Feature: IoT
Figure 2: Circuit for isolated LVDS using ADN4651
reproduction of a transformer that enables simple, space- saving isolation of digital signals. Te LVDS family also offers very precise time characteristics,
as well as extremely low jitter, also known as timing jitter. Jitter describes the variations in the rising and falling edges of a digital signal with respect to an ideal time reference. At high data rates, low jitter is extremely important because it takes just 1.6ns to transmit one bit at 600Mbps. Any jitter in the rising or falling edges of the signal must allow the ADC enough time for the actual high or low level so that sampling can be performed correctly. For the ADN465x family, jitter is typically 70ps. The LVDS modules also offer two isolated LVDS channels,
the transmission and reception channel of the ADN4651. The channels in the ADN4652 are arranged inversely to those of the ADN4651, whereas the ADN4650 offers either transmission or reception channels, depending on the wiring. The ADN465x family works internally with a supply voltage of 2.5V; industrial systems, unfortunately, do not often have this supply voltage but only 3.3V. For this reason, low dropout voltage regulators (LDOs) enabling an external supply voltage of 3.3V at the inputs are integrated into the ADN465x family. The supply to the module or its inputs and its isolated output side can, for example, be accomplished with the isolated ADuM5000 DC-DC converter. This can selectively generate an isolated output voltage of either 5V or 3.3V, with a maximum power output of 500mW; see Figure 1. In combination with the ADuM5000, this device family
can meet the numerous demands placed on isolated LVDS interfaces in today’s industrial applications. This highly- integrated solution also meets all the prerequisites for standardised bus communications. LVDS interfaces are frequently used in energy-saving applications. For this, the combination of the ADN4651 and the ADuM5000 represents an extremely power-saving alternative to traditional optocoupler solutions. It also meets the frequent need for simultaneous isolation of several channels.
32 November/December 2020
www.electronicsworld.co.uk
In LVDS applications, channels are used in parallel to maximise throughput and, with it, the baud rate
In LVDS applications, channels are used in parallel to
maximise the throughput and, with it, the baud rate. The described circuit with the above-mentioned modules from Analog Devices (Figure 2) offers one four-channel isolator, and two transmission and two reception channels. This permits signal transmission by means of two complete transmission and reception channels on one assembly with very high transmission rates. Data rates from almost DC to 600Mbps can easily be
reached with the ADN465x family, provided the specifications are adhered to for maximum pulse-width distortion. In addition, a few factors necessary for transmission of differential signals at high speeds must be taken into consideration in the layout. Thus, the input- and output- side traces should be matched and exhibit an approximate impedance of 50Ω with respect to ground, or 100Ω between the signal lines. Furthermore, it is advisable to attach 100Ω terminators to the LVDS inputs, as shown in Figure 2. Cable length and connector type also affect the maximum
data rate. Data rates up to 200Mbps in combination with connectors for higher data rates and shielded wire pairs enable cable lengths of several meters. Te ADN4650/ADN4651/ADN46521 are signal-isolated
LVDS buffers that operate at up to 600Mbps with very low jitter. Combined with the ADuM5000 they are ideal for high-speed signal transmission up to 600Mbps for short distances and 200Mbps for several meters.
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