ADBMS1818ASWZ-R7 datasheet(78/89 Pages) AD

78 / 89 page

Data Sheet

ADBMS1818

APPLICATIONS INFORMATION

analog.com

Rev. 0 | 78 of 89

Implementing a Modular isoSPI Daisy Chain

The hardware design of a daisy-chain isoSPI bus is identical for

each device in the network due to the daisy-chain point to point

architecture. The simple design as shown in Figure 103 is function-

al, but inadequate for most designs. The termination resistor, RM,

must be split and bypassed with a capacitor, as shown in Figure

104. This change provides both a differential and a common mode

termination, and as such, increases the system noise immunity.

Figure 104. Daisy Chain Interface Components

The use of cables between battery modules, particularly in hazard

applications, can lead to increased noise susceptibility in the com-

munication lines. For high levels of electromagnetic interference

(EMC), additional filtering is recommended. The circuit example in

Figure 104 shows the use of common-mode chokes (CMC) to add

common-mode noise rejection from transients on the battery lines.

The use of a center tapped transformer also provides additional

noise performance. A bypass capacitor connected to the center tap

creates a low impedance for common-mode noise (see Figure 104).

Since transformers without a center tap can be less expensive, they

may be preferred. In this case, the addition of a split termination

resistor and a bypass capacitor (see Figure 104) can enhance the

isoSPI performance. Large center tap capacitors greater than 10 nF

must be avoided as they may prevent the isoSPI common-mode

voltage from settling. Common-mode chokes similar to those used

in Ethernet or CANbus applications are recommended. Specific

examples are provided in Table 73.

An important daisy chain design consideration is the number of

devices in the isoSPI network. The length of the chain determines

the serial timing and affects data latency and throughput. The

maximum number of devices in an isoSPI daisy chain is strictly

dictated by the serial timing requirements. However, it is important

to note that the serial read back time, and the increased current

consumption, might dictate a practical limitation.

For a daisy chain, the following two timing considerations for proper

operation dominate (see Figure 86):

1. t6, the time between the last clock and the rising chip select,

must be long enough.

2. t5, the time from a rising chip select to the next falling chip

select (between commands), must be long enough.

Both t5 and t6 must be lengthened as the number of

ADBMS1818 devices in the daisy chain increases. The equa-

tions for these times are below:

t5 > (Number of Devices × 70 ns) + 900 ns, t6 > (Number of

Devices × 70 ns) + 950 ns

Connecting Multiple ADBMS1818s on the Same

PCB

When connecting multiple ADBMS1818 devices on the same PCB,

only a single transformer is required between the ADBMS1818

isoSPI ports. The absence of the cable also reduces the noise

levels on the communication lines and often only a split termination

is required. Figure 105 shows an example application that has

multiple ADBMS1818 devices on the same PCB, communicating to

the bottom MCU through an LTC6820 isoSPI driver. If a transformer

with a center tap is used, a capacitor can be added for improved

noise rejection. Additional noise filtering can be provided with dis-

crete common-mode chokes (not shown) placed on both sides of

the single transformer.

On single board designs with low noise requirements, it is possible

for a simplified capacitor isolated coupling as shown in Figure 106

to replace the transformer.

In this circuit, the transformer is directly replaced by two 10 nF

capacitors. An optional CMC provides noise rejection similar to

application circuits using transformers. The circuit is designed to

use IBIAS and ICMP settings identical to the transformer circuit.

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