DN266 - LT1880 SOT-23 Superbeta Op Amp Saves Board Space in Precision Applications

SOT-23 Superbeta Op Amp Saves Board Space
in Precision Applications – Design Note 266
Glen Brisebois
INTRODUCTION
The tiny new LT®1880 achieves precision unprecedented
in a SOT-23 package without resorting to autozeroing
techniques. Input offset voltage and drift are typically
40μV and 0.3μV/°C, respectively, with guarantees of
200μV and 1.2μV/°C maximum over temperature. The
device operates on total supplies from 2.7V to 40V with
rail-to-rail outputs, giving a dynamic range of 120dB.
Unlike some competitors’ SOT-23 op amps, which claim
to maintain good precision, the LT1880 supports its
input precision with a high open loop gain of 1.6 million, as well as 135dB CMRR and PSRR. It is available
in commercial and industrial temperature grades.
APPLICATIONS
Getting Rail-to-Rail Operation without
Rail-to-Rail Inputs
The LT1880 does not have rail-to-rail inputs, but for
most inverting applications and noninverting gain
applications, this is largely inconsequential. Figure 1
shows the basic op amp configurations, what happens
to the op amp inputs, and whether or not the op amp
must have rail-to-rail inputs.
divided-by-ten version of the input voltage. The gain
accuracy is limited by the resistors to 0.2%. Output
referred, this error becomes 2.7mV at 1.35V output.
The 40μV input offset voltage contribution, plus the additional error due to input bias current times the ~100k
effective source impedance, contribute negligible error.
Precision Photodiode Amplifier
Photodiode amplifiers usually employ JFET op amps
because of their low bias current; however, when precision is required, JFET op amps are generally inadequate
due to their relatively high input offset voltage and drift.
The LT1880 provides a high degree of precision with very
low bias current (IB = 150pA typical) and is therefore
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1.5V
+
LT1880
VIN
+
100k
0.1%
RG
VIN
–
DN266 F02
–1.5V
Figure 2. Extreme Inverting Case: Circuit Operates
Properly with Input Voltage Swing Well Outside
Op Amp Supply Rails
+
VIN
–
1M
0.1%
The circuit of Figure 2 shows an extreme example of the
inverting case. The input voltage at the 1M resistor can
swing ±13.5V and the LT1880 will output an inverted,
VREF
±1.35V
OUTPUT
SWING
±13.5V SWINGS
WELL OUTSIDE
SUPPLY RAILS
VIN
–
+
–
RF
RF
DN266 F01
RG
VREF
INVERTING: AV = –RF/RG
OP AMP INPUTS DO NOT MOVE,
BUT ARE FIXED AT DC BIAS
POINT VREF
NONINVERTING: AV = 1 + RF/RG
INPUTS MOVE BY AS MUCH AS
VIN, BUT THE OUTPUT MOVES
MORE
INPUT DOES NOT HAVE TO BE
RAIL-TO-RAIL
INPUT MAY NOT HAVE TO BE
RAIL-TO-RAIL
NONINVERTING: AV = 1
INPUTS MOVE AS MUCH AS
OUTPUT
INPUT MUST BE RAIL-TO-RAIL
FOR OVERALL CIRCUIT
RAIL-TO-RAIL PERFORMANCE
Figure 1. Some Op Amp Configurations Do Not Require Rail-to-Rail Inputs to Achieve Rail-to-Rail Outputs
09/01/266_conv
applicable to this demanding task. Figure 3 shows an
LT1880 configured as a transimpedance photodiode
amplifier. The transimpedance gain is set to 51.1kΩ
by RF. The feedback capacitor, CF, may be as large as
desired where response time is not an issue, or it may
be selected for maximally flat response and highest
possible bandwidth given a photodiode capacitance
CD. Figure 4 shows a chart of CF and rise time versus
CD for maximally flat response. Total output offset is
below 262μV, worst-case, over temperature (0°C to
70°C). With a 5V output swing this implies a minimum
86dB dynamic range, sustained over temperature (0°C
to 70°C), and a full-scale photodiode current of 98μA.
CF
WORST-CASE
OUTPUT OFFSET
≤196V AT 25°C
≤262V 0°C TO 70°C
≤323V –40°C TO 85°C
which then maintains its inverting input at the same
voltage by driving 1mA of current through the RTD and
the total 1.25k of resistance set by R1 and R2. Lower
precision components R4 and C1 ensure circuit stability, which would otherwise be excessively dependant
on the cable characteristics. R5 is also noncritical and
is included to improve ESD immunity and decouple
any cable capacitance from the LT1880’s output.
The 4-wire cable allows Kelvin sensing of the RTD
voltage while excluding the cable IR drops from the
voltage reading. With 1mA excitation, a 1kΩ RTD will
have 1V across it at 0°C, and 3.85mV/°C temperature
response. This voltage can be easily read in myriad ways,
with the best method depending on the temperature
region to be emphasized and the particular ADC that
will be reading the voltage.
RF 51.1k
R5
180Ω, 5%
5V
–
PHOTODIODE
(SEE TEXT)
CD
+
+
OUT
LT1880
DN266 F03
VOUT = 1.00V AT 0°C + 3.85mV/°C
– –50°C TO 600°C
1k
AT 0°C
RTD*
–5V
Figure 3. Precision Photodiode Amplifier
R4
1k, 5%
100
RISE TIME (μs), CF (pF)
R1
1.24k
0.1%
5V
–
+
LT1880
R3
150k
1%
R2
10Ω
1%
CF
10
C1
0.1μF
LT1634ACS8-1.25
5V
RISE TIME
1
*OMEGA F3141 1kΩ, 0.1% PLATINUM RTD
(800) 826-6342
DN266 F05
100mV OUTPUT STEP
0.1
0.1
1
10
CD (pF)
100
1000
DN266 F04
Figure 4. Feedback CF and Rise Time vs Photodiode CD
Single-Supply Current Source for Platinum RTD
The precision, low bias current input stage of the
LT1880 makes it ideal for precision integrators and
current sources. Figure 5 shows the LT1880 providing
a simple precision current source for a remote 1kΩ
RTD on a 4-wire connection. The LT1634 reference
places 1.25V at the noninverting input of the LT1880,
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Figure 5. Single Supply Current Source for Platinum RTD
Conclusion
The precision, low bias current input stage of the
LT1880 makes it ideal for precision and high impedance circuits. The rail-to-rail output stage renders the
op amp capable of driving other devices as simply as
possible with extended dynamic range, while the 2.7V
to 40V operation means that it will work on almost
all supplies. The small SOT-23 package makes it a
compelling choice where board space is at a premium
or where a composite amplifier is competing against a
larger single-chip solution. For applications help,
call (408) 432-1900
dn266f_conv LT/TP 0901 371.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2001
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