ADR395 AD [Analog Devices], ADR395 Datasheet - Page 21

• Manufacturer Part Number: ADR395
• Description: Complete Dual, 16-Bit, High Accuracy, Serial Input, Bipolar Voltage Output DACs
• Manufacturer: AD [Analog Devices]

Preliminary Technical Data
V
THEORY OF OPERATION
The AD5762R is a dual, 16-bit, serial input, bipolar voltage output
DAC and operates from supply voltages of ±11.4 V to ±16.5 V and
has a buffered output voltage of up to ±10.5263 V. Data is written to
the AD5762R in a 24-bit word format, via a 3-wire serial interface.
The AD5762R also offers an SDO pin, which is available for daisy
chaining or readback.
The AD5762R incorporates a power-on reset circuit, which
ensures that the DAC registers power up loaded with 0x0000.
The AD5762R features a digital I/O port that can be
programmed via the serial interface, an analog die temperature
sensor, on-chip 10 ppm/°C voltage reference, on-chip reference
buffers and per channel digital gain and offset registers.
DAC ARCHITECTURE
The DAC architecture of the AD5762R consists of a 16-bit
current mode segmented R-2R DAC. The simplified circuit
diagram for the DAC section is shown in Figure 38.
The four MSBs of the 16-bit data word are decoded to drive 15
switches, E1 to E15. Each of these switches connects one of the
15 matched resistors to either AGND or IOUT. The remaining
12 bits of the data word drive switches S0 to S11 of the 12-bit R-
2R ladder network.
REFERENCE BUFFERS
The AD5762R can operate with either an external or an internal
reference. The reference input has an input range up to 7 V. This
input voltage is then used to provide a buffered positive and
negative reference for the DAC cores. The positive reference is
given by
While the negative reference to the DAC cores is given by
These positive and negative reference voltages (along with the
gain register values) define the output ranges of the DACs.
REF
+ V
−V
E15
2R
4 MSBs DECODED INTO
15 EQUAL SEGMENTS
REF
REF
E14
2R
= −2 × V
= 2 × V
E1
Figure 38. DAC Ladder Structure
REFIN
REFIN
2R
R
S11
12-BIT, R-2R LADDER
2R
R
S10
2R
R
S0
2R
2R
AGND
R/8
I
OUT
V
OUT
Rev. PrA | Page 21 of 33
SERIAL INTERFACE
The AD5762R is controlled over a versatile 3-wire serial
interface that operates at clock rates of up to 30 MHz and is
compatible with SPI®, QSPI™, MICROWIRE™, and DSP
standards.
Input Shift Register
The input shift register is 24 bits wide. Data is loaded into the
device MSB first as a 24-bit word under the control of a serial
clock input, SCLK. The input register consists of a read/write
bit, three register select bits, three DAC address bits and 16 data
bits as shown in Table 7. The timing diagram for this operation
is shown in Figure 2.
Upon power-up, the DAC registers are loaded with zero code
(0x0000) and the outputs are clamped to 0 V via a low
impedance path. The outputs can be updated with the zero code
value by asserting either LDAC or CLR . The corresponding
output voltage depends on the state of the BIN/ 2sCOMP pin. If
the BIN/ 2sCOMP pin is tied to DGND, then the data coding is
twos complement and the outputs update to 0 V. If the
BIN/ 2sCOMP pin is tied to DV
binary and the outputs update to negative full scale. To have the
outputs power-up with zero code loaded to the outputs, the
CLR pin should be held low during power-up.
Standalone Operation
The serial interface works with both a continuous and noncon-
tinuous serial clock. A continuous SCLK source can only be
used if SYNC is held low for the correct number of clock cycles.
In gated clock mode, a burst clock containing the exact number
of clock cycles must be used and SYNC must be taken high after
the final clock to latch the data. The first falling edge of SYNC
starts the write cycle. Exactly 24 falling clock edges must be
applied to SCLK before SYNC is brought back high again. If
SYNC is brought high before the 24
the data written is invalid. If more than 24 falling SCLK edges
are applied before SYNC is brought high, then the input data is
also invalid. The input register addressed is updated on the
rising edge of SYNC . In order for another serial transfer to take
place, SYNC must be brought low again. After the end of the
serial data transfer, data is automatically transferred from the
input shift register to the addressed register.
When the data has been transferred into the chosen register of
the addressed DAC, all DAC registers and outputs can be
updated by taking LDAC low.
CC
, then the data coding is offset
th
falling SCLK edge, then
AD5762R