DDC232 Datasheet, PDF (26/28 Pages) Burr-Brown (TI) – 32-Channel, Current-Input Analog-to-Digital Converter

DDC232
SBAS331C – AUGUST 2004 – REVISED SEPTEMBER 2006
POWER-UP SEQUENCING
Prior to power-up, all digital and analog inputs must
be low. At the time of power-up, all of these signals
should remain low until the power supplies have
stabilized, as shown in Figure 24. At this time, begin
supplying the master clock signal to the CLK pin.
Wait for time t_POR, then give a RESET pulse. After
releasing RESET, the configuration register must be
programmed. Table 12 shows the timing for the
power-up sequence.
Power Supplies
RESET
t_POR
t_RST
Figure 24. DDC232 Timing Diagram at Power-Up
Table 12. Timing for DDC232 Power-Up Sequence
SYMBOL DESCRIPTION
t_POR
Wait After Power-Up
Until Reset
t_RST Reset Low Width
MIN TYP MAX UNITS
250
ms
1
µs
www.ti.com
LAYOUT
POWER SUPPLIES AND GROUNDING
Both AVDD and DVDD should be as quiet as
possible. It is particularly important to eliminate noise
from AVDD that is non-synchronous with the
DDC232 operation. Figure 25 illustrates how to
supply power to the DDC232. Each supply of the
DDC232 should be bypassed with 10µF solid
tantalum capacitors. It is recommended that both the
analog and digital grounds (AGND and DGND) be
connected to a single ground plane on the printed
circuit board (PCB).
VA
10µF
VD
AVDD AGND
DDC232
DVDD
10µF
DGND
Figure 25. Power-Supply Connections
Shielding Analog Signal Paths
As with any precision circuit, careful PCB layout will
ensure the best performance. It is essential to make
short, direct interconnections and avoid stray wiring
capacitance—particularly at the analog input pins
and QGND. These analog input pins are
high-impedance and extremely sensitive to
extraneous noise. The QGND pin should be treated
as a sensitive analog signal and connected directly
to the supply ground with proper shielding. Leakage
currents between the PCB traces can exceed the
input bias current of the DDC232 if shielding is not
implemented. Digital signals should be kept as far as
possible from the analog input signals on the PCB.
26
Submit Documentation Feedback