Datasheet

–11–
REV.
3
AD7564
GENERAL DESCRIPTION
D/A Section
The AD7564 contains four 12-bit current output D/A convert-
ers. A simplified circuit diagram for one of the D/A converters
is shown in Figure 15.
V
REF
2R 2R 2R 2R 2R 2R 2R
CBA
S9
S8 S0
R
FB
I
OUT1
I
OUT2
R
R
R
R/2
SHOWN FOR ALL 1s ON DAC
Figure 15. Simplified D/A Circuit Diagram
A segmented scheme is used whereby the 2 MSBs of the 12-bit
data word are decoded to drive the three switches A, B and C.
The remaining 10 bits of the data word drive the switches S0 to
S9 in a standard R-2R ladder configuration.
Each of the switches A to C steers 1/4 of the total reference
current with the remaining current passing through the R-2R
section.
All DACs have separate V
REF
, I
OUT1
, I
OUT2
and R
FB
pins.
When an output amplifier is connected in the standard configu-
ration of Figure 17, the output voltage is given by:
V
OUT
= D ×V
REF
where D is the fractional representation of the digital word
loaded to the DAC. Thus, in the AD7564, D can be set from 0
to 4095/4096.
Interface Section
The AD7564 is a serial input device. Three input signals con-
trol the serial interface. These are
FSIN, CLKIN and SDIN.
The timing diagram is shown in Figure 1.
Data applied to the SDIN pin is clocked into the input shift reg-
ister on each falling edge of CLKIN. SDOUT is the shift regis-
ter output. It allows multiple devices to be connected in a daisy
chain fashion with the SDOUT pin of one device connected to
the SDIN of the next device.
FSIN is the frame synchronization
for the device.
When the sixteen bits have been received in the input shift regis-
ter, DB2 and DB3 (A0 and A1) are checked to see if they corre-
spond to the state on pins A0 and A1. If it does, then the word
is accepted. Otherwise, it is disregarded. This allows the user
to address a number of AD7564s in a very simple fashion. DB1
and DB0 of the 16-bit word determine which of the four DAC
input latches is to be loaded. When the
LDAC line goes low, all
four DAC latches in the device are simultaneously loaded with
the contents of their respective input latches and the outputs
change accordingly.
Bringing the
CLR line low resets the DAC latches to all 0s. The
input latches are not affected so that the user can revert to the
previous analog output if desired.
16-BIT INPUT
SHIFT REGISTER
CLKIN
FSIN
SDIN
SDOUT
Figure 16. Input Logic
UNIPOLAR BINARY OPERATION
(2-Quadrant Multiplication)
Figure 17 shows the standard unipolar binary connection dia-
gram for one of the DACs in the AD7564. When V
IN
is an ac
signal, the circuit performs 2-quadrant multiplication. Resistors
R1 and R2 allow the user to adjust the DAC gain error. Offset
can be removed by adjusting the output amplifier offset voltage.
Figure 17. Unipolar Binary Operation
A1 should be chosen to suit the application. For example, the
AD707 is ideal for very low bandwidth applications while the
AD843 and AD845 offer very fast settling time in wide band-
width applications. Appropriate multiple versions of these am-
plifiers can be used with the AD7564 to reduce board space
requirements.
The code table for Figure 17 is shown in Table III.
Table III. Unipolar Binary Code Table
Digital Input Analog Output
MSB . . . LSB (V
OUT
as Shown in Figure 17)
1111 1111 1111 –V
REF
(4095/4096)
1000 0000 0001 –V
REF
(2049/4096)
1000 0000 0000 –V
REF
(2048/4096)
0111 1111 1111 –V
REF
(2047/4096)
0000 0000 0001 –V
REF
(1/4096)
0000 0000 0000 –V
REF
(0/4096) = 0
NOTE
Nominal LSB size for the circuit of Figure 17 is given by: V
REF
(1/4096).
DAC A
A1
AD7564
V
REF
A
V
IN
NOTES
1. ONLY ONE DAC IS SHOWN FOR CLARITY.
2. DIGITAL INPUT CONNECTIONS ARE OMITTED.
3. C1 PHASE COMPENSATION (5–15pF) MAY BE
REQUIRED WHEN USING HIGH SPEED AMPLIFIER.
R2 10Ω
R1 20Ω
SIGNAL
GND
A1: AD707
AD711
AD843
AD845
C1
R
FB
A
I
OUT2
A
I
OUT1
A
V
OUT
B