dm00105150

DT0015
Design tip
Chop away input offsets with TSZ121/TSZ122/TSZ124
By Preet Sibia
Main components
TSZ121
Single very high accuracy (5 μV) zero drift micropower 5 V
operational amplifier
TSZ122
Dual very high accuracy (5 μV) zero drift micropower 5 V
operational amplifiers
TSZ124
Quad very high accuracy (5 μV) zero drift micropower 5 V
operational amplifiers
Purpose and benefits

Review of input offset voltage

Overview of TSZ12x chopper design to provide highest precision

Example applications using TSZ family to achieve highest accuracy
Description
The TSZ121/TSZ122/TSZ124 series of high precision operational amplifiers
offer very low input offset voltages with virtually zero drift. These devices also
feature rail-to-rail input and output and an excellent speed/power consumption
ratio. The 5uV input offset voltage and near zero drift are key parameters of
interest for small signal precision system designs.
Input offset voltage basics:
For the textbook ideal op amp, if both inputs of an op amp are at exactly the same voltage,
then the output of the amplifier will be zero volts. In the real world, due to process
variations of each input stage, a small differential voltage is needed on the inputs to force
the output to zero. This voltage called the input offset voltage, VOS as shown in Figure 1.
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Figure 1. Input offset voltage circuit model
Input offset voltage also varies with temperature, and its temperature
coefficient is known as ΔVos/ΔT or commonly as drift. Typical drift values for
general precision op amps are in the range 1-10 μV/°C. Drift is specified in ST
op amp datasheets such as the TSZ12x family of products. Table 1 highlights
typical offset and drift values for commonly used op amp process
technologies.
Table 1.
Typical Input offset voltage and drift ranges by amplifier technology
Amplifier Type
Input offset voltage
ranges
Drift ΔVos/ΔT
TSZ121/TSZ122/TSZ124
5μV
30nV/°C
Chopper-Stabilized Op Amps
<20μV
50-200nV/°C
General Precision Op Amps
50μV - 500μV
<1-10μV/°C
Bipolar Op Amps
100μV – 10mV
1-10μV/°C
JFET Input Op Amps
100μV – 3mV
1-30μV/°C
CMOS Input Op Amps
500μV – 10mV
1-10μV/°C
In small signal, low-speed applications, such as sensor signal conditioning or
high-resolution ADC front ends, input offset often becomes a major obstacle to
overcome for the precision designer. For these applications and others where
the lowest offset and drift performance are required, specialized chopperstabilized amplifiers such as the TSZ12x products are often the only viable
options.
TSZ family Chopper Design Technique
Chopper-stabilized amps constantly correct low-frequency errors across the
inputs of the amplifier. The TSZ12x family uses a 400kHz clock for chopper
synchronization. The analysis of the frequency domain provides a good look at
the modulation technique of the TSZ chopper-stabilized architecture as shown
in Figure 2.
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Figure 2. Block diagram of frequency domain of TSZ chopper-stabilized architecture
The modulation technique transposes the signal to a higher frequency where
there is no 1/f noise, and demodulates it back after amplification.
1. Figure 2 shows the input signal Vin is modulated once (Chop1) so the entire input
signal is transposed to the high frequency domain.
2. The amplifier adds its own error (Vos (output offset voltage) + the noise Vn (1/f noise))
to this modulated signal.
3. This signal is then demodulated (Chop2), but since the noise and the offset are
modulated only once, they are transposed to the high frequency, leaving the output
signal of the amplifier without any offset and low frequency noise. Consequently, the
input signal is amplified with a very low offset and 1/f noise.
4. To get rid of the un-necessary high frequency component of the output signal, a low
pass filter is implemented providing the near zero offset output.
To further suppress the remaining ripple down to a desired level, another low
pass filter may be added externally on the output of the TSZ121, TSZ122, or
TSZ124 device.
Applications:
Several applications benefit significantly from the low input offset voltages and
low noise provided by chopper-stabilized amplifiers. Using chopper-stabilized
amplifiers in designs is really not much different from using any operational
amplifier. Most new designs, such as with the TSZ121/122/124, have similar
pin outs and functionality as any other amplifier. Resistors are used to set the
DC closed- loop gain just as in any other op amp. Typical designs such as
filtering and integration can be done in the same way. In this section, we will
highlight a couple of key applications such as precision current shunt amplifier
and precision ADC front-ends.
Precision Current Shunt Amplifier
Shunt current sensors are often used in precision current sources for feedback
control in power systems. Battery fuel gauging, electric power steering and
precision power metering are just some of the other common applications
where low-side current sensing is found.
In these applications it is desirable to use a shunt with very low resistance to
minimize power losses and to accurately measure currents, generally < 0.1 Ω.
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To measure low current values in the 1mA range, such as in battery fuel
gauging, the μV range output voltage of the shunt requires ultra-low offset
voltage and zero drift to maintain absolute accuracy. Low input bias currents
are also needed so that “injected” bias current does not become a significant
percentage of the measured current. Therefore, CMOS-input chopper amps
such as the TSZ121/2/4 are an excellent fit for this application.
Figure 3. TSZ12x as a precision current shunt amplifier
Equation 1:
( )
(
)
Due to the ultra-low offset and low input offset currents of the TSZ12x
products, Equation 1 simplifies to
Equation 2:
( )
TSZ121 as a precision ADC front-end
Many manufacturers quote eye-catching bit counts for their Analog-to-Digital
Converters (ADC), however achieving this performance is tied to selecting the
best front-end amplifier and the inherent linearity and offset errors of the ADC.
Even for now-standard 12-bit+ ADCs found in most microcontrollers, the right
amplifier can make all of the difference in ensuring all of your LSBs get
resolved, especially with ever decreasing voltage ranges for full-scale signals.
Figure 4 and Table 2 show how a simple circuit can go wrong if we choose an
amplifier with too much input offset voltage. In this example with a 12-bit ADC,
the TSZ121 ensures no LSBs are lost and the full signal is resolved.
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Figure 4. ADC front-end with a gain of -10.
Table 2.
Device
LSBs lost due to input offset voltage.
VOS (max)
Max offset at ADC
LSB lost
TSZ121
5 μV
55 μV
0
TS507
100 μV
1.1mV
1
TS512A
500 μV
5.5mV
7
TS512
2.5 mV
27.5mV
34
Conclusion
In this design tip, we reviewed the basics of op amp input offset voltage and
the range of offset values by process technology. STMicroelectronics’ new
chopper-stabilized amplifiers, as part of the TSZ family of products, achieves
input offset of nearly zero with zero drift. This family of precision amplifiers is
well suited to achieve high accuracy designs in low-side current monitoring
applications, sensor signal conditioning, and high-accuracy ADC front-end
needs.
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Support material
Documentation
Datasheet TSZ121, TSZ122, TSZ124. Very high accuracy (5 μV) zero drift micropower 5 V
operational amplifiers
Application note, AN4348 Signal Conditioning for Electrochemical Sensors
www.st.com/opamps/
Revision history
Date
16-Jan-2014
January 2014
Version
1
Changes
Initial release
DT0015 Rev 1
6/7
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