AN614: A Simple Alternative to Analog Isolation Amplifiers

AN614
A SIMPLE ALTERNATIVE
TO
ANALOG ISOLATION AMPLIFIERS
1. Introduction
Analog circuits sometimes require linear (analog) signal isolation for safety, signal level shifting, and/or ground loop
elimination. Linear signal isolation is typically difficult to implement, costly, and often exhibits mediocre
performance. While the design community thirsts for a flexible and inexpensive linear isolator solution, it is the
analog isolation amplifier (ISOamp) that most often captures the socket.
ISOamps are hybridized devices that contain linear input and output circuits separated by an internal isolation
barrier. ISOamps typically modulate the linear input signal and transmit the resulting digital information across the
isolation barrier to a demodulator where it is converted back to analog (Figure 1). ISOamps typically employ
transformers and high-voltage capacitors or optocouplers in the isolation barrier design. While ISOamps are a
convenient single-package linear isolation solution, the industry's limited device offerings often force the designer
to make difficult trade-offs or add external circuitry for a complete solution. In addition, transformer-based ISOamps
are susceptible to signal corruption from external field interference and errors due to poor common-mode transient
immunity (CMTI). Optocoupler-based ISOamp versions also often suffer from poor linearity and even worse CMTI.
This application note provides insight into a robust linear isolator reference design based on the Silicon Labs
Si86xx family of CMOS digital isolators.
ISOLATION
BARRIER
Modulator
or Driver
Input
Demodulator
or
Conditioner
Input Amp
Output
Output Amp
Figure 1. ISOamp Block Diagram
Rev. 0.2 2/13
Copyright © 2013 by Silicon Laboratories
AN614
AN614
2. Silicon Labs’ ISOlinear Reference Design Overview
Silicon Labs offers the ISOlinear reference design (Si86IsoLin), a simple linear isolation circuit that is enabled by
the Si86xx isolator family, offering industry-leading integration, operating performance, and reliability. The Si86xx
isolators are available in UL/VDE/CSA certified isolation ratings of 2.5 or 5 kV. The ISOlinear reference design
architecture is shown in Figure 2. A self-oscillating pulse width modulator (PWM) converts the linear input voltage
into a series of fixed-frequency, variable duty, cycle pulses in which the width of each pulse is proportional to the
sampled input signal amplitude as shown in the waveform diagrams at the bottom of Figure 2. This modulated
(digital) signal is passed through the CMOS digital isolator and then restored to analog format by a fourth-order
analog filter.
Unlike ISOamps, the ISOlinear reference design gives the designer flexibility to craft low-cost, linear isolators that
meet the needs of the end application. For example, the user may elect to relax output ripple requirements to use a
lower order (lower cost) output filter; or modulator performance can be enhanced if higher slew rate op-amps and/
or low-delay comparators are used, and so on.
SELF-OSCILLATING MODULATOR
RFB
RF
CINT
VIN
RINT
Si86xx
Isolator
4th ORDER FILTER (DEMODULATOR)
RIN1
RIN
VDD
R1
VDD
VREF
VDD
VOUT
RIN2
CF1
CF2
R2
VDD
VSS
C1
C2
RREF1
RREF2
t0
Modulator
Input
t1
t2
t3
t4
t0
t5
Modulator
Samples
t1
t2
t3
t4
Filter
Output
Modulator
Output
Filter
Output
Figure 2. ISOlinear Reference Design Schematic and Operation
2
Rev. 0.2
t5
AN614
Si8663
Amp
Modulator
Amp
Modulator
Amp
Modulator
Amp
Filter
Amp
Filter
Amp
Filter
Isolator Channel
Isolator Channel
Isolator Channel
Isolator Channel
Isolator Channel
Isolator Channel
Filter
Amp
Filter
Amp
Filter
Amp
Modulator
Amp
Modulator
Amp
Modulator
Amp
Figure 3. Six-Channel Linear Isolator Block Diagram
In addition, a single Si86xx isolator can host multiple linear isolators by adding additional modulator/filter circuits to
a multi-channel Si86xx digital isolator, as shown in Figure 3. In this example, an Si8663 six-channel digital isolator
provides three forward and three reverse linear isolator channels, amortizing the isolator across all six channels.
9‐Bit Version
(Circuit # 3)
10‐Bit Version
(Circuit #2)
12‐Bit Version
(Circuit #1)
Figure 4. ISOlinear Reference Design Evaluation Board
Figure 4 shows the board included in the ISOlinear reference design kit. This board contains reference design
configurations for 9-bit, 10-bit, and 12-bit linear isolator performance, enabling the user to optimize isolator cost/
performance trade-offs. (For detailed circuit ISOlinear reference design schematics, see the “ISOlinear Reference
Design Users Guide” available for download at www.silabs.com/isolation.)
Rev. 0.2
3
AN614
Figure 5 shows total harmonic distortion and nonlinearity measurements from the evaluation board of Figure 4.
Waveform A shows the ISOlinear input and output oscilloscope waveforms. The FFT of waveform A reveals a total
harmonic distortion (THD) of 0.096% (waveform B). The performance of this design rounds to 12-bit performance
with 0.125% nonlinearity. (Note: for more information about the ISOlinear Reference Design, please see
application note “AN559: Isolating Analog Signals Using the Si86xx CMOS Isolator Family”. This application note
provides detailed technical information for modifying the stock ISOlinear reference design, including modulator and
filter design equations, design guidelines and more. The ISOlinear Reference Design Kit (Si86IsoLin)is available
for purchase at www.silabs.com/isolation.)
Input
FFT
Output
0.096% THD
A
B
0.125% Nonlinearity
Linearity
2.85
Vout (V)
2.75
2.65
2.55
2.45
2.35
2.25
C
- 0.25
-0.2
-0.15
-0.1
-0.05
0
Vin (V)
0.05
0.1
0.15
0.2
0.25
Figure 5. ISOlinear Reference Design Performance Example
Table 1 compares the ISOlinear reference design performance against several commercially-available ISOamps.
The ISOlinear reference designs offer higher signal-to-noise (sans the 9-bit version); higher CMTI, greater isolation
rating flexibility and lower cost compared to competing ISOamps. (Note: a higher cost, faster comparator can be
used to reduce nonlinearity to less than 0.1%.)
4
Rev. 0.2
AN614
Table 1. ISOlinear vs. Analog Isolation Amplifiers
Device
SNR (dB)
BW (kHz)
Linearity
Isolation
Rating (kV)
CMTI
(kV/µs)
Price at 1 kU
($)
Silicon Labs ISOlinear
(12-Bit Reference Design)
71.8
100
0.125
2.5 or 5.0
35 (min),
50 (typ)
2.50 to 3.50
Silicon Labs ISOlinear
(10-Bit Reference Design)
62
250
0.125
2.5 or 5.0
35 (min),
50 (typ)
2.00 to 2.50
Silicon Labs ISOlinear
(9-Bit Reference Design)
50.8
500
0.125
2.5 or 5.0
35 (min),
50 (typ)
1.75 to 2.25
Avago HCPL-7840
Not Specified
100
0.1
2.5
15
3.05
Avago ACPL-790
60
200
0.05
5.0
15
6.63
Avago ACPL-780
Not Specified
100
0.004
5.0
15
7.30
Avago ACPL-C79A
60
200
0.05
5.0
15
10.00
Avago ACPL-C790 ()
60
200
0.05
5.0
15
4.65
TI ISO-124
Not Specified
50
0.01
1.5
Not Specified
8.30
ADIAD202
Not Specified
5
0.05
1.0
Not Specified
29.82
Rev. 0.2
5
AN614
3. Application Examples
The electrocardiograph (ECG) application shown in Figure 6 requires safety isolation to protect the patient from
dangerous leakage currents. Medical applications of this type typically use conductive gels at sensor sites, which
lower human body impedance to the extent that a few milliamps of leakage current can cause injury or death. To
mitigate this issue, ECGs typically have multiple stages of isolation to prevent downstream leakage current from
flowing into the patient. The ECG of Figure 6 shows the patient connected to an instrumentation amplifier powered
by a low-current floating supply, V1. The ISOlinear circuit (shown as a functional block in the center of the diagram)
galvanically isolates the instrumentation amplifier input side from potential down-stream leakage currents
generated by the voltage source, V2. The lower voltage, higher-current DSP circuit is also isolated from the ADC/
filter circuit by a separate Si86xx digital isolator, again to ensure no leakage current paths into the ECG inputs.
V2
Demodulator
Si86xx
Isolator
Instrumentation Amplifier
Modulator
ISOlinear Reference Design
V3
Filter(s)
V1
Right Leg Driver
FG
FG
Figure 6. ECG (Safety Isolation) Application
6
Rev. 0.2
ADC
Si86xx
Isolator
To DSP
AN614
ISOlinear
Ref Design
1.5V
INPUT
R1
R4
C1
R5
R12
D1
R13
C4
U2
3.0V
LDO
C5
Demodulator
R3
ISOLATION
R2
Modulator
BLACK
WHITE
Single-Phase
AC Line
VDDA
VDDB
GNDA
GNDB
R14
1.5V
R15
OUTPUT
3.3V
Mixed-Signal
Circuits
3.3V
Output Side
Bias Supply
+1,000V
Common Mode
Transient
‐1,000V
Figure 7. AC Line Current Monitor (Safety Isolation and Level Shift) Application
Figure 7 shows an ac line current monitor that demonstrates both safety isolation and level shifting. Safety isolation
galvanically isolates the 110 VAC line from the low-voltage, ground-based circuits. This circuit uses resistive shunt
R1 to sense ac current. The high-voltage interface circuit is referenced to the ac neutral (white) wire in a two-wire
(non-earth grounded) single-phase ac service.
ISOlinear input-side bias voltage is line-derived and uses a 3 V linear regulator. The low-voltage output-side
circuits are biased by a ground-referenced supply. Because there is no earth ground in this system, an ac line
perturbation can potentially cause high voltage to appear on the neutral (white) wire. This elevated neutral line
common-mode voltage is rejected by the Si86xx isolator's high CMTI of 35 kV/µs minimum, 50 kV/µs typical. For
more isolated level shifting application examples, see Silicon Labs Application Note “AN598: High-Speed Level
Shifting Using Si8xxx Isolators”.
Figure 8 shows a common ground loop in the transmission path of two linear circuits (a common scenario in test
and measurement, audio and other applications that use cable interconnects). The ground loop in the top diagram
circulates between the connector grounds while parasitic inductance (Z) causes ringing that generates output
noise. The bottom circuit of Figure 8 inserts the ISOlinear circuit between the signal source and receiver, breaking
the ground loop and dramatically reducing the local ground path lengths and associated parasitic inductance.
Rev. 0.2
7
AN614
VDD1
VDD2
Connector
Connector
Cable
Signal Source
Signal Receiver
Ground Loop
Noisy Output
Signal
Z
VDD1
VDD2
Signal Source
GND1
Demodulator
Modulator
IN
Si86xx Isolator
ISOlinear Reference Design
OUT
Signal Receiver
GND2
Figure 8. Ground Loop Elimination Circuit
8
Rev. 0.2
Clean
Output Signal
AN614
4. Summary
Many analog systems require isolation to provide safety, level shifting or to mitigate ground noise. ISOamps are a
common but expensive solution that lack flexibility and force the designer to compromise and/or add supplemental
circuitry to a design.
Silicon Labs' ISOlinear reference design (Si86IsoLin-EVB) offers a lower-cost, more flexible linear isolation
solution. This design is enabled by the Silicon Labs Si86xx industry-leading digital isolator and requires only four
operational amplifiers and some passive components in addition to the isolator. The ISOlinear reference design
benefits the user with a robust, competitive solution and the opportunity to create an optimum solution for the
application at hand at a fraction of the price of an ISOamp.
4.1. Related Documents
AN559: “Isolating Analog Signals Using the Si86xx CMOS Isolator Family”, Silicon Labs, 2011
 ISOlinear Users Guide, Silicon Labs, 2011
 AN598: “High-Speed Level Shifting Using Si8xxx Isolators”

Rev. 0.2
9
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