MP7641
THEORY OF OPERATION
The MP7641 is equipped with a serial data 3-wire standard
µ-processor logic interface to reduce pin count, package size,
andboardwire(space). ThisinterfaceconsistsofLDwhichcon-
trolsthetransferofdatatotheselectedDACchannel, SDI(serial
data/address input), CLK (shift register clock) and SDO (serial
data output). When the LD signal is high, CLK signal loads the
digital input bits (SDI) into the 12-bit shift register (4 bits address
A3 to A0, then 8 bits data D7 to D0). The LD signal going low
loads this data into the selected DAC. The LD signal going low
also disables the serial data input (SDI), output (SDO 3-stated)
and the CLK input. This design tremendously reduces digital
noise, and glitch transients into the DACs due to free running
CLK and SDI. Also, 3-stating the SDO output with LD signal
would allow read back of pre-stored digital data of the selected
package using one SDO wire for all DAC ICs on the board. Note
alsothattheresetsignal(RST)resetsallanalogoutputsto1/2of
VREF, regardless of any digital inputs. Note that the input VRi is
referenced to AGND.
Function
A3 A2 A1 A0
LD
CLK
RST
SDI
SDO
0→1
Shift Data In
and Out
Data Input
Valid
Data Output
Valid
X
X
X
X
X
X
X
X
1
1
1
Repeat
Stop Shifting
Data In and
Out
Hi-Z
0
X
X
Load DACs
0
0
0
0
0
0
0
0
1
0
0
0
0
1
1
1
1
0
0
0
1
1
0
0
1
1
0
0
1
0
1
0
1
0
1
0
No Operation
1→0
Hi-Z
Hi-Z
Hi-Z
Hi-Z
Hi-Z
Hi-Z
Hi-Z
Hi-Z
DAC 0
DAC 1
DAC 2
DAC 3
DAC 4
DAC 5
DAC 6
DAC 7
X
X
X
X
X
X
X
X
X
X
X
X
X
1
1
1
1
1
1
1
1
X
X
X
X
X
X
X
X
1→0
1→0
1→0
1→0
1→0
1→0
1→0
No Operation
No Operation
No Operation
Hi-Z
Hi-Z
X
X
1
1
1
1
1
1
0
1
1
1
Reset all DACs to
X
X
X
X
X
X
0
X
X
V
REF/2
Table 1. Digital Function Truth Table
Serial In/Serial Out
DAC Output Voltage
VOi = AGND + (VRi – AGND) (
DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0
D
256
)
MSB
LSB
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
AGND
1
(V – AGND) (
) + AGND
Ri
256
254
256
(V – AGND) (
) + AGND
) + AGND
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
1
Ri
255
256
(V – AGND) (
Ri
Table 2. DAC Transfer Function
Analog Output vs. Digital Code
Rev. 2.00
9
EXAR [ EXAR CORPORATION ]
