AD7849
t
1
SCLK
t
2
SYNC
t
3
BIN/COMP
t
4
SDIN
(AD7849B/C/T)
DB15
t
5
DB0
t
4
SDIN
(AD7849A)
LDAC, CLR
DB13
t
5
DB0
t
7
DCEN IS TIED PERMANENTLY LOW
Figure 12. Timing Diagram (Stand-Alone Mode)
DIGITAL INTERFACE
The AD7849 contains an input serial to parallel shift register
and a DAC latch. A simplified diagram of the input loading
circuitry is shown in Figure 12. Serial data on the SDIN input
is loaded to the input register under control of DCEN,
SYNC
and SCLK. When a complete word is held in the shift register it
may then be loaded into the DAC latch under control of
LDAC.
Only the data in the DAC latch determines the analog
output on the AD7849.
The DCEN (daisy-chain enable) input is used to select either a
stand-alone mode or a daisy-chain mode. The loading format is
slightly different depending on which mode is selected.
Serial Data Loading Format (Stand-Alone Mode)
There are two ways in which the DAC latch and hence the ana-
log output may be updated. The status of the
LDAC
input is
examined after
SYNC
is taken low. Depending on its status,
one of two update modes is selected.
If
LDAC
= 0 then the automatic update mode is selected. In
this mode the DAC latch and analog output are updated auto-
matically when the last bit in the serial data stream is clocked
in. The update thus takes place on the sixteenth falling SCLK
edge.
If
LDAC
= 1 then the automatic update is disabled. The DAC
latch update and output update are now separate. The DAC
latch is updated on the falling edge of
LDAC.
However, the
output update is delayed for a further 5
µs
by means of an inter-
nal track-and-hold amplifier in the output stage. This function
results in lower digital-to-analog glitch impulse at the DAC
output. Note that the
LDAC
input must be taken back high
again before the next data transfer is initiated.
With DCEN at Logic 0 the stand-alone mode is selected. In this
mode a low
SYNC
input provides the frame synchronization
signal which tells the AD7849 that valid serial data on the SDIN
input will be available for the next 16 falling edges of SCLK. An
internal counter/decoder circuit provides a low gating signal so
that only 16 data bits are clocked into the input shift register.
After 16 SCLK pulses the internal gating signal goes inactive
(high) thus locking out any further clock pulses. Therefore ei-
ther a continuous clock or a burst clock source may be used to
clock in the data.
The
SYNC
input is taken high after the complete 16-bit word is
loaded in.
The AD7849B, AD7849C and AD7849T versions are 16-bit
resolution DACS and have a straight 16-bit load format, with
the MSB (DB15) being loaded first. The AD7849A is a 14-bit
DAC but the loading structure is still 16-bit. The MSB (DB13)
is loaded first and the final two bits of the 16-bit stream must
be 0s.
DCEN
SYNC
RESET
EN
SCLK
GATED
SIGNAL
GATED
SCLK
INPUT
SHIFT REGISTER
(16 BITS)
SDOUT
SDIN
AUTO-UPDATE
CIRCUITRY
LDAC
CLR
DAC LATCH
(14/16 BITS)
÷
16
COUNTER/
DECODER
Figure 13. Simplified Loading Structure
REV. B
–9–
AD [ ANALOG DEVICES ]
