Quad/Octal, 2-Wire Serial 8-Bit DACs

with Rail-to-Rail Outputs

14

______________________________________________________________________________________

Early Stop Conditions

The addressed device recognizes a STOP condition at

any point in a transmission. If the STOP occurs during a

command byte, all previous uninterrupted command

and output byte pairs are accepted, the interrupted

command byte is ignored, and the transmission ends

(Figure 13a). If the STOP occurs during an output byte,

all previous uninterrupted command and output byte

pairs are accepted, the final command byte’s PD and

RST bits are accepted, the interrupted output byte is

ignored, and the transmission ends (Figure 13b).

Analog Section

DAC Operation

The MAX520 contains four matched voltage-output

DACs, and the MAX521 contains eight. The DACs are

inverted R-2R ladder networks that convert 8-bit digital

words into equivalent analog output voltages in propor-

tion to the applied reference voltages. For both

devices, DAC0–DAC3 each have separate reference

inputs, while the MAX521’s DAC4–DAC7 all share a

common reference input. Figure 14 shows a simplified

diagram of one DAC.

Reference Inputs

The MAX520/MAX521 can be used for multiplying appli-

both DC and AC signals. The voltage at each REF input

sets the full-scale output voltage for its respective

DAC(s). The reference voltage must be positive. The

DAC’s input impedance is code dependent, with the

lowest value occurring when the input code is 55 hex or

0101 0101, and the maximum value occurring when the

input code is 00 hex. Since the REF input resistance

(

)

(

)

SDA

0

START

CONDITION

ADDRESS BYTE

(DEVICE 0)

ACK

1

0

1

000

00

0

0

00

000

00

1

0

1

0

1

0

0

1

00

1

REPEATED START

CONDITION

STOP

CONDITION

COMMAND BYTE

ADDRESSING DAC1

COMMAND BYTE

(ADDRESSING DAC2)

ACK

OUTPUT BYTE

(FULL SCALE)

ACK

ADDRESS BYTE

(DEVICE 1)

ACK

DEVICE 0's

DAC1 INPUT LATCH

SET TO FULL SCALE

DEVICE 1's

DAC2 INPUT LATCH

SET TO FULL SCALE

SDA

ACK

ACK

OUTPUT BYTE

(FULL SCALE)

ONLY DEVICE 1's DAC2 OUTPUT LATCH SET TO FULL

SCALE. DEVICE 0's OUTPUT LATCHES UNCHANGED.

111

1

1

11

0

000

00

0

0

0

11

111

1

1

11

Figure 12. Repeated START Conditions

(

)

SDA

0

0

START

CONDITION

ADDRESS BYTE

ACK

1

1

AD1 AD0 0

0

0 0

0

011

(RST) (PD)

(PD)

EARLY

STOP CONDITION

INTERRUPTED

COMMAND BYTE

MAX520/MAX521's STATES

REMAIN UNCHANGED

(

)

SDA

0

START

CONDITION

ADDRESS BYTE

ACK

10

1

AD1 AD0 0

0 000

000

0

1

1

100

RST 1

COMMAND BYTE

(POWER DOWN)

ACK

INTERRUPTED

OUTPUT BYTE

(a)

(b)

MAX520/MAX521 POWER DOWN;

INPUT LATCHES UNCHANGED IF

RST = 0, DAC OUTPUTS RESET IF

RST = 1.

EARLY

STOP CONDITION

0 OR AD2

0 OR AD2

Figure 13. Early STOP Conditions