DN331 - Dual 25µV Micropower Op Amp Fits in 3mm x 3mm Package

Dual 25µV Micropower Op Amp Fits in 3mm × 3mm Package
Design Note 331
Glen Brisebois
Introduction
Conventional monolithic micropower op amps with a
wide supply voltage range require a large die area and
therefore, a large package and footprint. The unconventional LT®6011 dual op amp fits 25μV input precision
micropower operation and wide 2.7V to 36V supply range
in a tiny new package—its 3mm × 3mm DFN package
is so small it doesn’t even have leads. The LT6011 also
provides rail-to-rail output swing and utilizes superbeta
input transistors to achieve picoampere input currents.
Hall Sensor Amplifier
Figure 1 shows the LT6011 applied as a low power Hall
sensor amplifier. The magnetic sensitivity of a Hall sensor is proportional to the DC excitation voltage applied
across it. With a 1V bias voltage, the sensitivity of this
Hall sensor is specified as 4mV/mTesla of magnetic
field. At that level of DC bias, however, the 400Ω bridge
consumes 2.5mA. Reducing the excitation voltage would
reduce the power consumption, but it would also reduce
the sensitivity. This is where the beauty of precision
micropower amplification becomes especially apparent.
The LT1790-1.25 micropower reference provides a
stable 1.25V reference voltage. The 7.87k:100k resistive
ladder attenuates this to about 90mV across the 7.87k
and the LT1782 acts as a buffer. When this 90mV is
applied as excitation across the Hall bridge, the current
is only 230μA. This is less than 1/10 of the original
value. (Just imagine if all your batteries could last 10
times longer than they do.) But as mentioned earlier,
the sensitivity is now likewise reduced by the same
factor, down to 0.4mV/mT.
The way back to high sensitivity is to take gain with a
precision micropower amplifier. The LT6011 is therefore
configured as an instrumentation amplifier in a gain of
101. Such high gains are permissible and advantageous
using an LT6011 because of its exceptional input precision and low drift. The output sensitivity of the circuit
is raised to a whopping 40mV/mT, while consuming a
total supply current of only 600μA. To have achieved
this sensitivity by increasing the bridge excitation would
have required a prohibitive 25mA from the supply! (As an
interesting note, here in Milpitas, California, the Earth’s
50μT field is about 60° from horizontal and causes a
2mV shift in the circuit’s output.)
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100k
4
1, 2
VS
HALL ELEMENT
ASAHI-KASEI
HW-108A (RANK D)
www.asahi-kasei.co.jp
VS
LT1790-1.25
1k
6
+
100k
1%
VS = 3V TO 18V
IS = ~600A
VOUT = ~40mV/mT
8
3
1
+
1k
400
w4
100k
4
3
LT1782
–
–
1/2 LT6011
2
1
10k
OFFSET
VS
ADJUST
7.87k
1%
2
6
26.7k
1%
–
1/2 LT6011
5
+
7
4
DN331 F01
Figure 1. Hall Sensor Amplifier Optimizes Sensitivity vs Supply Current
02/04/331_conv
VOUT
1.25V + 40mV
mT
DAC Amplifier
Figure 2 shows the LT6011 applied as both a reference
amplifier and I-to-V converter with the LTC1592 16-bit
DAC. Whereas faster amplifiers such as the LT1881
and LT1469 are also suitable for use with this DAC,
the LT6011 is desirable when power consumption is
more important than speed. The total supply current of
this application varies from 1.6mA to 4mA, depending
on code, and is almost entirely dominated by the DAC
resistors and the reference.
The DAC itself is powered only from a single 5V supply.
Op amp B of the LT6011 inverts the 5V reference using
the DAC’s internal precision resistors R1 and R2, thus
providing the DAC with a negative reference allowing
bipolar output polarities. Op amp A provides the I-to-V
conversion and buffers the final output voltage. The
precision required of the I-to-V converter function is
critical because the DAC output resistor network is
obviously very code dependent, so the noise gain which
the op amp sees is also code dependent. An imprecise
op amp in this function would have its input errors
amplified somewhat chaotically versus code.
The speed of the circuit is shown in Figure 3. Settling
is achieved within 250μs. Because the outputs of the
LT6011 swing to within 40mV of either supply rail, the
supply voltages to the amplifier need to be only barely
wider than the desired ±10V output.
CS/LD
5V/DIV 0V
10V
VOUT 0V
5V/DIV
–10V
DN331 F03
100μs/DIV
Figure 3. 20V Output Step Time Domain Response
VS+
LT1236-5
5
6
+
B
1/2 LT6011
7
SUPPLY CURRENT % 1.6mA TO 4mA
DEPENDING ON CODE
–
C2
270pF
2
1
16 15
R1
RCOM
R2 REF ROFS RFB
R1
9
5V
13
12
11
10
4
C1
270pF
R2
VS+
VCC
0.1μF
14
3
5
2
–
IOUT2 6
3
+
IOUT1
CLR
16-BIT DAC
CS/LD
SCK
AGND
SDI
GND
LTC1592
SDO
7
8
A
1/2 LT6011
1
VOUT
4
VS–
8
DN331 F02
VS = ±15V
Figure 2. DAC Reference Inverter and I-to-V Converter
Data Sheet Download
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dn331f_conv LT/TP 0204 305K • PRINTED IN THE USA
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