DN163 - 1µA Op Amp Permits Precision Portable Circuitry

1µA Op Amp Permits Precision Portable Circuitry – Design Note 163
Mitchell Lee and Jim Williams
The circuit’s internal AC coupling prevents A1’s DC characteristics from influencing overall DC performance,
accounting for the extremely low offset errors noted.
A new dual op amp with only 1μA power consumption and precision DC specifications permits high
performance portable applications. The LT ®1495
has 375μV offset, 2μV/°C drift, 1nA bias current and
100dB of open-loop gain. These attributes, combined
with careful design, make portable, high performance
circuitry possible.
The desired micropower operation and A1’s bandwidth
dictate the 5Hz clock rate. As such, resultant overall
bandwidth is low. Full power bandwidth is 0.05Hz with
a slew rate of about 1V/s. Clock related noise, about
5μV, can be reduced by increasing CCOMP, with commensurate bandwidth reduction.
5.5μA, 0.05μV/°C Chopped Amplifier
Figure 1 shows a chopped amplifier requiring only 5.5μA
supply current. Offset voltage is 5μV, with 0.05μV/°C
drift. Gain exceeding 108 affords high accuracy, even
at large closed-loop gains.
0.03% Linear V/F Converter with 13μA Power Drain
Figure 2’s voltage-to-frequency converter takes full
advantage of the LT1495’s low power consumption.
A 0V to 2.5V input produces a 0Hz to 10kHz output,
with 0.03% linearity, 250ppm/°C drift and 10ppm/V
supply rejection. Maximum current consumption is
only 13μA, 200 times lower than currently available ICs.
Comparator C1 switches a charge pump comprising
D1, D2 and the 100pF capacitor to maintain its negative input at 0V. A1 and associated components form
a temperature compensating reference for the charge
pump. The 100pF capacitor charges to a fixed voltage;
Micropower comparators C1A and C1B form a biphase
5Hz clock. The clock drives the input-related switches,
causing an amplitude modulated version of the DC input
to appear at A1A’s input. AC-coupled A1A takes a gain of
1000, presenting its output to a switched demodulator
similar to the aforementioned modulator.
The demodulator output, a reconstructed, DC amplified
version of the circuit’s input, feeds DC gain stage A1B.
A1B’s output is fed back, via gain setting resistors, to
the input modulator, closing a feedback loop around the
entire amplifier. Amplifier gain is set by the feedback
resistor’s ratio, in this case 1000.
CCOMP
0.2μF
1μF
K1
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of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
5V
+
INPUT
1RF
K2
A1A
1/2 LT1495
1M
–
K2
1M
–
A1B
1/2 LT1495
10M
– 5V
K1
OUTPUT
+
10k
10M
10M
10k
K1
–
0.047μF
C1A
1/2
LTC
+ ®1441
–
10M
10M
5V
C1B
1/2 LTC1441
+
– 5V
K2
GAIN: 1000
OFFSET: 5μV
DRIFT: 50nV/°C
SUPPLY CURRENT: 5.5μA
BANDWIDTH: 0.2Hz
CLOCK RATE: 5Hz
CD4016 QUAD
Figure 1. 0.05μV/°C Chopped Amplifier Consumes 5.5μA Supply Current
09/97/163_conv
DN163 F01
270k
10M*
3.9M*
5V
–
390Ω
562k*
1μF
–
20M
3.9M
+
VIN
0V TO 2.5V
A1
1/2 LT1495
+
100k
5V
C1
LTC1440
0.0082μF
+
1.2
REFERENCE
LTC1440
12pF
1N4148
3.6M TYP
SELECT FOR 100Hz
AT VIN = 0.025V
360k
Q1: ZTX-849
*1% METAL FILM
†
POLYSTYRENE
D1
15k
39k
100pF†
Q1
10M
OUTPUT
0kHz TO 10kHz
D2
0V – 2.5V = 0kHz – 10kHz
SUPPLY CURRENT = 6.2μA QUIESCENT
= 13.3μA AT f = 10kHz
LINEARITY: 0.03%
PSRR (4.4V TO 36V): 10ppm/V
TEMPERATURE DRIFT: 250ppm/°C
0.05μF
–
1N5712
A2
1/2 LT1495
+
DN163 F02
Figure 2. 0kHz to 10kHz Voltage to Frequency Converter Consumes Only 13μA
Portable Reference
A final circuit is Figure 3’s unique portable reference,
which draws only 16μA from a pair of AAA alkaline
cells. Battery life is five years—equivalent to shelf life.
Two outputs are provided: a buffered, 1.5V voltage
output and a regulated 1.5μA current source. The current source compliance ranges from approximately
1V to – 43V.
The LT1634A reference is self-biased, completely
eliminating line regulation as a concern. Start-up is
guaranteed by the LT1495 op amp, whose output initially
saturates at 11mV from the negative rail. The 1μA current output is derived from a fraction of the reference
voltage impressed across R3.
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Once it is powered, there is no reason to turn the circuit
off. One AAA alkaline contains 1200mAH capacity,
enough to power the circuit through the five year shelf
life of the battery.
The voltage output accuracy is about 0.17% and the
current output accuracy is about 1.2%. Trim R1 to
calibrate voltage (0.1%/kΩ) and R3 to calibrate the
output current (0.4%/kΩ).
R3
249k
0.1%
2 AAA
ALKALINE
CELLS
Q1
ZTX214C
R4
24k
5%
1/2 LT1495
+
Start-up or input overdrive can cause the circuit’s
AC-coupled feedback to latch. If this occurs, C1’s output
goes low; A2, detecting this via the 10M/0.05μF lag,
goes high. This lifts C1’s positive input and grounds the
negative input with Q1, initiating normal circuit action.
Note that the portable reference’s current output can
be pulled well below common, limited only by Q1’s
45V breakdown. The 1.5V output can source or sink
up to 700μA and is current limited to protect batteries
in case of a short circuit.
–
hence, the switching repetition rate is the circuit’s only
degree of freedom to maintain feedback. Comparator C1
pumps uniform packets of charge to its negative input
at a repetition rate precisely proportional to the input
voltage derived current. This action ensures that circuit
output frequency is strictly and solely determined by
the input voltage.
+
1/2 LT1495
–
LT1634A-1.25
R1 TO R3: MAR5 SERIES
IRC (512) 992-7900
1μA
COM
1.5V
R1
200k
0.1%
R2
1M
0.1%
DN163 F03
Figure 3. Portable reference Operates Five Years
on One Pair of AAA Cells
For applications help,
call (408) 432-1900
dn163f_conv LT/TP 0997 340K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1997
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