DN56 - 3V Operation of Linear Technology Op Amps

3V Operation of Linear Technology Op Amps - Design Note 56
George Erdi
The latest trend in digital electronics is the introduction
of numerous ICs operating on regulated 3V or 3.3V
power supplies. This is a logical development to increase
circuit densities and to reduce power dissipation. In addition, many systems are directly powered by two AA
cells or 3V lithium batteries. Clearly, analog ICs which
work on 3V with good dynamic range to complement
these digital circuits are, and will be, in great demand.
Many Linear Technology operational amplifiers work
well on a 3V supply. The purpose of this design note
is to list these devices and their performance when
powered by 3V. The op amps can be divided into two
groups: single and dual supply devices. The single supply
op amps are optimized for, and fully specified at, a 5V
positive supply with the negative supply terminal tied
to ground. Input common mode voltage range goes
below ground, and the output swings to within a few
millivolts of ground while sinking current. Members of
the single supply family are the micropower LT®1077/
LT1078/LT1079 single, dual and quad op amps with
40μA supply current per amplifier, the LT1178/LT1179
dual and quad with 13μA per amplifier. The LT1006/
LT1013/LT1014 single, dual and quad have faster speed
and lower voltage noise, at the expense of 300μA per
amplifier.
The performance of these devices at 3V is quite similar
to the 5V specs. Clearly, input voltage range and output
voltage swing have to be reduced by 2V since the supply is 2V less. Offset voltage change from 5V to 3V is
determined by the power supply rejection ratio specs.
At 114dB or 2μV/V the degradation in offset voltage is
only 4μV (= 2V • 2μV/V). Input bias and offset currents,
voltage and current noise, as well as offset voltage drift
with temperature, are practically unchanged compared
to the 5V specifications.
Table 1 summarizes the performance of the low cost
grades of these single supply devices at 3V. One note of
caution: the minimum operating voltage for the LT1013/
LT1014 is 2.95V. All other devices work on lower supplies, ranging from 1.7V to 2.6V.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
Table 1. Single Supply Op Amps: Low Cost Grade Specifications VS = 3V, 0V. TA = 25°C
LT1077CN8
LT1078CN8
LT1079CN
PARAMETER
LT1178CN8
LT1179CN
LT1006CN8
LT1013CN8
LT1014CN
TYP
MIN/MAX
TYP
MIN/MAX
TYP
MIN/MAX
15
45
80
140/170
—
45
—
140/170
35
95
95
470
μV
μV
Input Voltage Range
–0.3
1.8
0
1.7
–0.3
1.9
0
1.7
–0.3
1.8
0
1.7
V
V
Output Swing
No Load
0.003
2.4
0.006
2.2
0.006
2.4
0.009
2.2
0.015
2.4
0.025
2.2
V
V
2k to Ground
0.0006
2.1
0.0010
1.9
0.0002
2.0
0.0006
1.8
0.007
2.3
0.015
2.0
V
V
500
110
180
60
1000
500
V/mV
0.1Hz to 10Hz Noise
0.6
—
1.0
—
0.5
—
μVP-P
Minimum Supply Voltage.
With 300μV VOS Degradation
—
—
2.3
1.8
—
—
2.2
1.7
—
—
2.6/2.95
—
Gain-Bandwidth Product
160
—
50
—
700
—
Offset Voltage
Voltage Gain
02/92/56_conv
Single
Dual/Quad
RL = 50k
UNITS
V
V
kHz
The LT1101 micropower (= 75μA) instrumentation amplifier completes the single supply family. Again, this in
amp in 8-pin packages is fully specified at 5V. Minimum
supply voltage is 1.8V; the performance change in going
from 5V to 3V supply is minimal.
The second group of devices are dual supply op amps,
i.e., the common mode input voltage and the output
swing are limited to a diode voltage (= 600mV) above
the negative supply terminal for proper operation. In
addition, dual supply op amps are traditionally optimized
for ±15V operation. Thus, reducing the total supply
voltage to 3V represents a significant change. Table 2
lists the performance of four op amps: the LT1008 and
LT1012 are actually fully tested at reduced supplies. The
LT1097 and LT1001 performance is inferred from device
evaluation data. Dual versions in 14-pin packages are
also available: the LT1002 is a dual LT1001; the LT1024
is a dual version of the LT1012.
of Table 1 are all at least 6nA. The traditional ways of
achieving pico-ampere bias current are not available
either: JFET input or CMOS chopper-stabilized op amps
do not function at 3V supply.
Figure 1 shows an application using the LT1078 to
monitor the condition of the 3V battery. One output
warns that the battery voltage is dropping, the other
output shuts the system down as the battery voltage
falls below the threshold value.
.
.
L
7
+
-5
-0#"55&3:-08
*'7S7
–
5
8
+
7
-5
In most 3V applications the single supply op amps of
Table 1 are more flexible and desirable, since no special biasing is needed to shift the input and the output
into the operating range. However, the offset voltage
drift with temperature performance of the dual supply
devices is better. And, most importantly, when picoampere input bias currents are needed, the LT1008/
LT1012/LT1097 have no competition. The op amps
.
.
#"55&3:
065165
+
6
–
-5
LO = 4:45&.
4)65%08/
*'7S7
505"-4611-:$633&/5ó˜"
%/t5"
Figure 1. Low Battery Detector with System Shutdown
Table 2. Dual Supply Op Amps at VS = 3V, 0V. TA = 25°C. Low Cost Grade Electrical Characteristics
LT1097CN8
PARAMETER
TYP
MIN/MAX
Offset Voltage
20
Drift with Temperature
0.3
Input Bias Current
Input Offset Current
LT1008CN8
TYP
MIN/MAX
100
40
1.3
0.3
40
280
40
260
Input Voltage Range
0.65
2.3
Output Swing
LT1012CN8
LT1001CN8
TYP
MIN/MAX
TYP
MIN/MAX
180
25
1.6
0.3
40
150
30
150
0.80
2.2
0.65
2.3
0.62
2.25
0.8
2.1
Voltage Gain RL = 10k
600
0.1Hz to 10Hz Noise
Minimum Supply Voltage
120
40
150
μV
1.3
0.3
1.3
μV/°C
40
200
600
3500
pA
30
200
350
3200
pA
0.80
2.2
0.65
2.3
0.80
2.2
0.75
2.2
0.90
2.1
V
V
0.62
2.25
0.8
2.1
0.62
2.25
0.8
2.1
0.55
2.2
0.7
2.05
V
V
250
500
200
500
200
300
150
V/mV
0.5
—
0.5
—
0.5
—
0.35
—
μVP-P
—
2.4
—
2.4
—
2.4
—
1.9
V
Supply Current
350
560
380
600
380
600
390
550
μA
Gain-Bandwidth Product
500
—
500
—
500
—
600
—
kHz
Data Sheet Download
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Linear Technology Corporation
UNITS
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© LINEAR TECHNOLOGY CORPORATION 1992
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