REV. A

ADM1025/ADM1025A

–8–

When reading data from a register there are two possibilities:

1. If the ADM1025/ADM1025A’s Address Pointer Register

value is unknown or not the desired value, it is first necessary

to set it to the correct value before data can be read from the

desired data register. This is done by performing a write to

the ADM1025/ADM1025A as before, but only the data byte

containing the register address is sent, as data is not to be

written to the register. This is shown in Figure 9b.

A read operation is then performed consisting of the serial bus

address, R/

W bit set to 1, followed by the data byte read from

the data register. This is shown in Figure 9c.

2. If the Address Pointer Register is known to be already at the

desired address, data can be read from the corresponding

data register without first writing to the Address Pointer

Register, so Figure 9b can be omitted.

NOTES

1. Although it is possible to read a data byte from a data register

without first writing to the Address Pointer Register, if the

Address Pointer Register is already at the correct value, it is

not possible to write data to a register without writing to the

Address Pointer Register, because the first data byte of a

write is always written to the Address Pointer Register.

2. In Figures 9a to 9c, the serial bus address is shown as the

default value 01011(A1)(A0), where A1 and A0 are set by

the three-state ADD pin.

3. In addition to supporting the Send Byte and Receive Byte

protocols, the ADM1025/ADM1025A also supports the

Read Byte protocol (see System Management Bus specifica-

tions Rev. 1.1 for more information).

4. If Reset or Interrupt functionality is required, address pin

cannot be strapped to GND, as this would keep the ADD/

RST/INT/NTO pin permanently low.

MEASUREMENT INPUTS

The ADM1025/ADM1025A has six external measurement

inputs, five for voltage and one (two pins) for temperature.

on-chip temperature sensor.

A-TO-D CONVERTER

These inputs are multiplexed into the on-chip, successive ap-

proximation, analog-to-digital converter. This has a resolution

of eight bits. The basic input range is zero to 2.5 V, but the

inputs have built-in attenuators to allow measurement of 2.5 V,

any external components. To allow for the tolerance of these

supply voltages, the A-to-D converter produces an output of

3/4 full scale (decimal 192) for the nominal input voltage, and

so has adequate headroom to cope with overvoltages. Table II

shows the input ranges of the analog inputs and output codes of

the A-to-D converter.

When the ADC is running, it samples and converts an input

every 11.6 ms, except for the external temperature (D+ and D–)

input. This has special input signal conditioning and is averaged

over 16 conversions to reduce noise; a measurement on this

input takes nominally 34.8 ms.

INPUT CIRCUITS

The internal structure for the analog inputs are shown in Figure

10. Each input circuit consists of an input protection diode, an

attenuator, plus a capacitor to form a first-order low-pass filter

which gives the input immunity to high frequency noise.

35pF

22.7k

122.2k

25pF

55.2k

91.6k

25pF

82.4k

62.6k

25pF

111.2k

36.7k

10pF

105k

19.6k

MUX

Figure 10. Structure of Analog Inputs

R/W

0

SCL

SDA

1

0

1

1

A1

A0

D7

D6

D5

D4

D3

D2

D1

D0

ACK. BY

ADM1025

START BY

MASTER

19

1

ACK. BY

ADM1025

9

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

ADDRESS POINTER REGISTER BYTE

STOP BY

MASTER

Figure 9b. Writing to the Address Pointer Register Only

R/

W

0

SCL

SDA

1

0

1

1

A1

A0

D7

D6

D5

D4

D3

D2

D1

D0

NO ACK.

BY MASTER

START BY

MASTER

19

1

ACK. BY

ADM1025

9

FRAME 1

SERIAL BUS ADDRESS BYTE

FRAME 2

DATA BYTE FROM ADM1025

STOP BY

MASTER

Figure 9c. Reading Data from a Previously Selected Register