AD7834SQ AD [Analog Devices], AD7834SQ Datasheet - Page 9

• Manufacturer Part Number: AD7834SQ
• Description: LC2MOS Quad 14-Bit DAC
• Manufacturer: AD [Analog Devices]

REV. A
GENERAL DESCRIPTION
DAC Architecture—General
Each channel consists of a segmented 14-bit R-2R voltage-mode
DAC. The full- scale output voltage range is equal to the entire
reference span of V
straight binary; all 0s produces an output of V
duces an output of V
The analog output voltage of each DAC channel reflects the
contents of its own DAC latch. Data is transferred from the ex-
ternal bus to the input register of each DAC latch on a per
channel basis. The AD7835 has a feature whereby using the A2
pin, data can be transferred from the input data bus to all four
input registers simultaneously.
Bringing the CLR line low switches all the signal outputs,
V
nal outputs are held at this level after the removal of the CLR
signal and will not switch back to the DAC outputs until the
LDAC signal is exercised.
Data Loading—AD7834, Serial Input Device
A write operation transfers 24 bits of data to the AD7834. The
first 8 bits are control data and the remaining 16 bits are DAC
data (see Figure 12). The control data identifies the DAC chan-
nel to be updated with new data and which of 32 possible pack-
ages the DAC resides in. In any communication with the device
the first 8 bits must always be control data.
Note that the DAC output voltages, V
updated to reflect new data in the DAC input registers in one of
two ways. The first method normally keeps LDAC high and
only pulses LDAC low momentarily to update all DAC latches
simultaneously with the contents of their respective input regis-
ters. The second method ties LDAC low, and channel updating
occurs on a per channel basis after new data has been clocked
into the AD7834. With LDAC low, the rising edge of FSYNC
transfers the new data directly into the DAC latch, updating the
analog output voltage.
Data being shifted into the AD7834 enters a 24-bit long shift
register. If more than 24 bits are clocked in before FSYNC goes
high, the last 24 bits transmitted are used as the control data
and DAC data.
Individual bit functions are discussed below.
D23: Determines whether the following 23-bits of address and
data should be used or should be ignored. This is effectively a
software Chip Select bit. D23 is the first bit to be transmitted in
the 24-bit long word.
OUT
1 to V
FOLLOWING 23 BITS OF INFORMATION
PACKAGE ADDRESS MSB, PA4
CONTROL BIT TO USE/IGNORE
PACKAGE ADDRESS LSB, PA0
OUT
CHANNEL ADDRESS MSB, D1
CHANNEL ADDRESS LSB, D2
PACKAGE ADDRESS, PA3
PACKAGE ADDRESS, PA2
PACKAGE ADDRESS, PA1
4, to the voltage level on the DSG pin. The sig-
REF
REF
(+) – V
(+) – 1 LSB.
REF
Figure 12. Bit Assignments for 24-Bit Data Stream of AD7834
(–). The DAC coding is
D23 D22 D21 D20 D19 D18 D17 D16 D15 D14 D13 D12 D11 D10 D9
OUT
NOTE: D23 IS THE FIRST BIT TRANSMITTED IN THE SERIAL WORD.
1 to V
REF
OUT
(–); all 1s pro-
4, can be
FIRST LEADING ZERO
–9–
SECOND LEADING ZERO
D22 and D21: Decoded to select one of the four DAC channels
within a device. The truth table for D22 and D21 is as shown
below in Table II.
D20–D16: Determines the package address. The five address
bits allow up to 32 separate packages to be individually de-
coded. Successful decoding is accomplished when these five bits
match up with the five hardwired pins on the physical package.
D15–D0: DAC Data to be loaded into identified DAC Input
Register. This data must have two leading 0s followed by 14 bits
of data, MSB first. The MSB is in location D13 of the 24-bit
data stream.
Data Loading—AD7835, Parallel Loading Device
Data can be loaded into the AD7835 in either straight 14-bit
wide words or in two 8-bit bytes.
In systems which can transfer 14-bit wide data, the BYSHF
input should be hardwired to V
as a straight 14-bit parallel-loading DAC.
In 8-bit bus systems where it is required to transfer data in two
bytes, it is necessary to have the BYSHF input under logic con-
trol. In such a system the top 6 pins of the device data bus,
DB8–DB13, must be hardwired to DGND. New low byte data
is loaded into the lower 8 places of the selected input register by
carrying out a write operation while holding BYSHF high. A
second write operation is subsequently executed with BYSHF
low and the 6 MSBs on the DB0–DB5 inputs (DB5 = MSB).
MSB, DB13
SECOND MSB, DB12
D23
0
1
THIRD MSB, DB11
DB10
D22
0
0
1
1
DB9
Control Function
Ignore following 23 bits of information.
Use following 23 bits of address and
data as normal.
Table II. D22, D21 Control
DB8
D8
Table I. D23 Control
DB7
D7
D21
0
1
0
1
DB6
D6
DB5
D5
CC
DB4
D4
Control Function
Select Channel 1
Select Channel 2
Select Channel 3
Select Channel 4
. This sets up the AD7835
AD7834/AD7835
DB3
D3
THIRD LSB, DB2
D2
SECOND LSB, DB1
D1
LSB, DB0
D0