DAC8512 Datasheet, PDF (14/20 Pages) Analog Devices – % V, Serial Input Complete 12-Bit DAC

DAC8512
LOAD DAC
COUNTER
CLK
Q_D
Q_C
Q_B
Q_A
LOAD
(X)
DAC8512
CLK (Y)
LOAD = Q_C · Q_D
DAC8512 CLK = LOAD · SCLK
Figure 36. Opto-lsolated Two-Wire Serial Interface Timing Diagram
The timing diagram of Figure 36 can be used to understand the
operation of the circuit. Only two opto-couplers are used in the
circuit; one for SCLK and one for SDI. The 74HC161 counter
in incremented on every rising edge of the clock. Additionally,
the data is loaded into the DAC8512 on the falling edge of the
clock by inverting the serial clock using gate “Y.” The timing
diagram shows that after the twelfth bit has been clocked the
output of the counter is binary 1011. On the very next rising
clock edge, the output of the counter changes to binary 1100
upon which the output of gate “X” goes LOW to generate the
LD pulse. The LD signal is connected to both the DAC’s LD
and the counter’s LOAD pins to prevent the thirteenth rising
clock edge from advancing the DAC’s internal shift register.
This prevents false loading of data into the DAC8512. Inverting
the DAC’s serial clock allows sufficient time from the CLK edge
to the LD edge, and from the LD edge to the next clock pulse
all of which satisfies the timing requirements for loading the
DAC8512.
After loading one address of the DAC, the entire process can re-
peated to load another address. If the loading is complete, then
the clock must stop after the thirteenth pulse of the final load.
The DAC’s clock input will be pulled high and the counter reset
to zero. As was shown in Figure 35, both the 74HC161’s and
the DAC8512’s CLR pins are connected to a simple R-C timing
circuit that resets both ICs when the power in turned on. The
circuit’s time constant should be set longer than the power sup-
ply turn-on time and, in this circuit, is set to 10 ms, which
should be adequate for most systems. This same two-wire inter-
face can be used for other three-wire serial input DACs.
Decoding Multiple DAC8512s
The CS function of the DAC8512 can be used in applications
to decode a number of DACs. In this application, all DACs re-
ceive the same input data; however, only one of the DAC’s CS
input is asserted to transfer its serial input register contents into
the destination DAC register. In this circuit, shown in Figure 37,
the CS timing is generated by a 74HC139 decoder and should
follow the DAC8512’s standard timing requirements. To pre-
vent timing errors, the 74HC139 should not be activated by its
ENABLE input while the coded address inputs are changing. A
simple timing circuit, R1 and C1, connected to the DACs’ CLR
pins resets all DAC outputs to zero during power-up.
SCLK
SDI
LD
+5V
R1
C1
1k
0.1µF
6
3
DAC8512
4
#1
VOUT1
5
8
2
+5V
74HC139
16
4
VCC 1Y0
ENABLE
1
5
1G 1Y1
CODED
ADDRESS
2
1A
3
1B
6
1Y2
7
1Y3
15
12
+5V
2G 2Y0
NC
1kΩ ₁₄
2A
11
2Y1
NC
13
10
2B 2Y2
NC
8
9
GND 2Y3
NC
6
3
DAC8512
4
#2
VOUT2
5
8
2
6
3
DAC8512
4
#3
VOUT3
5
8
2
6
3
DAC8512
4
#4
VOUT4
5
8
2
Figure 37. Decoding Multiple DAC8512s Using the CS Pin
–14–
REV. A