Texas Instruments | Zero-drift Amplifiers: Features and Benefits (Rev. B) | Application notes | Texas Instruments Zero-drift Amplifiers: Features and Benefits (Rev. B) Application notes

Texas Instruments Zero-drift Amplifiers: Features and Benefits (Rev. B) Application notes
Zero-Drift Amplifiers: Features and Benefits
Errol Leon, Richard Barthel, Tamara Alani
Introduction
VDD
Applications suitable for zero-drift amplifiers
Zero-drift amplifiers are suitable for a wide variety of
general-purpose and precision applications that benefit
from stability in the signal path. The excellent offset
and drift performance of these amplifiers make it
especially useful early in the signal path, where high
gain configurations and interfacing with micro-volt
signals are common. Common applications that benefit
from this technology include precision strain gauge
and weight scales, current shunt measurement,
thermocouple-, thermopile-, and bridge-sensor
interfaces.
Rail-to-rail zero-drift amplifiers
System performance can be optimized by using
standard continuous-time amplifiers plus a systemlevel auto-calibration mechanism. However, this
additional auto-calibration requires complicated
hardware and software which results in increased
development time, cost and board space. The
alternative and more efficient solution is to use a zerodrift amplifier, such as the OPA388.
A traditional rail-to-rail input CMOS architecture has
two differential pairs; one PMOS transistor pair (blue)
and one NMOS transistor pair (red). Zero-drift
amplifiers with rail-to-rail input operation use the same
complementary p-channel (blue) and n-channel (red)
input configuration shown below in Figure 1.
SBOA182B – February 2017 – Revised August 2018
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VIN-
Q2
Q3
Q4
VIN+
To Voltage
Amplification Stage
Q1
VSS
Figure 1. Simplified PMOS / NMOS Differential Pair
The result of this input architecture exhibits some
degree of crossover distortion (for more information on
crossover distortion, see Zero-crossover Amplifiers:
Features and Benefits). However, the offset of the
amplifier is corrected through internal periodic
calibration, so the magnitude of the offset transition
and the crossover distortion is greatly diminished.
Figure 2 shows a comparison of the offset between a
standard CMOS rail-to-rail and a zero-drift amplifier.
200
Input Offset Voltage (µV)
Zero-drift amplifiers employ a unique, self-correcting
technology which provides ultra-low input offset
voltage (VOS) and near-zero input offset voltage drift
over time and temperature (dVOS/dT) suitable for
general and precision applications. TI’s zero-drift
topology also delivers other advantages including no
1/f noise, low broadband noise, and low distortion –
simplifying development complexity and reducing cost.
This may be done 1 of 2 ways; chopper or autozeroing. This tech note will explain the differences
between standard continuous-time and zero-drift
amplifiers.
100
0
Zero-Drift Amplifier
- 100
- 200
Standard CMOS Rail-toRail Amplifier
- 300
0.0
1.0
2.0
3.0
4.0
5.0
Common Mode Voltage (V)
Figure 2. CMOS and Zero-drift Input Offset Voltage
Comparison
How zero-drift works
Chopping zero-drift amplifiers' internal structure can
have as many stages as continuous-time amplifiers –
the main difference is that the input and output of the
first stage has a set of switches that inverts the input
signal every calibration cycle. Figure 3 shows the first
half cycle. In the first half cycle, both sets of switches
are configured to flip the input signal twice, but the
offset flips once. This keeps the input signal in phase
but the offset error polarity is reversed.
Zero-Drift Amplifiers: Features and Benefits Errol Leon, Richard Barthel, Tamara Alani
Copyright © 2017–2018, Texas Instruments Incorporated
1
www.ti.com
+IN
+
VOS
t
VOUT
+
GM1
-a
Voltage Noise Spectral Density (nV/rtHz)
±IN
t
CC
Figure 5 shows the 1/f and broadband voltage noise
spectral density for a zero-drift (red) and continuoustime (black) amplifier. Notice the zero-drift curve has
no 1/f voltage noise.
Figure 3. First Half-cycle of Internal Structure
Figure 4 shows the second half cycle. Here, both sets
of switches are configured to pass the signal and
offset error through unaltered. Effectively, the input
signal is never out of phase, remaining unchanged
from end to end. Since the offset error from the first
clock phase and second clock phase are opposite in
polarity, the error is averaged to zero.
1000
100
Continuous-time Amplifier
10
Zero-Drift Amplfier
1
1
10
100
+IN
+
10k
100k
±IN
CC
VOUT
-a
Figure 4. Second Half-cycle of Internal Structure
A synchronous notch filter is used at the same
frequency of switching to attenuate any residual error.
This principle continues to be in effect throughout the
amplifier’s operation across its input, output and
environment. In essence, TI’s zero-drift technology
delivers ultra-high performance and outstanding
precision owing to this self-correcting mechanism.
Table 1 shows a comparison of VOS and dVOS/dT of a
continuous-time and zero-drift amplifier. Notice that the
VOS and dVOS/dT are three orders of magnitude smaller
on the zero-drift amplifier.
Again, why zero-drift?
Zero-drift amplifiers provide ultra-low input offset
voltage, near-zero input offset voltage drift over
temperature and time, and no 1/f voltage noise –
design factors which are crucial to general purpose
and precision applications.
Additional Resources
Table 2 below highlights some of TI’s zero-drift
amplifiers. For a full list, see our parametric search tool
results by visiting: ti.com/opamps.
Table 2. TI's Zero-Drift Amplifiers
Device
Table 1. Input Offset Voltage and Drift Comparison
OPA388
(Zero-drift)
OPA2325
(Continuous-time)
Vos (µV)
dVOS/dT (µV/°C)
0.25
0.005
max
5
0.05
typ
40
2
max
150
7.5
typ
C001
Figure 5. Voltage Noise Comparison
t
GM1
Device
1M
+
VOS
t
Auto-zeroing requires a different topology but results in
similar functionality. The auto-zeroing technique has
less distortion at the output. Chopping results in lower
broadband noise.
Optimized Parameters
OPA388
Zero-crossover, Offset (max): 5μV, Drift (max):
2.5V<Vs<5.5V 0.05μV/°C, GBW: 10MHz, Noise: 7nV/√Hz, RRIO
OPA2333P
2mm × 2mm SON package, Offset (max): 10μV,
1.8V<Vs<5.5V Drift (max): 0.05μV/°C , IQ(max): 25μA/Ch, RRIO
OPA333
Offset (max): 10μV, Drift (max): 0.05μV/°C , IQ(max):
1.8V<Vs<5.5V 25μA/Ch, RRIO
OPA188
4V<Vs<36V
Offset (max): 25μV, Drift (max): 0.085μV/°C,
GBW: 2MHz, Noise: 8.8nV/√Hz, RRO
OPA317
Offset (max): 20μV, Drift (max): 0.05μV/°C, IQ(max):
1.8V<Vs<5.5V 35μA/Ch, RRIO
OPA189
Offset (max): 3μV, Drift (max): 0.02μV/°C, GBW:
4.5V<Vs<36V 14MHz, Noise: 5.2nV/√HzRRO
Noise in zero-drift amplifiers
Table 3. Related Documentation
In general, zero-drift amplifiers offer the lowest 1/f
noise (0.1Hz – 10Hz). 1/f noise (also referred to as
flicker or pink noise) is the dominant noise source at
low frequencies and can be detrimental in precision
DC applications. Zero-drift technology effectively
cancels slow varying offset errors (such as
temperature drift and low frequency noise) using the
periodic self-correcting mechanism.
2
1k
Frequency (Hz)
SBOA181
Zero-Drift Amplifiers: Features and Benefits Errol Leon, Richard Barthel, Tamara Alani
Zero-crossover Amplifiers: Features and Benefits
SBOA182B – February 2017 – Revised August 2018
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