Datasheet

AD5398
Rev. B | Page 12 of 16
05034-017
PD X D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 X X X X
00
SCL
SD
A
START BY
MASTER
ACK BY
AD5398
1191
ACK BY
AD5398
ACK BY
AD5398
STOP BY
MASTER
FRAME 3
LEAST SIGNIFICANT
DATA BYTE
FRAME 2
MOST SIGNIFICANT
DATA BYTE
FRAME 1
SERIAL BUS
ADDRESS BYTE
0 1 1 X X R/W
9
Figure 19. Write Operation
05034-018
PD X D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 X X X X
00
SCL
SD
A
START BY
MASTER
ACK BY
AD5398
1191
ACK BY
AD5398
ACK BY
AD5398
STOP BY
MASTER
FRAME 3
LEAST SIGNIFICANT
DATA BYTE
FRAME 2
MOST SIGNIFICANT
DATA BYTE
FRAME 1
SERIAL BUS
ADDRESS BYTE
0 1 1 X X R/W
9
Figure 20. Read Operation
Table 7. Data Format
1
Serial Data-Words High Byte Low Byte
Serial Data Bits SD7 SD6 SD5 SD4 SD3 SD2 SD1 SD0 SD7 SD6 SD5 SD4 SD3 SD2 SD1 SD0
Input Register R15 R14 R13 R12 R11 R10 R9 R8 R7 R6 R5 R4 R3 R2 R1 R0
Function PD X D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 X X X X
1
PD = soft power-down; X = unused/don’t care; D9 to D0 = DAC data.
POWER SUPPLY BYPASSING AND GROUNDING
When accuracy is important in an application, it is beneficial to
consider power supply and ground return layout on the PCB.
The PCB for the AD5398 should have separate analog and
digital power supply sections. Where shared AGND and DGND
is necessary, the connection of grounds should be made at only
one point, as close as possible to the AD5398.
Special attention should be paid to the layout of the AGND
return path and track between the voice coil motor and I
SINK
to
minimize any series resistance.
Figure 21 shows the output
current sink of the AD5398 and illustrates the importance of
reducing the effective series impedance of AGND, and the track
resistance between the motor and I
SINK
. The voice coil is
modelled as inductor L
C
and resistor R
C
. The current through
the voice coil is effectively a dc current that results in a voltage
drop, V
C
, when the AD5398 is sinking current; the effect of any
series inductance is minimal. The maximum voltage drop
allowed across R
SENSE
is 400 mV, and the minimum drain to
source voltage of Q1 is 200 mV. This means that the AD5398
output has a compliance voltage of 600 mV. If V
DROP
falls below
600 mV, the output transistor, Q1, can no longer operate
properly and I
SINK
might not be maintained as a constant.
05034-019
7
AGND
V
G
Q1
GROUND
RESISTANCE
GROUND
INDUCTANCE
8
I
SINK
R
SENSE
3.3Ω
R
G
L
G
V
DROP
V
T
V
C
V
BAT
R
T
R
C
L
C
TRACE
RESISTANCE
VOICE
COIL
ACTUATOR
Figure 21. Effect of PCB Trace Resistance and Inductance