LTC8554
P-1
General Description
The LTC8554 of quad-channel amplifiers provides input offset voltage correction for very
low offset and drift through the use of patented fast step response chopper stabilized
techniques. This method constantly measures and compensates the input offset,
eliminating drift over time and temperature and the effect of 1/f noise. This design
breakthrough allows the combination of a GBWP of 1.5 MHz and a high SR of 1.2 V/μs,
while only drawing 125 μA supply current. These devices are unity gain stable and have
good Power Supply Rejection Ratio (PSRR) and Common Mode Rejection Ratio (CMRR).
The LTC8554 amplifiers are perfectly suited for applications that require precision
amplification of low level signals, in which error sources cannot be tolerated, even in
which high bandwidth and fast transition are needed. The rail-to-rail input and output
swings make both high-side and low-side sensing easy. The LTC8554 can operate with a
single supply voltage as low as 1.8 V for 2-cell battery applications.
The LTC8554 op-amps have enhanced EMI protection to minimize any electromagnetic
interference from external sources, and have high electro-static discharge (ESD)
protection (5-kV HBM). All models are specified over the extended industrial temperature
range of −40 ℃ to +125 ℃.
Features and Benefits
High DC Precision:
– ±10 μV (maximum) VOS with a Drift of ±50 nV/℃ (maximum)
– AVOL: 112 dB (minimum, VDD = 5.5V)
– PSRR: 112 dB (minimum, VDD = 5.5V)
– CMRR: 112 dB (minimum, VDD = 5.5V)
– Vn: 0.45 μVPP (typical, f = 0.1 to 10 Hz)
1.5 MHz Bandwidth and 1.2 V/μs Slew Rate
Settling Time to 0.1% with 1V Step: 1.2 μs
Overload Recovery Time to 0.1%: 35 μs
Micro-Power 125 μA per Amplifier and 1.8 V to 5.5 V Wide Supply Voltage Range
Operating Temperature Range: −40℃ to +125℃
Applications
Precision current sensing
Resistor thermal detectors
Temperature, position and pressure sensors
Medical equipment
Electronic scales
Strain gage amplifiers
Thermocouple amplifiers
Pin Configurations (Top View)
OUTA
OUTD
–IND
LTC8554
SOIC-14L / TSSOP-14L
–INA
LTC8554
QFN3x3-16L
16
15
14
13
OUTA
1
2
11
–VS
+INB
3
10
+INC
–INB
4
9
–INC
5
6
7
8
OUTC
+VS
NC
+IND
NC
12
OUTB
1
OUTD
–INA
2
13
–IND
+INA
3
12
+IND
+VS
4
11
–VS
+INB
5
10
+INC
–INB
6
9
–INC
OUTB
7
8
OUTC
A
+INA
14
B
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
D
C
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-2
Pin Description
Symbol
Description
–IN
Inverting input of the amplifier.
+IN
Non-inverting input of the amplifier.
+VS
Positive (highest) power supply.
–VS
Negative (lowest) power supply.
OUT
Amplifier output.
Ordering Information
Orderable
Type Number
Package
Name
Package
Quantity
Eco Class(1)
Operating
temperature
Marking
Code
LTC8554XS14/R5
SOIC-14L
2500
Green
–40℃ to +125℃
AZ4 X
LTC8554XF16/R6
QFN3x3-16L
3000
Green
–40℃ to +125℃
AZ4 X
LTC8554XT14/R6
TSSOP-14L
3000
Green
–40℃ to +125℃
AZ4 X
(1) Eco Class - The planned eco-friendly classification: Pb-Free (RoHS) or Green (RoHS & Halogen Free).
(2) Please contact to your Linearin representative for the latest availability information and product content
details.
Limiting Value
In accordance with the Absolute Maximum Rating System (IEC 60134).
Parameter
Absolute Maximum Rating
Supply Voltage, VS+ to VS–
10.0 V
Signal Input Terminals: Voltage, Current
VS– – 0.3 V to VS+ + 0.3 V, ±10 mA
Output Short-Circuit
Continuous
Storage Temperature Range, Tstg
–65 ℃ to +150 ℃
Junction Temperature, TJ
150 ℃
Lead Temperature Range (Soldering 10 sec)
260 ℃
ESD Rating
Parameter
Electrostatic
Discharge Voltage
Item
Value
Human body model (HBM), per MIL-STD-883J / Method 3015.9
±5 000
Charged device model (CDM), per ESDA/JEDEC JS-002-2014
±2 000
Machine model (MM), per JESD22-A115C
±250
(1) JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
(2) JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
Unit
V
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-3
Electrical Characteristics
VS = 5.0V, TA = +25℃, VCM = VS /2, VO = VS /2, and RL = 10kΩ connected to VS /2, unless otherwise noted.
Boldface limits apply over the specified temperature range, TA = −40 to +125 ℃.
Symbol
Parameter
Conditions
Min.
Typ.
Max.
Unit
OFFSET VOLTAGE
VOS
Input offset voltage
VOS TC
Offset voltage drift
TA = −40 to +125 ℃
PSRR
Power supply
rejection ratio
VS = 2.0 to 5.5 V, VCM < VS+ − 2V
112
TA = −40 to +125 ℃
106
±2
±10
μV
±5
±50
nV/℃
126
dB
INPUT BIAS CURRENT
±70
IB
IOS
Input bias current
TA = +85 ℃
±150
TA = +125 ℃
±700
pA
±100
Input offset current
pA
NOISE
Vn
Input voltage noise
en
Input voltage noise
density
In
Input current noise
density
f = 0.01 to 1 Hz
0.1
f = 0.1 to 10 Hz
0.45
f = 10 kHz
15
f = 1 kHz
19
f = 1 kHz
10
μVP-P
nV/√Hz
fA/√Hz
INPUT VOLTAGE
VCM
CMRR
Common-mode
voltage range
Common-mode
rejection ratio
VS––0.1
VS = 5.5 V, VCM = −0.1 to 5.5 V
112
VCM = 0 to 5.3 V, TA = −40 to +125 ℃
106
VS = 2.0 V, VCM = −0.1 to 2.0 V
102
VCM = 0 to 1.8 V, TA = −40 to +125 ℃
96
VS++0.1
V
130
122
dB
INPUT IMPEDANCE
RIN
Input resistance
CIN
Input capacitance
100
GΩ
Differential
2.0
Common mode
3.5
pF
OPEN-LOOP GAIN
AVOL
Open-loop voltage
gain
FREQUENCY RESPONSE
Gain bandwidth
GBW
SR
product
Slew rate
tS
Settling time
tOR
Overload recovery
time
RL = 20 kΩ, VO = 0.05 to 3.5 V
112
TA = −40 to +125 ℃
106
RL = 2 kΩ, VO = 0.15 to 3.5 V
101
TA = −40 to +125 ℃
95
132
120
dB
f = 1 kHz
1.5
MHz
G = +1, CL = 100 pF, VO = 1.5 to 3.5 V
1.2
V/μs
To 0.1%, G = +1, 1V step
1.2
To 0.01%, G = +1, 1V step
1.5
To 0.1%, VIN * Gain > VS
35
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
μs
μs
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-4
Electrical Characteristics (continued)
VS = 5.0V, TA = +25℃, VCM = VS /2, VO = VS /2, and RL = 10kΩ connected to VS /2, unless otherwise noted.
Boldface limits apply over the specified temperature range, TA = −40 to +125 ℃.
Symbol
Parameter
Conditions
Min.
Typ.
Max.
Unit
OUTPUT
VOH
High output voltage
swing
RL = 20 kΩ
VOL
Low output voltage
swing
RL = 20 kΩ
VS–+4
RL = 2 kΩ
VS–+40
ZOUT
Open-loop output
impedance
ISC
Short-circuit current
RL = 2 kΩ
VS+–6
VS+–100
mV
VS+–60
f = 350 kHz, IO = 0
2
Source current through 10Ω
40
Sink current through 10Ω
50
VS–+66
mV
kΩ
mA
POWER SUPPLY
VS
Operating supply
voltage
IQ
Quiescent current
(per amplifier)
TA = 0 to +70 ℃
1.8
5.5
TA = −40 to +125 ℃
2.0
5.5
80
125
V
190
μA
+125
℃
THERMAL CHARACTERISTICS
TA
Operating
temperature range
θJA
Package Thermal
Resistance
-40
SOIC-14L
115
QFN3x3-16L
65
TSSOP-14L
112
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
℃/W
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-5
Typical Performance Characteristics
4,925 Sampels
VS = 5V
VCM = –VS
800
Number of Amplifiers
700
600
AOL (dB)
500
400
300
125
100
100
75
75
50
50
25
25
0
200
0
-25
100
-25
-50
0
-10 -8
-6
-4
-2
0
2
4
6
8
-50
10
10
1k
Input Offset Voltage (μV)
Open-loop Gain and Phase as a function of
Frequency.
140
140
120
120
100
100
PSRR (dB)
CMRR (dB)
-75
10M
100k
Frequency (Hz)
Input Offset Voltage Production Distribution.
80
60
80
60
40
40
20
20
0
0
1
100
10k
1M
1
100
Frequency (Hz)
10k
1M
Frequency (Hz)
Common-mode Rejection Ratio as a function of
Frequency.
Power Supply Rejection Ratio as a function of
Frequency.
200
Quiescent Current (μA)
200
Quiescent Current (μA)
Phase (deg)
At TA=+25℃, VS=±2.5V, VCM=VS /2, RL=10kΩ connected to VS /2, and CL=100pF, unless otherwise noted.
175
150
125
100
75
50
175
150
125
100
75
50
25
25
0
0
1.5
2
2.5
3
3.5
4
4.5
5
5.5
-50
0
25
50
75
100
125
Temperature (℃)
Supply Voltage (V)
Quiescent Current as a function of Supply Voltage.
-25
Quiescent Current as a function of Temperature.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-6
Typical Performance Characteristics (continued)
At TA = +25℃, VCM = VS /2, and RL = 10kΩ connected to VS /2, unless otherwise noted.
60
Sourcing
Current
Output Voltage (V)
4
3
–40℃
+125℃
+25℃
2
1
Sinking Current
Short-circuit Current (mA)
5
0
50
40
–ISC
30
+ISC
20
10
0
0
10
20
30
40
50
60
2
2.5
3
3.5
4
4.5
5
Supply Voltage (V)
Output Current (mA)
Output Voltage Swing as a function of Output
Current.
Short-circuit Current as a function of Supply
Voltage.
1V/div
20 mV/div
CL=100pF
CL = 100pF
AV = +1
2.5μs/div
Large Signal Step Response (4V Step).
5.5
0.25 µs/div
Small Signal Step Response (100mV Step).
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
P-7
LTC8554
Application Notes
The LTC8554 operational amplifiers are unity-gain
stable and free from unexpected output phase
reversal. These devices use a proprietary autocalibration technique to provide low offset voltage
and very low drift over time and temperature. For
lowest offset voltage and precision performance,
optimize circuit layout and mechanical conditions.
Avoid
temperature
gradients
that
create
thermoelectric
(Seebeck)
effects
in
the
thermocouple junctions formed from connecting
dissimilar conductors. Cancel these thermallygenerated potentials by assuring they are equal on
both input terminals. Other layout and design
considerations include:
• Use low thermoelectric-coefficient conditions
(avoid dissimilar metals).
• Thermally isolate components from power supplies
or other heat sources.
• Shield operational amplifier and input circuitry
from air currents, such as cooling fans.
Following these guidelines reduces the likelihood of
junctions being at different temperatures, which can
cause thermoelectric voltages of 0.1μV/℃ or higher,
depending on materials used.
OPERATING VOLTAGE
The LTC8554 op-amps are fully specified and
ensured for operation from 2.0V to 5.5V (±1.0V to
±2.75V). In addition, many specifications apply from
–40℃ to +125℃. Parameters that vary significantly
with operating voltages or temperature are
illustrated in the Typical Characteristics graphs.
NOTE: Supply voltages (VS+ to VS–) higher than +10V
can permanently damage the device.
INPUT VOLTAGE
The input common-mode voltage range of the
LTC8554 extends 100mV beyond the negative supply
rail and reaches the positive supply rail. This
performance is achieved with a complementary input
stage: an N-channel input differential pair in parallel
with a P-channel differential pair. The N-channel pair
is active for input voltages close to the positive rail,
typically VS+–1.4V to the positive supply, whereas the
P-channel pair is active for inputs from 200mV below
the negative supply to approximately VS+–1.4V. There
is a small transition region, typically VS+–1.2V to VS+–
1V, in which both pairs are on. This 200mV transition
region can vary up to 200mV with process variation.
Thus, the transition region (both stages on) can range
from VS+–1.4V to VS+–1.2V on the low end, up to VS+–1V
to VS+–0.8V on the high end. Within this transition
region, PSRR, CMRR, offset voltage, offset drift, and
THD can be degraded compared to device operation
outside this region.
The typical input bias current of the LTC8554 during
normal operation is approximately 70pA. In overdriven conditions, the bias current can increase
significantly. The most common cause of an
overdriven condition occurs when the operational
amplifier is outside of the linear range of operation.
When the output of the operational amplifier is driven
to one of the supply rails, the feedback loop
requirements cannot be satisfied and a differential
input voltage develops across the input pins. This
differential input voltage results in activation of
parasitic diodes inside the front-end input chopping
switches that combine with electromagnetic
interference (EMI) filter resistors to create the
equivalent circuit. Notice that the input bias current
remains within specification in the linear region.
INPUT EMI FILTER AND CLAMP CIRCUIT
Figure 1 shows the input EMI filter and clamp circuit.
The LTC8554 op-amps have internal ESD protection
diodes (D1, D2, D3, and D4) that are connected
between the inputs and each supply rail. These
diodes protect the input transistors in the event of
electrostatic discharge and are reverse biased
during normal operation. This protection scheme
allows voltages as high as approximately 300mV
beyond the rails to be applied at the input of either
terminal without causing permanent damage. See the
table of Absolute Maximum Ratings for more
information.
Operational amplifiers vary in susceptibility to EMI. If
conducted EMI enters the operational amplifier, the
dc offset at the amplifier output can shift from its
nominal value when EMI is present. This shift is a
result of signal rectification associated with the
internal semiconductor junctions. Although all
operational amplifier pin functions can be affected by
EMI, the input pins are likely to be the most
susceptible. The EMI filter of the LTC8554 op-amps is
composed of two 5-kΩ input series resistors (RS1 and
RS2), two common-mode capacitors (CCM1 and CCM2),
and a differential capacitor (CDM). These RC networks
set the −3dB low-pass cutoff frequencies at 35-MHz
for common-mode signals, and at 22-MHz for
differential signals.
VS+
D1
RS1
500Ω
IN+
D2
D3
CCM1
RS2 CDM
500Ω
IN–
D4
CCM2
VS–
Figure 1. Input EMI Filter and Clamp Circuit
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-8
Application Notes (continued)
INTERNAL OFFSET CORRECTION
The LTC8554 operational amplifiers use an autocalibration technique with a time-continuous 500kHz
operational amplifier in the signal path. This amplifier
is zero-corrected every 2μs using a proprietary
technique. Upon power up, the amplifier requires
approximately 100μs to achieve specified VOS
accuracy. This design has no aliasing or flicker noise.
parallel with the feedback resistor to counteract the
parasitic capacitance associated with inverting node.
CF
RF
RISO
VIN
CL
CAPACITIVE LOAD AND STABILITY
The LTC8554 op-amps can safely drive capacitive
loads of up to 500pF in any configuration. As with
most amplifiers, driving larger capacitive loads than
specified may cause excessive overshoot and ringing,
or even oscillation. A heavy capacitive load reduces
the phase margin and causes the amplifier frequency
response to peak. Peaking corresponds to overshooting or ringing in the time domain. Therefore, it
is recommended that external compensation be used
if the LTC8554 op-amps mus4 drive a load exceeding
500pF. This compensation is particularly important in
the unity-gain configuration, which is the worst case
for stability.
A quick and easy way to stabilize the op-amp for
capacitive load drive is by adding a series resistor,
RISO, between the amplifier output terminal and the
load capacitance, as shown in Figure 2. RISO isolates
the amplifier output and feedback network from the
capacitive load. The bigger the RISO resistor value, the
more stable VOUT will be. Note that this method
results in a loss of gain accuracy because RISO forms
a voltage divider with the RL.
RISO
LTC8554
VOUT
LTC8554
VOUT
VIN
CL
Figure 2. Indirectly Driving Heavy Capacitive Load
An improvement circuit is shown in Figure 3. It
provides DC accuracy as well as AC stability. The RF
provides the DC accuracy by connecting the inverting
signal with the output.
The CF and RISO serve to counteract the loss of phase
margin by feeding the high frequency component of
the output signal back to the amplifier’s inverting
input, thereby preserving phase margin in the overall
feedback loop.
For no-buffer configuration, there are two others
ways to increase the phase margin: (a) by increasing
the amplifier’s gain, or (b) by placing a capacitor in
RL
Figure 3. Indirectly Driving Heavy Capacitive Load
with DC Accuracy
OVERLOAD RECOVERY
Overload recovery is defined as the time required for
the operational amplifier output to recover from a
saturated state to a linear state. The output devices
of the operational amplifier enter a saturation region
when the output voltage exceeds the rated operating
voltage, either because of the high input voltage or
the high gain. After the device enters the saturation
region, the charge carriers in the output devices
require time to return back to the linear state. After
the charge carriers return back to the linear state,
the device begins to slew at the specified slew rate.
Thus, the propagation delay in case of an overload
condition is the sum of the overload recovery time
and the slew time. The overload recovery time for
the LTC8554 op-amps is approximately 35μs.
EMI REJECTION RATIO
Circuit performance is often adversely affected by
high frequency EMI. When the signal strength is low
and transmission lines are long, an op-amp must
accurately amplify the input signals. However, all opamp pins — the non-inverting input, inverting input,
positive supply, negative supply, and output pins —
are susceptible to EMI signals. These high frequency
signals are coupled into an op-amp by various
means, such as conduction, near field radiation, or
far field radiation. For example, wires and printed
circuit board (PCB) traces can act as antennas and
pick up high frequency EMI signals.
Amplifiers do not amplify EMI or RF signals due to
their relatively low bandwidth. However, due to the
nonlinearities of the input devices, op-amps can
rectify these out of band signals. When these high
frequency signals are rectified, they appear as a dc
offset at the output.
The LTC8554 op-amps have integrated EMI filters at
their input stage. A mathematical method of
measuring EMIRR is defined as follows:
EMIRR = 20 log (VIN_PEAK / ΔVOS)
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-9
Application Notes (continued)
MAXIMIZING PERFORMANCE THROUGH PROPER
LAYOUT
To achieve the maximum performance of the
extremely high input impedance and low offset
voltage of the LTC8554 op-amps, care is needed in
laying out the circuit board. The PCB surface must
remain clean and free of moisture to avoid leakage
currents between adjacent traces. Surface coating of
the circuit board reduces surface moisture and
provides a humidity barrier, reducing parasitic
resistance on the board. The use of guard rings
around the amplifier inputs further reduces leakage
currents. Figure 4 shows proper guard ring
configuration and the top view of a surface-mount
layout. The guard ring does not need to be a specific
width, but it should form a continuous loop around
both inputs. By setting the guard ring voltage equal to
the voltage at the non-inverting input, parasitic
capacitance is minimized as well. For further
reduction of leakage currents, components can be
mounted to the PCB using Teflon standoff insulators.
Guard
Ring
+IN
–IN
+VS
Figure 4. Use a guard ring around sensitive pins
Other potential sources of offset error are
thermoelectric voltages on the circuit board. This
voltage, also called Seebeck voltage, occurs at the
junction of two dissimilar metals and is proportional
to the temperature of the junction. The most common
metallic junctions on a circuit board are solder-toboard trace and solder-to-component lead. If the
temperature of the PCB at one end of the component
is different from the temperature at the other end,
the resulting Seebeck voltages are not equal,
resulting in a thermal voltage error.
This thermocouple error can be reduced by using
dummy components to match the thermoelectric
error source. Placing the dummy component as
close as possible to its partner ensures both
Seebeck voltages are equal, thus canceling the
thermocouple error. Maintaining a constant ambient
temperature on the circuit board further reduces this
error. The use of a ground plane helps distribute heat
throughout the board and reduces EMI noise pickup.
INPUT-TO-OUTPUT COUPLING
To minimize capacitive coupling, the input and output
signal traces should not be parallel. This helps
reduce unwanted positive feedback.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-10
Typical Application Circuits
PRECISION LOW-SIDE CURRENT SHUNT SENSING
Many applications require the sensing of signals near the positive or negative rails. Current shunt sensing is
one such application and is mostly used for feedback control systems. It is also used in a variety of other
applications, including power metering, battery fuel gauging, and feedback controls in industrial applications. In
such applications, it is desirable to use a shunt with very low resistance to minimize series voltage drop. This
configuration not only minimizes wasted power, but also allows the measurement of high currents while
saving power.
A typical shunt may be 100mΩ. At a measured current of 2, the voltage produced from the shunt is 100mV, and
the amplifier error sources are not critical. However, at low measured current in the 1mA range, the 100μV
generated across the shunt demands a very low offset voltage and drift amplifier to maintain absolute
accuracy.
The unique attributes of a zero drift amplifier provide a solution. Figure 5 shows a low-side current sensing
circuit using the LTC8554 op-amp. The LTC8554 is configured as difference amplifiers with a gain of 1000.
Although the LTC8554 op-amps have high CMRR, the CMRR of the system is limited by the external resistors.
Therefore, the key to high CMRR for the system is resistors that are well matched from both the resistive ratio
and relative drift, where R1/R2 = R3/R4. The transfer function is given by:
VOUT = VSHUNT × GainDiff_Amp = (RSHUNT × ILOAD) × (R2 / R1) = 100 × ILOAD
VBUS
ILOAD
100Ω
R3
VSHUNT
0.1Ω
RSHUNT
100kΩ
R2
LTC8554
100Ω
R1
VOUT
VDD
100kΩ
R2
Figure 5. Low-Side Current Sensing Circuit
Any unused channel of the LTC8554 op-amp must be configured in unity gain with the input common-mode
voltage tied to the midpoint of the power supplies.
BIDIRECTIONAL CURRENT-SENSING
This single-supply, low-side, bidirectional current-sensing solution detects load currents from –2 to +2. The
single-ended output spans from 110mV to 3.19V. This design uses the LTC8554 because of its low offset voltage
and rail-to-rail input and output. One of the amplifiers is configured as a difference amplifier and the other
amplifier provides the reference voltage.
Figure 6 shows the solution. This solution has the following requirements:
•
Supply voltage: 3.3V
•
Input: –2 to +2
•
Output: 1.65V±1.54V (110mV to 3.19V)
There are two types of errors in this design: offset and gain. Gain errors are introduced by the tolerance of the
shunt resistor and the ratios of R4 to R3 and, similarly, R2 to R1. Offset errors are introduced by the voltage
divider (R5 and R6) and how closely the ratio of R4/R3 matches R2/R1. The latter value affects the CMRR of the
difference amplifier, ultimately translating to an offset error.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-11
Typical Application Circuits
VDD
VBUS
VREF
U1B
LTC8554
ILOAD
R2
VDD
VDD
R5
R6
R1
VSHUNT
RSHUNT
R3
U1A
LTC8554
VOUT
R4
Figure 6. Bidirectional Current-Sensing Schematic
The load current, ILOAD, flows through the shunt resistor (RSHUNT) to develop the shunt voltage, VSHUNT. The shunt
voltage is then amplified by the difference amplifier consisting of U2 and R1 through R4. The gain of the
difference amplifier is set by the ratio of R4 to R3. To minimize errors, set R2 = R4 and R1 = R3. The reference
voltage, VREF, is supplied by buffering a resistor divider using U1B. The transfer function is given by Equation 1.
VOUT = VSHUNT × GainDiff_Amp ＋ VREF
Where
•
VSHUNT = ILOAD × RSHUNT
•
GainDiff_Amp = R4 / R3
•
VREF = VDD × [R6 / (R5 ＋ R6)]
(1)
There are two types of errors in this design: offset and gain. Gain errors are introduced by the tolerance of the
shunt resistor and the ratios of R4 to R3 and, similarly, R2 to R1. Offset errors are introduced by the voltage
divider (R5 and R6) and how closely the ratio of R4/R3 matches R2/R1. The latter value affects the CMRR of the
difference amplifier, ultimately translating to an offset error.
The value of VSHUNT is the ground potential for the system load because VSHUNT is a low-side measurement.
Therefore, a maximum value must be placed on VSHUNT. In this design, the maximum value for VSHUNT is set to
100mV. Equation 2 calculates the maximum value of the shunt resistor given a maximum shunt voltage of
100mV and maximum load current of 2.
RSHUNT(MAX) = VSHUNT(MAX) / ILOAD(MAX) = 100mV / 2 = 100 mΩ
(2)
The tolerance of RSHUNT is directly proportional to cost. For this design, a shunt resistor with a tolerance of 0.5%
was selected. If greater accuracy is required, select a 0.1% resistor or better.
The load current is bidirectional; therefore, the shunt voltage range is –100mV to +100mV. This voltage is
divided down by R1 and R2 before reaching the operational amplifier, U2. Take care to ensure that the voltage
present at the non-inverting node of U2 is within the common-mode range of the device. Therefore, use an
operational amplifier, such as the LTC8554, that has a common-mode range that extends below the negative
supply voltage. Finally, to minimize offset error, note that the LTC8554 has a typical offset voltage of merely
±2μV (±10μV maximum).
Given a symmetric load current of –2 to +2, the voltage divider resistors (R5 and R6) must be equal. To be
consistent with the shunt resistor, a tolerance of 0.5% was selected. To minimize power consumption, 10kΩ
resistors were used.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-12
Typical Application Circuits
To set the gain of the difference amplifier, the common-mode range and output swing of the LTC8554 must be
considered. Equation 3 and Equation 4 depict the typical common-mode range and maximum output swing,
respectively, of the LTC8554 given a 3.3V supply.
•
–100mV < VCM < 3.4V
(3)
•
100mV < VOUT < 3.2V
(4)
The gain of the difference amplifier can now be calculated as shown in Equation 5.
GainDiff_Amp = (VOUT_MAX – VOUT_MIN) / [RSHUNT × (IMAX – IMIN)]
GainDiff_Amp = (3.2V – 100mV) / 100mΩ × [2 – (–2)] = 15.5 V/V
(5)
The resistor value selected for R1 and R3 was 1kΩ. 15.4kΩ was selected for R2 and R4 because this number is the
nearest standard value. Therefore, the ideal gain of the difference amplifier is 15.4V/V.
The gain error of the circuit primarily depends on R1 through R4. As a result of this dependence, 0.1% resistors
were selected. This configuration reduces the likelihood that the design requires a two-point calibration. A
simple one-point calibration, if desired, removes the offset errors introduced by the 0.5% resistors.
Output Voltage (V)
3.3
1.65
0
-1
-0.5
0
0.5
Input Current (A)
1
Figure 7. Bidirectional Current-Sensing Circuit Performance: Output Voltage vs. Input Current
HIGH-SIDE VOLTAGE-TO-CURRENT (V-I) CONVERTER
The circuit shown in Figure 8 is a high-side voltage-to-current (V-I) converter. It translates in input voltage of
0V to 2V to and output current of 0mA to 100mA.
V＋
RS2
470Ω
VRS2
RS3
4.7Ω
IRS2
C7
R3 200Ω
VRS3
2200pF
U1B
LTC8554
V＋
U1A
LTC8554
IRS3
R4 10kΩ
Q2
R5 330Ω
Q1
VIN
VDD
C6
1000pF
VRS1
R2 10kΩ
RS1
2kΩ
IRS1
Figure 8. Bidirectional Current-Sensing Schematic
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
VLOAD
RLOAD
ILOAD
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-13
Typical Application Circuits
The design requirements are as follows:
•
Supply Voltage: 5V DC
•
Input: 0V to 2V DC
•
Output: 0mA to 100mA DC
The V-I transfer function of the circuit is based on the relationship between the input voltage, VIN, and the three
current sensing resistors, RS1, RS2, and RS3. The relationship between VIN and RS1 determines the current that
flows through the first stage of the design. The current gain from the first stage to the second stage is based
on the relationship between RS2 and RS3.
For a successful design, pay close attention to the dc characteristics of the operational amplifier chosen for
the application. To meet the performance goals, this application benefits from an operational amplifier with low
offset voltage, low temperature drift, and rail-to-rail output. The LTC8554 CMOS operational amplifier is a
high-precision, typically 2μV offset, 5nV/℃ drift amplifier optimized for low-voltage, single-supply operation
with an output swing to within 15mV (at RL = 10kΩ) of the positive rail. The LTC8554 family uses chopping
techniques to provide low initial offset voltage and near-zero drift over time and temperature. Low offset
voltage and low drift reduce the offset error in the system, making these devices appropriate for precise dc
control. The rail-to-rail output stage of the LTC8554 ensures that the output swing of the operational amplifier
is able to fully control the gate of the MOSFET devices within the supply rails.
Figure 9 shows the measured transfer function for this circuit. The low offset voltage and offset drift of the
LTC8554 facilitate excellent dc accuracy for the circuit.
Output Current (mA)
100
75
50
25
0
0
0.5
1
1.5
2
Input Voltage (V)
Figure 9. Measured Transfer Function for High-Side V-I Converter
SINGLE-SUPPLY INSTRUMENTATION AMPLIFIER
The extremely low offset voltage and drift, high open-loop gain, high common-mode rejection, and high power
supply rejection of the LTC8554 op-amps make them excellent op-amp choices as discrete, single-supply
instrumentation amplifiers.
Figure 10 shows the classic 3-op-amp instrumentation amplifier using the LTC8554 op-amp. The key to high
CMRR for the instrumentation amplifier are resistors that are well matched for both the resistive ratio and
relative drift. For true difference amplification, matching of the resistor ratio is very important, where:
•
R5/R2 = R6/R4
•
RG1 = RG2, R1 = R3, R2 = R4, R5 = R6
•
VOUT = (VIN1 – VIN2) × (1 ＋ R1 / RG1) × (R5 / R2)
The resistors are important in determining the performance over manufacturing tolerances, time, and
temperature. Assuming a perfect unity-gain difference amplifier with infinite common-mode rejection, a 1%
tolerance resistor matching results in only 34dB of common-mode rejection. Therefore, at least 0.01% or better
resistors are recommended.
To build a discrete instrumentation amplifier with external resistors without compromising on noise, pay close
attention to the resistor values chosen. RG1 and RG2 each have thermal noise that is amplified by the total noise
gain of the instrumentation amplifier and, therefore, a sufficiently low value must be chosen to reduce thermal
noise contribution at the output while still providing an accurate measurement.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-14
Typical Application Circuits
VIN1
A1
R5
RG1
R1
R2
A3
RG2
R3
VOUT
R4
A2
R6
VIN2
Figure 10. Discrete 3-Op-amp Instrumentation Amplifier
Figure 11 shows the external resistors noise contribution referred to the output .
Resistor
Value
Resistor Thermal Noise
Thermal Noise Referred to Output
RG1
RG2
R1
R2
R3
R4
R5
R6
0.4 kΩ
0.4 kΩ
10 kΩ
10 kΩ
10 kΩ
10 kΩ
20 kΩ
20 kΩ
2.57 nV/√Hz
2.57 nV/√Hz
12.83 nV/√Hz
12.83 nV/√Hz
12.83 nV/√Hz
12.83 nV/√Hz
18.14 nV/√Hz
18.14 nV/√Hz
128.30 nV/√Hz
128.30 nV/√Hz
25.66 nV/√Hz
25.66 nV/√Hz
25.66 nV/√Hz
25.66 nV/√Hz
18.14 nV/√Hz
18.14 nV/√Hz
Figure 11. Thermal Noise Contribution Example
Note that A1 and A2 have a high gain of 1 + R1/RG1. Therefore, use a high precision, low offset voltage and low
noise amplifier for A1 and A2, such as the LTC8554 op-amp. Conversely, A3 operates at a much lower gain and
has a different set of op-amp requirements. Its input noise, referred to the overall instrumentation amplifier
input, is divided by the first stage gain and is not as important. Note that the input offset voltage and the input
voltage noise of the amplifiers are also amplified by the overall noise gain.
Any unused channel of the LTC8554 op-amp must be configured in unity gain with the input common-mode
voltage tied to the midpoint of the power supplies.
Understanding how noise impacts a discrete instrumentation amplifier or a difference amplifier (the second
stage of a 3-op-amp instrumentation amplifier) is important, because they are commonly used in many
different applications.
LOAD CELL (STRAIN GAGE) SENSOR SIGNAL CONDITIONING
The LTC8554, with its ultralow offset, drift, and noise, is well suited to signal condition a low level sensor
output with high gain and accuracy. A weigh scale (load cell) is an example of an application with such
requirements. Figure 12 shows a configuration for a single-supply, precision, weigh scale measurement
system. The LTC8554 is used at the front end for amplification of the low level signal from the load cell.
Current flowing through a PCB trace produces an IR voltage drop; with longer traces, this voltage drop can be
several millivolts or more, introducing a considerable error. A 1 inch long, 0.005 inch wide trace of 1 oz copper
has a resistance of approximately 100mΩ at room temperature. With a load current of 10mA, the resistance can
introduce a 1mV error.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-15
Typical Application Circuits
Therefore, a 6-wire load cell is used in the circuit. The load cell has two sense pins, in addition to excitation,
ground, and two output connections. The sense pins are connected to the high side (excitation pin) and low side
(ground pin) of the Wheatstone bridge. The voltage across the bridge can then be accurately measured
regardless of voltage drop due to wire resistance. The two sense pins are also connected to the analog-todigital converter (ADC) reference inputs for a ratio-metric configuration that is immune to low frequency
changes in the power supply excitation voltage.
The LTC8554 is configured as the first stage of a 3-op-amp instrumentation amplifier to amplify the low level
amplitude signal from the load cell by a factor of 1 + 2R1/RG. Capacitors C1 and C2 are placed in the feedback
loops of the amplifiers and interact with R1 and R2 to perform low-pass filtering. This filtering limits the amount
of noise entering the Σ-Δ ADC. In addition, C3, C4, C5, R3 and R4 provide further common-mode and differential
mode filtering to reduce noise and unwanted signals.
VIN1
U1A
LTC8554
REF+
REF–
VEXC
LOAD
CELL
OUT–
R1 11.3kΩ
SENSE+
C1
OUT+
R3 1kΩ
R2
RG
60.4Ω
C2
AIN+
3.3μF
3.3μF
R4 1kΩ
SENSE–
R2 11.3kΩ
C5
10μF
AIN–
C4
C3
1μF
1μF
U1B
LTC8554
Figure 12. Precision Weigh Scale Measurement System
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
A/D
Converter
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-16
Package Outlines
DIMENSIONS, SOIC-14L
A3
A2
A
A1
D
b
C
e
L1 L
E
Symbol
E1
A
A1
A2
A3
b
C
D
E
E1
e
L1
L
θ
Dimensions
In Millimeters
Min
Max
1.450
1.850
0.100
0.300
1.350
1.550
0.550
0.750
0.406 TYP.
0.203 TYP.
8.630
8.830
5.840
6.240
3.850
4.050
1.270 TYP.
1.040 REF.
0.350
0.750
2°
8°
Dimensions
In Inches
Min
Max
0.057
0.073
0.004
0.012
0.053
0.061
0.022
0.030
0.016 TYP.
0.008 TYP.
0.340
0.348
0.230
0.246
0.152
0.159
0.050 TYP.
0.041 REF.
0.014
0.030
2°
8°
θ
RECOMMENDED SOLDERING FOOTPRINT, SOIC-14L
14X
5.40
0.213
(1.50)
MAX
(0.059)
(3.90)
MIN
(0.154)
1
(0.60)
MAX 14X
(0.024)
PITCH
1.270
0.050
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
mm
( inches
)
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-17
Package Outlines
DIMENSIONS, QFN3x3-16L
SIDE VIEW
A
TOP VIEW
Symbol
A3
D
A
A3
b
D
D1
E
E1
e
h
K
L
BOTTOM VIEW
K
9
L
12
8
E
13
E1
e
D1
h
5
16
PIN#1
4
Min.
0.70
0.20
2.90
1.60
2.90
1.60
0.20
0.225
0.35
Millimeters
Nom.
0.75
0.210 REF.
0.25
3.00
1.65
3.00
1.65
0.50 BSC.
0.25
0.275
0.40
1
b
RECOMMENDED SOLDERING FOOTPRINT, QFN3x3-16L
0.65
0.20
3.50
1.70
1.70
1.80
0.30
0.25
0.50
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
Max.
0.80
0.30
3.10
1.70
3.10
1.70
0.30
0.325
0.45
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
LTC8554
P-18
Package Outlines (continued)
DIMENSIONS, TSSOP-14L
A3 A2
A
Symbol
A1
D
b
e
C
L1 L
E
E1
A
A1
A2
A3
b
C
D
E
E1
e
L1
L
θ
Dimensions
In Millimeters
Min
Max
1.200
0.050
0.150
0.900
1.050
0.390
0.490
0.200
0.290
0.130
0.180
4.860
5.060
6.200
6.600
4.300
4.500
0.650 TYP.
1.000 REF.
0.450
0.750
0°
8°
Dimensions
In Inches
Min
Max
0.047
0.002
0.006
0.035
0.041
0.015
0.019
0.008
0.011
0.005
0.007
0.191
0.199
0.244
0.260
0.169
0.177
0.026 TYP.
0.039 REF.
0.018
0.030
0°
8°
θ
RECOMMENDED SOLDERING FOOTPRINT, TSSOP-14L
14X
(1.45)
MAX
(0.057)
(4.40)
MIN
(0.173)
PITCH
0.65
0.026
1
5.90
0.232
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
14X
(0.45)
MAX
(0.018)
mm
( inches
)
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
P-19
LTC8554
IMPORTANT NOTICE
Linearin is a global fabless semiconductor company specializing in advanced high-performance high-
quality analog/mixed-signal IC products and sensor solutions. The company is devoted to the innovation
of high performance, analog-intensive sensor front-end products and modular sensor solutions, applied
in multi-market of medical & wearable devices, smart home, sensing of IoT, and intelligent industrial &
smart factory (industrie 4.0). Linearin’s product families include widely-used standard catalog products,
solution-based application specific standard products (ASSPs) and sensor modules that help customers
achieve faster time-to-market products. Go to http://www.linearin.com for a complete list of Linearin
product families.
For additional product information, or full datasheet, please contact with the Linearin’s Sales Department
or Representatives.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
Linearin and designs are registered trademarks of Linearin Technology Corporation.
© Copyright Linearin Technology Corporation. All Rights Reserved.
All other trademarks mentioned are the property of their respective owners.
FN1617-21Q.2 — Data Sheet
Zero-Drift, Micro-Power 1.5MHz, RRIO Operational Amplifiers
工商网监
湘ICP备2023018690号
