead plastic mini-DIP,
CERDIP, and SOIC.
*Not for new design, obsolete April 2002.
PRODUCT HIGHLIGHTS
1. A combination of low supply current, excellent dc and ac
performance and low drift makes the AD648 the ideal op
amp for high performance, low power applications.
2. The AD648 is pin compatible with industry standard dual
op amps such as the LF442, TL062, and AD642, enabling
designers to improve performance while achieving a reduc-
tion in power dissipation of up to 85%.
3. Guaranteed low input offset voltage (2 mV max) and drift
(20 µV/°C max) for the AD648J are achieved using Analog
Devices’ laser drift trimming technology.
4. Analog Devices specifies each device in the warmed-up
condition, insuring that the device will meet its published
specifications in actual use.
5. Matching characteristics are excellent for all grades. The
input offset voltage matching between amplifiers in the
AD648J is within 2 mV.
6. Crosstalk between amplifiers is less than –120 dB at 1 kHz.
REV. E
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties that
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 2002






AD648 Datasheet, Funktion
Typical Performance Characteristics—AD648
REV. E
–5–

6 Page









AD648 pdf, datenblatt
AD648
LOG RATIO AMPLIFIER
Log ratio amplifiers are useful for a variety of signal conditioning
applications, such as linearizing exponential transducer outputs
and compressing analog signals having a wide dynamic range.
The AD648’s picoamp level input current and low input offset
voltage make it a good choice for the front end amplifier of the
log ratio circuit shown in Figure 30. This circuit produces an
output voltage equal to the log base 10 of the ratio of the input
currents I1 and I2. Resistive inputs R1 and R2 are provided
for voltage inputs.
Input currents I1 and I2 set the collector currents of Q1 and Q2,
a matched pair of logging transistors. Voltages at points A and B
are developed according to the following familiar diode equation:
VBE = (kT/q) ln (IC/IES)
In this equation, k is Boltzmann’s constant, T is absolute
temperature, q is an electron charge, and IES is the reverse
saturation current of the logging transistors. The difference of
these two voltages is taken by the subtractor section and scaled
by a factor of approximately 16 by resistors R9, R10, and R8.
Temperature compensation is provided by resistors R8 and
R15,
which have a positive 3500 ppm/°C temperature coefficient.
The transfer function for the output voltage is:
VOUT = 1 V log10 (I2/I1)
Frequency compensation is provided by R11, R12, C1, and C2.
Small signal bandwidth is approximately 300 kHz at input cur-
rents above 100 µA and will proportionally decrease with lower
signal levels. D1, D2, R13, and R14 compensate for the effects
of the two logging transistors’ ohmic emitter resistance.
To trim this circuit, set the two input currents to 10 µA and
adjust VOUT to zero by adjusting the potentiometer on A3. Then
set I2 to 1 µA and adjust the scale factor such that the output
voltage is 1 V by trimming potentiometer R10. Offset adjust-
ment for A1 and A2 is provided to increase the accuracy of the
voltage inputs.
This circuit ensures a 1% log conformance error over an input
current range of 300 pA to l mA, with low level accuracy limited
by the AD648’s input current. The low level input voltage accu-
racy of this circuit is limited by the input offset voltage and drift
of the AD648.
REV. E
Figure 30. Precision Log Ratio Amplifier
–11–

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