AD AD548 Precision, low power bifet op amp Datasheet

a
FEATURES
Enhanced Replacement for LF441 and TL061
DC Performance:
200 A max Quiescent Current
10 pA max Bias Current, Warmed Up (AD548C)
250 V max Offset Voltage (AD548C)
2 V/C max Drift (AD548C)
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 and Hermetic Metal Can Packages
and in Chip Form
Available in Tape and Reel in Accordance with
EIA-481A Standard
MIL-STD-883B Parts Available
Dual Version Available: AD648
Surface-Mount (SOIC) Package Available
Precision, Low Power
BiFET Op Amp
AD548
CONNECTION DIAGRAMS
Plastic Mini-DIP (N) Package
and
SOIC (R)Package
OFFSET NULL 1
8 NC
INVERTING 2
INPUT
NONINVERTING 3
INPUT
V– 4
7 V+
The economical J grade has a maximum guaranteed input offset
voltage of less than 2 mV and an input offset voltage drift of less
than 20 µV/°C. This level of dc precision is achieved utilizing
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.
The AD548 is recommended for any dual supply op amp application requiring low power and excellent dc and ac performance.
In applications such as battery-powered, precision instrument
front ends and CMOS DAC buffers, the AD548’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 AD548 is pinned out in a standard op amp configuration
and is available in three performance grades. The AD548J and
AD548K are rated over the commercial temperature range of
0°C to 70°C. The AD548B is rated over the industrial temperature range of –40°C to +85°C.
AD548
TOP VIEW
5 OFFSET
NULL
NOTE: PIN 4 CONNECTED TO CASE
NC = NO CONNECT
10k
5
1
4 –15V
PRODUCT DESCRIPTION
The AD548 is a low power, precision monolithic operational
amplifier. It offers both low bias current (10 pA max, warmed
up) and low quiescent current (200 µA max) and is fabricated
with ion-implanted FET and laser wafer trimming technologies.
Input bias current is guaranteed over the AD548’s entire
common-mode voltage range.
6 OUTPUT
VOS TRIM
TOP VIEW
PRODUCT HIGHLIGHTS
1. A combination of low supply current, excellent dc and ac
performance and low drift makes the AD548 the ideal op
amp for high performance, low power applications.
2. The AD548 is pin compatible with industry standard op
amps such as the LF441, TL061, and AD542, enabling
designers to improve performance while achieving a reduction
in power dissipation of up to 85%.
3. Guaranteed low input offset voltage (2 mV max) and drift
(20 µV/°C max) for the AD548J are achieved utilizing
Analog Devices’ laser drift trimming technology, eliminating
the need for external trimming.
4. Analog Devices specifies each device in the warmed-up
condition, insuring that the device will meet its published
specifications in actual use.
5. A dual version, the AD648, is also available.
6. Enhanced replacement for LF441 and TL061.
The AD548 is available in an 8-lead plastic mini-DIP and
surface-mount (SOIC) packages.
REV. D
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
AD548–SPECIFICATIONS (@ 25C and V = 15 V dc unless otherwise noted.)
S
Parameter
INPUT OFFSET VOLTAGE1
Initial Offset
TMIN to TMAX
vs. Temperature
vs. Supply
vs. Supply, TMIN to TMAX
Long-Term Offset Stability
INPUT BIAS CURRENT
Either Input2, VCM = 0
Either Input2 at TMAX, VCM = 0
Max Input Bias Current Over
Common-Mode Voltage Range
Offset Current, VCM = 0
Offset Current at TMAX
INPUT IMPEDANCE
Differential
Common Mode
INPUT VOLTAGE RANGE
Differential3
Common Mode
Common-Mode Rejection
VCM = ± 10 V
TMIN to TMAX
VCM = ± 11 V
TMIN to TMAX
INPUT VOLTAGE NOISE
Voltage 0.1 Hz to 10 Hz
f = 10 Hz
f = 100 Hz
f = 1 kHz
f = 10 kHz
INPUT CURRENT NOISE
f = 1 kHz
FREQUENCY RESPONSE
Unity Gain, Small Signal
Full Power Response
Slew Rate, Unity Gain
Settling Time to ± 0.01%
OPEN LOOP GAIN
VO = ± 10 V, RL ≥ 10 kΩ
TMIN to TMAX, RL ≥ 10 kΩ
VO = ± 10 V, RL ≥ 5 kΩ
TMIN to TMAX, RL ≥ 5 kΩ
OUTPUT CHARACTERISTICS
Voltage @ RL ≥ 10 kΩ,
TMIN to TMAX
Voltage @ RL ≥ 5 kΩ,
TMIN to TMAX
Short Circuit Current
Min
AD548J
Typ
Max
0.75
Min
2.0
3.0/3.0/3.0
20
80
76/76/76
AD548K/B
Typ
0.3
Max
Unit
0.5
0.7/0.8
5
mV
mV
µV/°C
dB
dB
µV/Month
10
0.25/0.65
pA
nA
15
5
0.15/0.35
pA
pA
nA
86
80
15
15
5
20
0.45/1.3/20
3
30
10
0.25/0.65/10
5
2
1 × 1012储3
3 × 1012储3
1 × 1012储3
3 × 1012储3
Ω储pF
Ω储pF
± 11
± 20
± 12
± 11
± 20
± 12
V
V
76
76/76/76
70
70/70/70
90
90
84
84
82
82
76
76
92
92
86
86
dB
dB
dB
dB
2
80
40
30
30
2
80
40
30
30
µV p-p
nV/√Hz
nV/√Hz
nV/√Hz
nV/√Hz
1.8
1.8
fA/√Hz
1.0
30
1.8
8
MHz
kHz
V/µs
µs
300
300
150
150
1000
700
500
300
3V/mV
V/mV
V/mV
V/mV
± 12
± 12
± 11
± 11
± 13
V
± 12.3
V
15
mA
0.8
1.0
300
300/300/300
150
150/150/150
1.0
30
1.8
8
0.8
1.0
1000
700
500
300
± 12
± 13
± 12/± 12/± 12
± 11
± 12.3
± 11/± 11/± 11
15
–2–
REV. D
AD548
SPECIFICATIONS (continued)
Min
POWER SUPPLY
Rated Performance
Operating Range
Quiescent Current
TEMPERATURE RANGE
Operating, Rated Performance
Commercial (0°C to 70°C)
Industrial (–40°C to +85°C)
Military (–55°C to +125°C)
PACKAGE OPTIONS
SOIC (R-8)
Plastic (N-8)
Tape and Reel
± 4.5
AD548J
Typ
± 15
170
Max
Min
± 18
200
± 4.5
AD548J
AD548A
AD548S
AD548K/B
Typ
± 15
170
Max
Unit
± 18
200
V
V
µA
AD548K
AD548B
AD548KR4
AD548KN
AD548KR-REEL4
AD548JR
AD548JN4
AD548JR-REEL
NOTES
1
Input Offset Voltage specifications are guaranteed after five minutes of operation at TA = 25°C.
2
Bias Current specifications are guaranteed maximum at either input after five minutes of operation at TA = 25°C. For higher temperature, the current doubles every 10°C.
3
Defined as voltages between inputs, such that neither exceeds ± 10 V from ground.
4
Not recommended for new designs; obsolete April 2002.
Specifications subject to change without notice.
REV. D
–3–
AD548
ABSOLUTE MAXIMUM RATINGS l
NOTES
1
Stresses above those listed under Absolute Maximum Ratings may cause
permanent damage to the device. This is a stress rating only; functional operation
of the device at these or any other conditions above those indicated in the
operational sections of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect device reliability.
2
Thermal Characteristics: 8-Pin SOIC Package: θJA = 160°C/W, θJC = 42°C/W;
8-Lead Plastic Package: θJA = 90°C/W.
3
For supply voltages less than ± 18 V, the absolute maximum input voltage is equal
to the supply voltage.
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 18 V
Internal Power Dissipation2 . . . . . . . . . . . . . . . . . . . . 500 mW
Input Voltage3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 18 V
Output Short Circuit Duration . . . . . . . . . . . . . . . . . Indefinite
Differential Input Voltage . . . . . . . . . . . . . . . . . . +VS and –VS
Storage Temperature Range (Q, H) . . . . . . . –65°C to +150°C
(N, R) . . . . . . . . –65°C to +125°C
Operating Temperature Range
AD548J/K . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to 70°C
AD548B . . . . . . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C
Lead Temperature Range (Soldering 60 sec) . . . . . . . . . 300°C
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD548 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high-energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
–4–
WARNING!
ESD SENSITIVE DEVICE
REV. D
Typical Performance Characteristics– AD548
20
20
30
INPUT VOLTAGE – ±V
15
+VIN
10
–VIN
5
15
–VOUT
10
25°C
RL = 10k
5
0
0
0
5
10
15
SUPPLY VOLTAGE – V
OUTPUT VOLTAGE SWING – V p-p
OUTPUT VOLTAGE SWING – ±V
+VOUT
20
0
5
10
15
SUPPLY VOLTAGE – V
25
20
15
10
5
0
20
TPC 2. Output Voltage Swing
vs. Supply Voltage
TPC 1. Input Voltage Range
vs. Supply Voltage
100
1k
LOAD RESISTANCE –
10k
TPC 3. Output Voltage Swing
vs. Load Resistance
10
200
10
100nA
180
160
140
INPUT BIAS CURRENT
INPUT BIAS CURRENT – pA
QUIESCENT CURRENT – µA
10nA
8
6
4
1nA
100pA
10pA
1pA
2
100fA
0
0
5
10
15
SUPPLY VOLTAGE – V
20
10fA
0
4
8
12
16
SUPPLY VOLTAGE – V
–55
20
TPC 5. Input Bias Current
vs. Supply Voltage
10
30
1500
25
1250
OPEN LOOP GAIN – V/mV
8
20
6
4
15
10
2
0
–10
5
0
–6
–2
2
6
COMMON-MODE VOLTAGE – V
TPC 7. Input Bias Current vs.
Common-Mode Voltage
REV. D
10
–25
5
35
65
TEMPERATURE – °C
95
TPC 6. Input Bias Current vs.
Temperature
TPC 4. Quiescent Current vs.
Supply Voltage
I∆VOSI – V
INPUT BIAS CURRENT – pA
120
RL = 10k
1000
750
500
250
0
10
20
30
40
50
60
WARM-UP TIME – Sec
TPC 8. Change in Offset Voltage
vs. Warm-Up Time
–5–
70
0
–55
–25
5
35
65
95
TEMPERATURE – °C
TPC 9. Open-Loop Gain vs.
Temperature
125
125
AD548
GAIN
40
40
20
20
0
0
–20
–20
–40
1k
10k
100k
1M
120
POWER SUPPLY REJECTION – dB
60
60
PHASE IN DEGREES
80
OPEN LOOP VOLTAGE GAIN – dB
PHASE
80
OPEN LOOP GAIN – dB
120
100
100
110
100
90
80
70
–40
10M
60
0
2
4
8
10
12
14
16
80
60
40
–SUPPLY
20
0
–20
18
100
1k
10k
100k
FREQUENCY – Hz
SUPPLY VOLTAGE – V
FREQUENCY – Hz
TPC 10. Open-Loop Frequency
Response
TPC 11. Open-Loop Voltage Gain
vs. Supply Voltage
90
10
10mV
OUTPUT VOLTAGE – V p-p
60
50
40
30
OUTPUT VOLTAGE SWING – V
20
70
18
16
14
12
10
8
6
4
1mV
5
0
–5
1mV
2
20
1k
10k
100k
FREQUENCY – Hz
10
100
1k
10k
100k
–10
1M
0
0.1
0.01
UNITY GAIN
FOLLOWER
0.001
1k
10k
FREQUENCY – Hz
TPC 16. Total Harmonic
Distortion vs. Frequency
100k
140
INPUT NOISE VOLTAGE – µV p-p
INPUT NOISE VOLTAGE – nV/√Hz
FOLLOWER
WITH GAIN = 10
120
100
80
60
40
20
0
10
100
1k
10k
FREQUENCY – Hz
TPC 17. Input Noise Voltage
Spectral Density
–6–
6
8
TPC 15. Output Swing and Error
Voltage vs. Output Settling Time
160
1
4
SETTLING TIME – µs
TPC 14. Large Signal Frequency
Response
4
100
2
FREQUENCY – Hz
TPC 13. CMRR vs. Frequency
TOTAL HARMONIC DISTORTION – %
10mV
0
1M
1M
TPC 12. PSRR vs. Frequency
22
80
CMRR – dB
6
+SUPPLY
100
100k
WHENEVER JOHNSON NOISE IS GREATER THAN
AMPLIFIER NOISE, AMPLIFIER NOISE CAN BE
CONSIDERED NEGLIGIBLE FOR APPLICATION
10,000
1,000
1kHz BANDWIDTH
RESISTOR JOHNSON
NOISE
100
10
10Hz
BANDWIDTH
1
AMPLIFIER GENERATED NOISE
0
100k
1M
10M
100M 1G
10G
SOURCE IMPEDANCE – 100G
TPC 18. Total Noise vs. Source
Impedance
REV. D
AD548
TPC 19a. Unity Gain Follower
TPC 19b. Unity Gain Follower
Pulse Response (Large Signal)
TPC 19c. Unity Gain Follower
Pulse Response (Small Signal)
TPC 20a. Utility Gain Inverter
TPC 20b. Utility Gain Inverter
Pulse Response (Large Signal)
TPC 20c. Unity Gain Inverter
Pulse Response (Small Signal)
APPLICATION NOTES
The AD548 is a JFET-input op amp with a guaranteed maximum IB of less than 10 pA, and offset and drift laser-trimmed to
0.5 mV and 5 µV/°C, respectively (AD548B). AC specs include
1 MHz bandwidth, 1.8 V/µs typical slew rate and 8 µs settling time
for a 20 V step to ± 0.01%—all at a supply current less than
200 µA. To capitalize on the device’s performance, a number of
error sources should be considered.
The minimal power drain and low offset drift of the AD548
reduce self-heating or “warm-up” effects on input offset voltage,
making the AD548 ideal for on/off battery-powered applications. The power dissipation due to the AD548’s 200 µA supply
current has a negligible effect on input current, but heavy output loading will raise the chip temperature. Since a JFET’s
input current doubles for every 10°C rise in chip temperature,
this can be a noticeable effect.
The amplifier is designed to be functional with power supply
voltages as low as ± 4.5 V. It will exhibit a higher input offset
voltage than at the rated supply voltage of ± 15 V, due to power
supply rejection effects. The common-mode range of the AD548
extends from 3 V more positive than the negative supply to 1 V
more negative than the positive supply. Designed to cleanly
drive up to 10 kΩ and 100 pF loads, the AD548 will drive a 2
kΩ load with reduced open-loop gain.
OFFSET NULLING
Unlike bipolar input amplifiers, zeroing the input offset voltage
of a BiFET op amp will not minimize offset drift. Using balance
Pins 1 and 5 to adjust the input offset voltage as shown in
Figure 1 will induce an added drift of 0.24 µV/°C per 100 µV of
nulled offset. The low initial offset (0.5 mV) of the AD548B
results in only 0.6 µV/°C of additional drift.
REV. D
Figure 1. Offset Null Configuration
LAYOUT
To take full advantage of the AD548’s 10 pA max input current,
parasitic leakages must be kept below an acceptable level. The
practical limit of the resistance of epoxy or phenolic circuit
board material is between 1 × 1012 Ω and 3 × 1012 Ω. This can
result in an additional leakage of 5 pA between an input of 0 V
and a –15 V supply line. Teflon® or a similar low leakage material (with a resistance exceeding 1017 Ω) should be used to
isolate high impedance input lines from adjacent lines carrying
high voltages. The insulator should be kept clean, since contaminants will degrade the surface resistance.
A metal guard completely surrounding the high impedance nodes
and driven by a voltage near the common-mode input potential
can also be used to reduce some parasitic leakages. The guarding
pattern in Figure 2 will reduce parasitic leakage due to finite
board surface resistance; but it will not compensate for a low
volume resistivity board.
Teflon is a registered trademark of DuPont.
–7–
AD548
Figure 2. Board Layout for Guarding Inputs
INPUT PROTECTION
The AD548 is guaranteed to withstand input voltages equal to
the power supply potential. Exceeding the negative supply voltage on either input will forward bias the substrate junction of
the chip. The induced current may destroy the amplifier due to
excess heat.
Figure 4. AD548 Used as DAC Output Amplifier
That is:
Input protection is required in applications such as a flame
detector in a gas chromatograph, where a very high potential
may be applied to the input terminals during a sensor fault
condition. Figure 3 shows a simple current limiting scheme that
can be used. RPROTECT should be chosen such that the maximum overload current is 1.0 mA (l00 kΩ for a 100 V overload,
for example).
RFB is the feedback resistor for the op amp, which is internal to
the DAC. RO is the DAC’s R-2R ladder output resistance. The
value of RO is code dependent. This has the effect of changing
the offset error voltage at the amplifier’s output. An output
amplifier with a sub millivolt input offset voltage is needed to
preserve the linearity of the DAC’s transfer function.
 R 
VOS Output =VOS Input 1+ FB 
RO 

The AD548 in this configuration provides a 700 kHz small
signal bandwidth and 1.8 V/µs typical slew rate. The 33 pF
capacitor across the feedback resistor optimizes the circuit’s
response. The oscilloscope charts in Figures 5 and 6 show small
and large signal outputs of the circuit in Figure 4. Upper traces
show the input signal VIN. Lower traces are the resulting output
voltage with the DAC’s digital input set to all 1s. The AD548
settles to ± 0.01% for a 20 V input step in 14 µs.
Exceeding the negative common-mode range on either input
terminal causes a phase reversal at the output, forcing the
amplifier output to the corresponding high or low state. Exceeding the negative common-mode on both inputs simultaneously
forces the output high. Exceeding the positive common-mode
range on a single input does not cause a phase reversal, but if
both inputs exceed the limit the output will be forced high. In
all cases, normal amplifier operation is resumed when input
voltages are brought back within the common-mode range.
5V
20V
5µS
100
90
10
0%
Figure 5. Response to ± 20 V p-p Reference Square Wave
Figure 3. Input Protection of IV Converter
50mV
200mV
2µS
100
90
D/A CONVERTER OUTPUT BUFFER
The circuit in Figure 4 shows the AD548 and AD7545 12-bit
CMOS D/A converter in a unipolar binary configuration. VOUT
will be equal to VREF attenuated by a factor depending on the
digital word. VREF sets the full scale. Overall gain is trimmed by
adjusting RIN. The AD548’s low input offset voltage, low drift,
and clean dynamics make it an attractive low power output buffer.
10
0%
The input offset voltage of the AD548 output amplifier results
in an output error voltage. This error voltage equals the input
offset voltage of the op amp times the noise gain of the amplifier.
Figure 6. Response to ±100 mV p-p Reference Square Wave
–8–
REV. D
Application Hints–AD548
PHOTODIODE PREAMP
The performance of the photodiode preamp shown in Figure 7
is enhanced by the AD548’s low input current, input voltage
offset, and offset voltage drift. The photodiode sources a current
proportional to the incident light power on its surface. RF converts
the photodiode current to an output voltage equal to RF × IS.
Figure 7.
An error budget illustrating the importance of low amplifier
input current, voltage offset, and offset voltage drift to minimize
output voltage errors can be developed by considering the equivalent circuit for the small (0.2 mm2 area) photodiode shown in
Figure 7. The input current results in an error proportional to
the feedback resistance used. The amplifier’s offset will produce
an error proportional to the preamp’s noise gain (I + RF/RSH),
where RSH is the photodiode shunt resistance. The amplifier’s
input current will double with every 10°C rise in temperature,
and the photodiode’s shunt resistance halves with every 10°C
rise. The error budget in Figure 8 assumes a room temperature
photodiode RSH of 500 MΩ, and the maximum input current
and input offset voltage specs of an AD548C.
TEMP
C
–25
0
25
50
75
85
RSH (M)
VOS (V) (1+ RF/RSH) VOS
15,970
2,830
500
88.5
15.6
7.8
150
200
250
300
350
370
151 µV
207 µV
300 µV
640 µV
2.6 mV
5.1 mV
IB (pA)
IBRF
TOTAL
0.30
2.26
10.00
56.6
320
640
30 µV
262 µV
1.0 mV
5.6 mV
32 mV
64 mV
181 µV
469 µV
1.30 mV
6.24 mV
34.6 mV
69.1 mV
Figure 8. Photodiode Preamp Errors Over Temperature
The capacitance at the amplifier’s negative input (the sum of the
photodiode’s shunt capacitance, the op amp’s differential input
capacitance, stray capacitance due to wiring, etc.) will cause a
rise in the preamp’s noise gain over frequency. This can result in
excess noise over the bandwidth of interest. CF reduces the
noise gain “peaking” at the expense of bandwidth.
INSTRUMENTATION AMPLIFIER
The AD548C’s maximum input current of 10 pA makes it an
excellent building block for the high input impedance instrumentation amplifier shown in Figure 9. Total current drain for
this circuit is under 600 µA. This configuration is optimal for
conditioning differential voltages from high impedance sources.
The overall gain of the circuit is controlled by RG, resulting in
the following transfer function:
VOUT
VIN
REV. D
=1+
(R1 + R2 )
RG
Figure 9. Low Power Instrumentation Amplifier
Gains of 1 to 100 can be accommodated with gain nonlinearities
of less than 0.01%. Input errors, which contribute an output
error proportional to in amp gain, include a maximum untrimmed
input offset voltage of 0.5 mV and an input offset voltage drift
over temperature of 4 µV/°C. Output errors, which are independent of gain, will contribute an additional 0.5 mV offset and
4 µV/°C drift. The maximum input current is 15 pA over the
common-mode range, with a common-mode impedance of over
1 × 1012 Ω. Resistor pairs R3/R5 and R4/R6 should be ratio
matched to 0.01% to take full advantage of the AD548’s high
common-mode rejection. Capacitors C1 and C1′ compensate for
peaking in the gain over frequency caused by input capacitance
when gains of 1 to 3 are used.
The –3 dB small signal bandwidth for this low power instrumentation amplifier is 700 kHz for a gain of 1 and 10 kHz for a gain of
100. The typical output slew rate is 1.8 V/µs.
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 AD548’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 10. 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:
V BE = (kT/q) ln (I C /I ES )
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
–9–
AD548
compensation is provided by resistors R8 and R15 that have a
positive 3500 ppm/°C temperature coefficient. The transfer
function for the output voltage is:
VOUT = 1V log 10 (I 2 / I 1 )
Frequency compensation is provided by R11, R12, C1, and C2.
Small signal bandwidth is approximately 300 kHz at input currents 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 adjustment 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 1 mA, with low level accuracy
limited by the AD548’s input current. The low level input voltage
accuracy of this circuit is limited by the input offset voltage and
drift of the AD548.
Figure 10. Log Ratio Amplifier
–10–
REV. D
AD548
OUTLINE DIMENSIONS
Plastic Mini-DIP (N) Package
SOIC (R) Package
Dimensions shown in inches and (millimeters)
Dimensions shown in millimeters and (inches)
5.00 (0.1968)
4.80 (0.1890)
0.1574 (4.00)
0.1497 (3.80)
8
5
1
4
6.20 (0.2440)
5.80 (0.2284)
PIN 1
0.50 (0.0196)
45
0.25 (0.0099)
1.27 (0.0500)
BSC
COPLANARITY
0.25 (0.0098)
0.10 (0.0040)
SEATING
PLANE
1.75 (0.0688)
1.35 (0.0532)
0.51 (0.0201)
0.33 (0.0130)
8
0.25 (0.0098) 0 1.27 (0.0500)
0.41 (0.0160)
0.19 (0.0075)
CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
COMPLIANT TO JEDEC STANDARDS MS-012 AA
Revision History
Location
Page
Data Sheet changed from REV. C to REV. D.
Change to SOIC (R-8) Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Edits to FEATURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Deleted TO-99 CONNECTION DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Deleted AD548C from SPECIFICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Edits to ABSOLUTE MAXIMUM RATINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Deleted Metal Can from Figure 22 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Deleted TO-99 (H) and Cerdip (Q) Packages from OUTLINE DIMENSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
REV. D
–11–
–12–
PRINTED IN U.S.A.
C00510–0–5/02(D)