Datasheet

MCP4921/4922
DS21897A-page 20  2004 Microchip Technology Inc.
6.0 TYPICAL APPLICATIONS
The MCP492X devices are general purpose DACs
intended to be used in applications where a precision,
low-power DAC with moderate bandwidth is required.
Applications generally suited for the MCP492X devices
include:
• Set Point or Offset Trimming
• Sensor Calibration
• Digitally-Controlled Multiplier/Divider
• Portable Instrumentation (Battery Powered)
• Motor Feedback Loop Control
6.1 Digital Interface
The MCP492X utilizes a 3-wire syncronous serial
protocol to transfer the DACs’ setup and output values
from the digital source. The serial protocol can be inter-
faced to SPI™ or Microwire
peripherals common on
many microcontrollers, including Microchip’s
PICmicro
®
MCUs & dsPIC
TM
DSC family of microcon-
trollers. In addition to the three serial connections (CS
,
SCK and SDI), the LDAC
signal syncronizes when the
serial settings are latched into the DAC’s output from
the serial input latch. Figure 6-1 illustrates the required
connections. Note that LDAC
is active-low. If desired,
this input can be tied low to reduce the required con-
nections from 4 to 3. Write commands will be latched
directly into the output latch when a valid 16 clock
transmission has been received and CS has been
raised.
6.2 Power Supply Considerations
The typical application will require a by-pass capacitor
in order to filter high-frequency noise. The noise can
be induced onto the power supply's traces or as a result
of changes on the DAC's output. The bypass capacitor
helps to minimize the effect of these noise sources on
signal integrity. Figure 6-1 illustrates an appropriate
bypass strategy.
In this example, the recommended bypass capacitor
value is 0.1 µF. This capacitor should be placed as
close to the device power pin (V
DD
) as possible (within
4mm).
The power source supplying these devices should be
as clean as possible. If the application circuit has sep-
arate digital and analog power supplies, AV
DD
and
AV
SS
should reside on the analog plane.
FIGURE 6-1: Typical Connection
Diagram.
6.3 Layout Considerations
Inductively-coupled AC transients and digital switching
noise can degrade the input and output signal integrity,
potentially masking the MCP492X’s performance.
Careful board layout will minimize these effects and
increase the signal-to-noise ratio (SNR). Bench testing
has shown that a multi-layer board utilizing a low-induc-
tance ground plane, isolated inputs, isolated outputs
and proper decoupling are critical to achieving the
performance that the silicon is capable of providing.
Particularly harsh environments may require shielding
of critical signals.
Breadboards and wire-wrapped boards are not
recommended if low noise is desired.
Note: At the time of this data sheet’s release,
circuit examples had not completed
testing. Your results may vary.
MCP492X
V
DD
V
DD
V
DD
AV
SS
AV
SS
AV
SS
V
REFA
V
OUTA
V
REFB
V
OUTB
MCP492X
0.1 µF
PICmicro
®
Microcontroller
0.1 µF 0.1 µF
V
REFA
V
OUTA
V
REFB
V
OUTB
SDI
SDI
CS
1
SDO
SCK
LDAC
CS
0