ADBMS1818ASWZ-R7 datasheet(52/92 Pages) AD

52 / 92 page

Data Sheet

ADBMS1818

THEORY OF OPERATION

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Rev. B | 52 of 92

Selecting Bias Resistors

The adjustable signal amplitude allows the system to trade power

consumption for communication robustness, and the adjustable

comparator threshold allows the system to account for signal loss-

es.

The isoSPI transmitter drive current and comparator voltage thresh-

old are set by a resistor divider (RBIAS = RB1 + RB2) between IBIAS

and V–. The divided voltage is connected to the ICMP pin, which

sets the comparator threshold to half of this voltage (VICMP). When

either isoSPI is enabled (not idle), IBIAS is held at 2 V, causing IB to

flow out of the IBIAS pin. The IPx and IMx pin drive currents are 20

× IB.

As an example, if the divider resistor, RB1, is 2.8 kΩ and resistor

RB2 is 1.21 kΩ (so that RBIAS = 4 kΩ), then

IB= 2 V

RB1 + RB2

= 0.5 mA

IDRV = IIPx = IIMx = 20 × IB = 10 mA

VICMP = 2 V ×

RB2

RB1 + RB2

= IB × RB2 = 603 mV

VTCMP = 0.5 × VICMP = 302 mV

In this example, the pulse drive current IDRV is 10 mA, and the

receiver comparators detect pulses with IPx to IMx amplitudes

greater than ±302 mV.

If the isolation barrier uses 1:1 transformers connected by a twisted

pair and terminated with 120 Ω resistors on each end, then the

transmitted differential signal amplitude (±) is the following:

VA = IDRV ×

RM2

= 0.6 V

This calculation result ignores transformer and cable losses, which

may reduce the amplitude.

isoSPI Pulse Detail

Two ADBMS1818 devices can communicate by transmitting and

receiving differential pulses back and forth through an isolation

barrier. The transmitter can output three voltage levels: +VA, 0 V,

and –VA. A positive output results from IPx sourcing current and

IMx sinking current across the load resistor, RM. A negative voltage

is developed by IPx sinking and IMx sourcing. When both outputs

are off, the load resistance forces the differential output to 0 V.

To eliminate the dc signal component and enhance reliability, the

isoSPI uses two different pulse lengths. This allows four types of

pulses to be transmitted, as shown in Table 42. A +1 pulse is

transmitted as a positive pulse followed by a negative pulse. A

–1 pulse is transmitted as a negative pulse followed by a positive

pulse. The duration of each pulse is defined as t1/2PW because each

pulse is half of the required symmetric pair. (The total isoSPI pulse

duration is 2 × t1/2PW).

Table 42. isoSPI Pulse Types

Pulse Type

First Level

(t1/2PW)

Second Level

(t1/2PW)

Ending Level

Long +1

+VA (150 ns)

–VA (150 ns)

0 V

Long –1

–VA (150 ns)

+VA (150 ns)

0 V

Short +1

+VA (50 ns)

–VA (50 ns)

0 V

Short –1

–VA (50 ns)

+VA (50 ns)

0 V

The receiver is designed to detect each of these isoSPI pulse

types. For successful detection, the incoming isoSPI pulses (CSB

or data) must meet the following requirements:

►

t1/2PW of incoming pulse > tFILT of the receiver and

►

tINV of incoming pulse < tWNDW of the receiver

The worst-case margin (Margin 1) for the first condition is the

difference between the minimum t1/2PW of the incoming pulse and

the maximum tFILT of the receiver. Likewise, the worst-case margin

(Margin 2) for the second condition is the difference between

minimum tWNDW of the receiver and maximum tINV of the incoming

pulse. These timing relations are shown in Figure 83.

A host microcontroller does not have to generate isoSPI pulses to

use this 2-wire interface. The first ADBMS1818 in the system can

communicate to the microcontroller using the 4-wire SPI on its Port

A, then daisy chain to other ADBMS1818s using the 2-wire isoSPI

on its Port B. Alternatively, the LTC6820 can be used to translate

the SPI signals into isoSPI pulses.

Figure 83. isoSPI Pulse Detail

Operation with Port A Configured for SPI

When the ADBMS1818 is operating with Port A as a SPI (ISOMD

= V–), the SPI detects one of four communication events: CSB

falling, CSB rising, SCK rising with SDI = 0, and SCK rising with

SDI = 1. Each event is converted into one of the four pulse types

for transmission through the daisy chain. Long pulses are used to

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