Download DN230 - Rail-to-Rail Amplifiers Operate on 2.7V with 20µV Offset

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Rail-to-Rail Amplifiers Operate on 2.7V with 20µV Offset
Design Note 230
Glen Brisebois
The LT®1677 and LT1884 are the latest results of Linear
Technology’s quest for the “ideal” Op Amp1. Both of them
will operate with supplies down to 2.7V, have only 20µV
of input offset voltage and have rail-to-rail outputs. The
LT1677 features very low noise, 3.2nV/√Hz; the LT1884
features very low 150pA input bias current. Each of the
two application circuits shown below take advantage of
some particular subset of the features of these amplifiers.
made, are extremely small and require high gain. To
complicate matters, Geophones are often buried in order
to avoid interference from traffic and other surface
effects and so are often necessarily remote.
The circuit in Figure 1 applies a gain of ~100 to the
Geophone signal and transmits this back to the operator
by modulating its own supply current. U2 is an LT1635
configured as a stable current source of 5mA. This then
powers the LT1677 as well as another LT1635, this time
configured as a 3V shunt regulator. Resistors R6 and R7
set up a DC bias voltage of 1.85V, centering the output
swing offset by Q3 and keeping the LT1677 input common
Remote 2-Wire Geophone Preamp
Using the Low Noise LT1677
The LT1677 is optimized for lowest overall noise when
looking into transducers of 600Ω to 2700Ω impedance,
such as the Geophone shown in Figure 1. A low noise
amplifier is desired in this application because the seismic
signals that must be resolved, whether natural or man
1Not
only would the ideal op amp have zero noise, zero input offset, no parasitic
capacitances, infinite gain and bandwidth and supply its own power, but it would do
all this for free.
, LTC and LT are registered trademarks of Linear Technology Corporation.
12V
IMPOSSIBLE
TO MIS-WIRE
LOCAL END
2
–
3
+
7
6
U2
LT1635
4
1
R1
1k
R12
250Ω
5mA
7mA
CURRENT
SOURCE
8
R11
20Ω
R2
1k
R3
43.2Ω
Q2
2N3904
C2
0.01µF
Q1
2N3904
R4
14k
2
3
R5
1k
–
R8
866Ω
7
U1
LT1635
+
1
8
3V SHUNT
REGULATOR
R9
150k
REMOTE 3V SUPPLY
6
4
C5
47µF
2
R6
261k
3
R7
422k
–
6
+
D1 TO D4
1N4148
Q3
2N3906
4
C1
0.1µF
GEOSPACE CORP
GEOPHONE
MODEL 20DX
630Ω, 10Hz
(713) 939-7093
Figure 1. Remote 2-Wire Geophone Preamp
04/00/230
R10
165Ω
C3
1000pF
7
U3
LT1677
C4
2200pF
www.BDTIC.com/Linear
mode in its most precise range2. This places about 1.15V
across R10 thereby pulling an additional 7mA from the
main supply through Q2. It is this 7mA that will be
modulated by the AC signal. The total current of about
12mA puts 3V across the receiver resistor, with the 7mA
allowing a peak signal of ±1.5V about the 3V bias point.
10k
1M
–
1M
+
10k
The circuit operates as a current loop and so has good
interference immunity, with interference appearing across
U2 and Q2 rather than across R12. Q1 temperature
compensates Q2. C1 causes a boost in the gain below
10Hz where the geophone response is falling off. C3 limits
the bandwidth to 1kHz. D1 through D4 form a bridge
rectifier so that the local wiring is arbitrary. The LT1677
could drive R10 directly, but Q3 is used as an output buffer
so that the heavy currents do not eat into the LT1677’s
high open-loop gain.
REF
DN230 F02
Figure 2. Standard Difference Amplifier Can Handle High
Voltage CM Inputs, But at Cost of Differential Gain
The circuit in Figure 3 uses an LT1884 to achieve high
common mode input range and rejection without sacrificing differential gain. U1B samples the common mode
through R5 and R6 and nulls it through R3 and R4. The
R3-R1 ratio must be extremely well matched to the
R4-R2 ratio to avoid causing a common mode to differential mode translation at this point. Once the common
mode is nulled, then the differential mode input voltage is
converted to a differential input current and appears
unattenuated across R7. The common mode input voltage can theoretically be as high as about 250V (limited by
the output of U1B going to ground and the ÷100 ratio
maintaining common mode at 2.5V), but is limited in fact
by the working voltage of R1 and R2 and by the ratio
matching of R1-R3 and R2-R4.
Difference Amplifier Using the LT1884: ±42V CM
Input Range on a Single 5V Supply Without
Sacrificing Differential Gain
Measuring small voltages on top of large voltages can be
quite difficult. Often, the standard difference amplifier
topology is implemented with very high value input resistors and low value divide and feedback resistors, as
shown in Figure 2. However, this results in significant
differential mode attenuation.
2Note
that the LT1677 has rail-to-rail inputs.
R7*
1M
R1†
1M
–
VCM = 42VP-P
VDM = ±2.5V
+
5V
2
R2†
1M
3
–
8
U1A
1/2 LT1884
1
+
5V
4
†
R3†
10k
–
7
R8*
1M
R5*
10k
R4
10k
6
R6*
10k
VOUT
2.5V ±VDM
10k
LT1634B-2.5
U1B
1/2 LT1884
5
DN230 F03
+
*1% METAL FILM
†CADDOCK (541) 496-0700 T912 0.01% MATCHED RESISTOR PAIRS
Figure 3. Single Supply Difference Amplifier. U1B Nulls the Common Mode
So That U1A Can Concentrate on the Difference Mode
Data Sheet Download
http://www.linear-tech.com/go/dnLT1677
http://www.linear-tech.com/go/dnLT1884
Linear Technology Corporation
For literature on our Op Amps,
call 1-800-4-LINEAR. For applications help,
call (408) 432-1900, Ext. 2156
dn230f LT/TP 0400 340K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.BDTIC.com/Linear
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
 LINEAR TECHNOLOGY CORPORATION 2000