AD5040/AD5060
Rev. A | Page 15 of 24
THEORY OF OPERATION
The AD5040/AD5060 are single 14-/16-bit, serial input, voltage
output DACs. The parts operate from supply voltages of 2.7 V
to 5.5 V. Data is written to the AD5060 in a 24-bit word format,
and to the AD5040 in a 16-bit word format, via a 3-wire serial
interface.
Both the AD5040 and AD5060 incorporate a power-on reset
circuit that ensures the DAC output powers up to a known out-
put state (midscale or zero-scale, see the Ordering Guide). The
devices also have a software power-down mode that reduces the
typical current consumption to less than 1 µa.
DAC ARCHITECTURE
The DAC architecture of the AD5060 consists of two matched
DAC sections. A simplified circuit diagram is shown in
Figure 40. The 4 MSBs of the 16-bit data-word are decoded to
drive 15 switches, E1 to E15. Each of these switches connects
1 of 15 matched resistors to either DACGND or the V
REF
buffer
output.
The remaining 12 bits of the data-word drive switches
S0 to S11 of a 12-bit voltage mode R-2R ladder network.
2R
04767-027
S0
V
REF
2R
S1
2R
S11
2R
E1
2R
E2
2R
E15
2R
V
OUT
12-BIT R-2R LADDER FOUR MSBs DECODED INTO
15 EQUAL SEGMENTS
Figure 40. AD5060 DAC Ladder Structure
REFERENCE BUFFER
The AD5040 andAD5060 operate with an external reference.
The reference input (V
REF
) has an input range of 2 V to
V
DD
− 50 mV. This input voltage is then used to provide a
buffered reference for the DAC core.
SERIAL INTERFACE
The AD5060/AD5040 have a 3-wire serial interface (
SYNC
,
SCLK, and DIN), which is compatible with SPI, QSPI, and
MICROWIRE interface standards, as well as most DSPs.
shows a timing diagram of a typical AD5060 write
sequence.
Figure 2
The write sequence begins by bringing the
SYNC
line low. For
the AD5060, data from the DIN line is clocked into the 24-bit
shift register on the falling edge of SCLK. The serial clock
frequency can be as high as 30 MHz, making these parts
compatible with high speed DSPs. On the 24th falling clock
edge, the last data bit is clocked in and the programmed
function is executed (that is, a change in the DAC output or a
change in the mode of operation).
At this stage, the
SYNC
line can be kept low or be brought
high. In either case, it must be brought high for a minimum of
12 ns before the next write sequence so that a falling edge of
SYNC
can initiate the next write sequence. Because the
SYNC
buffer draws more current when V
IH
= 1.8 V than it does when
V
IH
= 0.8 V,
SYNC
should be idled low between write sequences
for an even lower power operation of the part. As previously
indicated, however, it must be brought high again just before
the next write sequence. The AD5040 requires 16 clock periods
to update the input shift register. On the 16th falling clock edge,
the last data bit is clocked in and the programmed function is
executed (that is, a change in the DAC output or a change in the
mode of operation).
Input Shift Register
The AD5060 input shift register is 24 bits wide; see Figure 41.
PD1 and PD0 are control bits that control the operating mode
of the part—normal mode or any one of three power-down
modes (see the Power-Down Modes section for more detail).
The next 16 bits are the data bits. These are transferred to the
DAC register on the 24th falling edge of SCLK.
DATA BITS
DB15 (MSB) DB0 (LSB)
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
NORMAL OPERATION
1kΩ TO GND
100kΩ TO GND
3-STATE
POWER-DOWN MODES
0
0
1
1
0
1
0
1
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0 0 0 0 0 0 PD1 PD0
Figure 41. AD5060 Input Register Content
