Datasheet

AD7245A/AD7248A
REV. B
–10–
APPLYING THE AD7245A/AD7248A
The internal scaling resistors provided on the AD7245A/
AD7248A allow several output voltage ranges. The part can
produce unipolar output ranges of 0 V to 5 V or 0 V to 10 V
and a bipolar output range of –5 V to +5 V. Connections for
the various ranges are outlined below.
UNIPOLAR (0 V TO 10 V) CONFIGURATION
The first of the configurations provides an output voltage range
of 0 V to 10 V. This is achieved by connecting the bipolar offset
resistor, R
OFS
, to AGND and connecting R
FB
to V
OUT
. In this
configuration the AD7245A/AD7248A can be operated single
supply (V
SS
= 0 V = AGND). If dual supply performance is
required, a V
SS
of –12 V to –15 V should be applied. Figure 8
shows the connection diagram for unipolar operation while the
table for output voltage versus the digital code in the DAC latch
is shown in Table III.
V
REF
2R
AD7245A/AD7248A*
V
OUT
R
FB
10⍀
REF OUT
R
OFS
10␮F
AGND
*DIGITAL CIRCUITRY
OMITTED FOR CLARITY
0.1␮F
V
DD
DGND
2R
DAC
REF
V
SS
Figure 8. Unipolar (0 to 10 V) Configuration
Table III. Unipolar Code Table (0 V to 10 V Range)
DAC Latch Contents
MSB LSB Analog Output, V
OUT
1 1 1 1 1 1 1 1 1 1 1 1 +2 V
REF
ⴛ
4095
4096






1 0 0 0 0 0 0 0 0 0 0 1 +2 V
REF
ⴛ
2049
4096






1 0 0 0 0 0 0 0 0 0 0 0 +2 V
REF
ⴛ
2048
4096






=+V
REF
0 1 1 1 1 1 1 1 1 1 1 1 +2 V
REF
ⴛ
2047
4096






0 0 0 0 0 0 0 0 0 0 0 1 +2 V
REF
ⴛ
1
4096






0 0 0 0 0 0 0 0 0 0 0 0 0 V
NOTE: 1 LSB = 2 ⴛ V
REF
(2
–12
) = V
REF
1
2048






The LDAC input controls the transfer of 12-bit data from the
input latch to the DAC latch. This LDAC signal is also level
triggered, and data is latched into the DAC latch on the rising
edge of LDAC. The LDAC input is asynchronous and indepen-
dent of WR. This is useful in many applications especially in
the simultaneous updating of multiple AD7248A outputs. How-
ever, in systems where the asynchronous LDAC can occur during
a write cycle (or vice versa) care must be taken to ensure that
incorrect data is not latched through to the output. In other words,
if LDAC goes low while WR and either CS input are low (or
WR and either CS go low while LDAC is low), then the LDAC
signal must stay low for t
7
or longer after WR returns high to
ensure correct data is latched through to the output. The write
cycle timing diagram for the AD7248A is shown in Figure 7.
CSLSB
CSMSB
WR
LDAC
DATA
IN
5V
0V
t
4
t
3
t
3
t
5
t
5
t
6
t
6
VA LI D
DATA
VA LI D
DATA
5V
0V
5V
0V
5V
0V
5V
0V
t
4
t
7
t
2
t
1
t
2
t
1
Figure 7. AD7248A Write Cycle Timing Diagram
An alternate scheme for writing data to the AD7248A is to tie
the CSMSB and LDAC inputs together. In this case exercising
CSLSB and WR latches the lower 8 bits into the input latch.
The second write, which exercises CSMSB, WR and LDAC
loads the upper 4-bit nibble to the input latch and at the same
time transfers the 12-bit data to the DAC latch. This automatic
transfer mode updates the output of the AD7248A in two write
operations. This scheme works equally well for CSLSB and
LDAC tied together provided the upper 4-bit nibble is loaded
to the input latch followed by a write to the lower 8 bits of
the input latch.
Table II. AD7248A Truth Table
CSLSB CSMSB WR LDAC Function
L H L H Load LS Byte into Input Latch
LH g H Latches LS Byte into Input Latch
g H L H Latches LS Byte into Input Latch
H L L H Loads MS Nibble into Input Latch
HL g H Latches MS Nibble into Input Latch
H g L H Latches MS Nibble into Input Latch
H H H L Loads Input Latch into DAC Latch
HH Hg Latches Input Latch into DAC Latch
H L L L Loads MS Nibble into Input Latch and
Loads Input Latch into DAC Latch
H H H H No Data Transfer Operation
H = High State, L = Low State