AN598 Datasheet, PDF (1/6 Pages) Silicon Laboratories – HIGH-SPEED LEVEL SHIFTING USING

AN598
HIGH-SPEED LEVEL SHIFTING USING Si8XXX ISOLATORS
1. Introduction
Voltage level shifting is common in analog and mixed-signal system applications. While several technologies
implement level shifting (e.g., HVIC processes, discrete circuits, dedicated level shifting ICs), recent advances in
CMOS isolation technology enables fast, reliable, and highly-integrated ICs that are ideal for a broad range of level
shifting applications. This application note discusses precision high-speed level shifting techniques using Silicon
Labs' Si8xxx Digital isolators and ISOdrivers. Specifically, this application note covers common-mode voltage level
shifting, logic threshold voltage level shifting, and gate driver common-mode inversion.
2. Addressing Common Mode Level Shifting
Many industrial applications require measurements of very low-level analog signals from sensors, such as thermo-
couples and pressure transducers. These sensors often ride on very high common-mode voltages (VCM) and, as a
result, have differential outputs. Typically, an instrumentation amplifier applies differential gain to amplify the sensor
output signal (typically <100 mV), amplifying the signal to a level compatible with the analog-to-digital converter
(ADC) input range (typically >1 V). Since the digitized signal is typically processed by an earth-ground-based MCU
or other controller, galvanic isolation is required between the ADC output and ground-based controller input to
prevent the high common-mode voltage from entering low-voltage, ground-based circuitry.
Bias
S
ADC
Instrumentation
Amp
VDD1
IN1
Si86xx Digital Isolator
Tx
Isolation
Barrier
Rx
VDD2
OUT1
BUF
S_Gi_N8_D6₁xx IsoINlaPtUoTr‐SIDE DIE
OUTPUT‐SIDE DIE _GND2
VDD2
VDD
Digital Input
MCU or Other
Processor
VCM
GND
Figure 1. Example Industrial Application
Figure 1 shows a Silicon Labs Si86xx digital isolator providing isolation and level-shifting between an analog data
acquisition circuit (riding on a high common-mode voltage) and a ground-referenced controller. The Si86xx isolator
operates by enabling its input-side transmitter when IN1 is logic high, causing the transmitter to generate a carrier
wave that propagates through the isolation barrier to the output-side receiver. The receiver asserts a logic high on
OUT1 when sufficient in-band energy is detected. Conversely, a logic low at IN1 results in no carrier, which, in turn,
results in a logic low at OUT1. This simple architecture carries with it industry-leading performance, reliability,
power savings, integration, cost effectiveness, and space savings. These isolators are very easy to use; the
designer need only choose the isolation working voltage rating, speed, and dc bias. (Note the difference between
working voltage and rated voltage; rated voltage is a component-level specification that specifies the isolator's
voltage withstand capability at 2500 V_RMS or 5000 V_RMS for one minute.) In Figure 1, the VCM is 560 V (peak or
dc) and is also the working voltage for the system, i.e. the voltage that the barrier must withstand continuously
throughout the system’s lifetime. As shown in Figure 2, the mean time-to-failure for these isolators at a working
voltage of 396 V_RMS (560 Vpk) is 60 years.
Rev. 0.1 5/11
Copyright © 2011 by Silicon Laboratories
AN598