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Texas Instruments Comparing isolated amplifiers and isolated modulators (Rev. A) Application notes
Application Report
SBAA359A – February 2019 – Revised February 2019
Comparing isolated amplifiers and isolated modulators
Krunal Maniar
ABSTRACT
Industrial applications such as motor drives, photo voltaic inverters, and uninterruptible power supplies
(UPS) and automotive applications such as onboard chargers (OBCs), traction inverters, and DC/DC
converters operate at high voltage and current levels to optimize overall efficiency and power throughput.
These systems are subjected to hostile environments such as electrical noise, vibration, mechanical
shock, extreme temperatures, ingress of contaminants, and so forth. Such systems demand robust,
reliable, galvanic isolation to isolate high voltages from low-voltage circuits. The feedback signals
measured on these high voltages are galvanically isolated from the low-voltage controllers by isolated
amplifiers or isolated modulators.
This document compares isolated amplifiers and isolated-modulator-based solutions and explains some
unique advantages of isolated-modulator-based solutions.
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2
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5
6
Contents
Introduction to Isolated Amplifiers .........................................................................................
Introduction to Isolated Modulators ........................................................................................
Performance Comparison Between Isolated Amplifiers and Isolated Modulators...................................
Isolated Modulators in Traction Inverters .................................................................................
Isolated Amplifier and Modulator Recommendations ...................................................................
Conclusion ....................................................................................................................
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2
3
4
5
5
List of Figures
1
Isolated Amplifier Implementation ......................................................................................... 2
2
Isolated Modulator Implementation ........................................................................................ 2
3
Implementation of Two Digital Filter in Parallel .......................................................................... 4
4
Current Measurement Using an Isolated Modulator
....................................................................
4
List of Tables
..................................
1
Performance Comparison Between Isolated Amplifiers and Isolated Modulators
3
2
Performance Trade-Off Between ENOB and Settling, Latency or Bandwidth for the AMC1306 at CLKIN =
20 MHz Using a Sinc3 Filter ................................................................................................ 3
3
Recommended Devices..................................................................................................... 5
Trademarks
All trademarks are the property of their respective owners.
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1
Introduction to Isolated Amplifiers
1
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Introduction to Isolated Amplifiers
Figure 1 shows the implementation of an isolated-amplifier-based measurement solution.
Isolation
Barrier
û Mod
Retiming and Low Pass Filter
Amplifier
Receiver
Analog
Input
AVDD2
Receiver
AVDD1
OUTP
-
VO
ADC
+
OUTN
Diff-to-SE
MCU/DSP
VREF
Isolated Amplifier
AGND2
AGND1
Figure 1. Isolated Amplifier Implementation
The input stage of an isolated amplifier consists of an input amplifier that drives a delta-sigma (ΔΣ)
modulator. The gain of the input amplifier is fixed and set by internal precision resistors. The ΔΣ modulator
uses the internal reference voltage and clock generator to convert the analog input signal to a digital bit
stream. The drivers transfer the output of the modulator across an isolation barrier that separates the high
and low voltage domains. The received bitstream and clock are synchronized and processed by an analog
low-pass filter on the low voltage side and presented as an analog output signal.
The differential output of the isolated amplifier is often converted to a single-ended analog output with an
op-amp-based circuit. This op-amp-based circuit can also implement a low-pass filter to further reduce the
signal bandwidth to a bandwidth of interest and thereby improve the system noise performance.
The analog-to-digital converter (ADC), either external or internal to the microcontroller (MCU) or digital
signal processor (DSP), receives this feedback analog output and converts this output back to the digital
domain.
2
Introduction to Isolated Modulators
Figure 2 shows the implementation of an isolated-modulator-based measurement solution.
AVDD
DOUT
Interface
Receiver
Isolation
Barrier
û Mod
Amplifier
Receiver
Analog
Input
DVDD
Digital
Filtering
CLKIN
MCU/
DSP/
FPGA
Isolated Modulator
AGND
DGND
Figure 2. Isolated Modulator Implementation
The input stage of an isolated modulator is similar to that of an isolated amplifier. The drivers transfer the
modulator output across the isolation barrier. The isolated data output DOUT provides a digital bit stream
of 1's and 0's at a much higher frequency (up to 20 MHz). The time average of this bit stream output is
proportional to the analog input voltage. The measured signal is reconstructed with a digital filter inside the
microcontroller families such as the TMS320F2807x and TMS320F2837x, a DSP, or a field programmable
gate array (FPGA).
2
Comparing isolated amplifiers and isolated modulators
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Performance Comparison Between Isolated Amplifiers and Isolated Modulators
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3
Performance Comparison Between Isolated Amplifiers and Isolated Modulators
Table 1 shows the basic difference in performance between isolated amplifiers and isolated modulators.
Table 1. Performance Comparison Between Isolated Amplifiers and Isolated Modulators
CATEGORY
ISOLATED AMPLIFIER
ISOLATED MODULATOR
Sample resolution
11 bits (bandwidth = 100 kHz)
> 14 bits achievable,
trade-off between resolution and bandwidth or
latency
Latency
2 µs to 3 µs (fixed)
< 1 µs achievable,
trade-off between resolution and bandwidth or
latency
Bandwidth
up to 300 kHz
> 1 MHz achievable,
trade-off between resolution and bandwidth or
latency
Accuracy and drift
performance
High
Very high
No. of components
needed
More
Less
In an isolated-amplifier-based solution, the measured analog signal undergoes several analog-to-digital
and digital-to-analog conversions. The stages within the isolated amplifier, the differential-to-single-ended
stage, and the ADC either external or internal to the MCU or DSP reduce overall accuracy and noise
performance and increase latency. The fixed low-pass filter implementation in the output stage of the
isolated amplifier limits the signal bandwidth. An external op-amp-based circuit used for differential-tosingle-ended conversions can be used to create an active low-pass filter to further limit the signal
bandwidth and thereby improve noise performance. The isolated amplifier has a fixed latency. Isolatedamplifier-based solutions are widely used because of familiarity and relative ease of implementation.
As shown in Figure 2, the measured analog signal in an isolated-modulator-based solution undergoes only
one analog-to-digital conversion. This solution eliminates the need for a differential-to-single-ended stage,
thereby reducing the number of components and solution size. The ADC used in an isolated-amplifierbased solution, which in many situations limits the maximum achievable sample resolution and accuracy,
is not needed anymore. This isolated-modulator-based approach has improved signal noise performance,
overall accuracy, and can achieve higher signal bandwidth and lower latency than an isolated-amplifierbased solution. Isolated modulators provide a much faster digital bitstream output, typically up to 20 MHz.
The sigma-delta filter module (SDFM) module inside the microcontroller families (such as the
TMS320F2807x and TMS320F2837x) provides an easy way to tune the noise performance and signal
bandwidth or latency. As shown in Table 2, a higher oversampling ratio (OSR) implementation leads to
better accuracy and higher sample resolution but less signal bandwidth and higher latency. Similarly,
lowering OSR reduces accuracy and sample resolution but increases bandwidth and reduces latency. A
similar DSP or an FPGA can also implement such a digital filter.
Table 2. Performance Trade-Off Between ENOB and Settling, Latency or Bandwidth for the
AMC1306 at CLKIN = 20 MHz Using a Sinc3 Filter
OSR
ENOB (Bits)
SETTLING (µs)
LATENCY (µs)
BANDWIDTH (kHz)
8
4.65
1.2
0.6
1250
16
7.57
2.4
1.2
625
32
10.02
4.8
2.4
312.5
64
12.3
9.6
4.8
156.25
128
13.51
19.2
9.6
78.13
256
14.11
38.4
19.2
39.06
512
14.39
76.8
38.4
19.53
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Isolated Modulators in Traction Inverters
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Additionally, as shown in Figure 3, multiple digital filters can be implemented in parallel to achieve higher
sample resolution, lower latency, and higher bandwidth, all at the same time. One of the digital filters can
implement a high OSR digital filter for better noise performance and another one can implement a lowlatency digital filter.
Isolated Modulator
Digital Filter (1)
High Resolution
High Latency
Low Bandwidth
DOUT
Analog
Input
Low Resolution
Low Latency
High Bandwidth
Digital Filter (2)
CLKIN
MCU/DSP/FPGA
Figure 3. Implementation of Two Digital Filter in Parallel
With the system advantages offered by an isolated-modulator-based solution, there is a resulting trend to
move to an isolated-modulator-based solution in high-performance systems.
4
Isolated Modulators in Traction Inverters
Figure 4 shows an implementation of an isolated-modulator-based solution in automotive traction
inverters.
Bus Voltage
ISENSE
RSHUNT
Isolated
Modulator
M
CLKIN
DOUT
Digital
Filter
MCU/
DSP/
FPGA
PWM
Signals
Figure 4. Current Measurement Using an Isolated Modulator
Traction inverters have a direct influence on the driving experience and require precise control of speed
and torque of the traction motor. A shunt coupled with an isolated delta-sigma modulator provides the
highest-quality feedback signals to the controller in order to establish the pulse-width modulation (PWM)
pattern for bridge transistors. The digital filter implementation allows the engineer to tune the quality of
traction motor controls.
As shown in Figure 3, the FPGA, MCU, and DSP can have multiple digital filters running in parallel. One
of the digital filters can be a high-performance digital filter that provides accurate feedback signals to
control the bridge transistors. Another digital filter can be a low-latency digital filter for detecting overload
or overcurrent conditions. A third-order (sinc3) filter with a different OSR can be used for both digital filters.
4
Comparing isolated amplifiers and isolated modulators
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Isolated Amplifier and Modulator Recommendations
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5
Isolated Amplifier and Modulator Recommendations
Table 3 lists the recommended devices for use with the isolated amplifier and modulator.
Table 3. Recommended Devices
6
DEVICE
ISOLATION
DESCRIPTION
AMC1306
Reinforced
±50-mV, ±250-mV small isolated modulators
AMC1305,
AMC1305-Q1
Reinforced
±50-mV, ±250-mV isolated modulators
AMC1301,
AMC1301-Q1
Reinforced
±250-mV isolated amplifiers
AMC1302,
AMC1302-Q1
Reinforced
±50-mV isolated amplifiers
AMC1311,
AMC1311-Q1
Reinforced
0-V to 2-V isolated amplifiers
Conclusion
Isolated modulators offer higher sample resolution and accuracy compared to isolated amplifiers. With the
combination of isolated modulators and custom digital filters, the engineer can trade system latency and
bandwidth with sample resolution. Isolated-modulator-based solutions require fewer components and
enable a smaller solution size at a reasonable cost. Isolated modulators are strongly recommended in
isolated measurement applications wherein high sample resolution or low latency is required.
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Revision History
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Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Original (February 2019) to A Revision .................................................................................................. Page
•
•
6
Changed digital bitstream output from up to 20 MHz to typically up to 20 MHz .................................................. 3
Changed Performance Trade-Off Between ENOB and Settling, Latency or Bandwidth table .................................. 3
Revision History
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