SBAS363 – DECEMBER 2009
Power-Down Modes and Acquisition Time
There are three power-down modes that reduce power dissipation: Nap, Deep, and Auto-Nap. The first two, Nap
and Deep Power-Down modes, are enabled/disabled by bits CFR_D3 and CFR_D2, respectively, in the
Configuration register (see Table 5 for details).
Deep Power-Down mode provides maximum power savings. When this mode is enabled, the analog core in the
converter is shut down, and the analog supply current falls from 6.6mA (VA = 5.0V) to 1μA in 2μs. The wakeup
time from Deep Power-Down mode is 1μs. The device can wake up from Deep Power-Down mode by either
disabling this mode, issuing the wakeup command, loading the default value into the CFR, or performing a reset
(either with the software reset command, CFR_D0 bit, or the external reset). See Table 4 and Table 5 along with
the Reset Function section for further information.
In Nap Power-Down mode, the bias currents for the analog core of the device are significantly reduced. Because
the bias currents are not completely shut off, the ADS8331/32 can wake up from this power-down mode much
faster than from Deep Power-Down mode. After Nap Power-Down mode is enabled, the analog supply current
falls from 6.6mA (VA = 5.0V) to 0.39mA in 200ns. The wakeup time from this mode is three conversion clock
cycles (CCLK). The device can wake up from Nap Power-Down mode in the same manner as waking up from
Deep Power-Down mode.
The third power-down mode, Auto-Nap, is enabled/disabled by bit CFR_D4 in the Configuration register (see
Table 5 for details). Once this mode is enabled, the device is controlled by the digital core logic on the chip. The
device is automatically placed into Nap Power-Down mode after the next end of conversion (EOC). The analog
supply current falls from 6.6mA (VA = 5.0V) to 0.39mA in 200ns. A conversion start wakes up the device in three
conversion clock cycles. Issuing the wake-up command, loading the default value into the CFR, disabling
Auto-Nap Power-Down mode, issuing a manual channel select command, or resetting the device can wake the
ADS8331/32 from Auto-Nap Power-Down mode. A comparison of the three power-down modes is listed in
(VA = 5.0V)
Table 2. Comparison of Power-Down Modes
EOC (end of
Wakeup command 1011b
Wakeup command 1011b
CONVST, any channel select command, default
command 1111b, or wakeup command 1011b.
The default acquisition time is three conversion clock (CCLK) cycles. Figure 41 shows the timing diagram for
CONVST, EOC, and auto-nap power-down signals in Manual-Trigger mode. As shown in the diagram, the device
wakes up after a conversion is triggered by the CONVST pin going low. However, a conversion is not yet started
at this time. The conversion start signal to the analog core of the chip is internally generated no less than six
conversion clock (CCLK) cycles later, to allow at least three CCLKs for wake up and three CCLKs for acquisition.
The ADS8331/32 enters Nap Power-Down mode one conversion cycle after the end of conversion (EOC).
3 + 3 = 6 Cycles
Figure 41. Timing for CONVST, EOC, and Auto-Nap Power-Down Signals in Manual-Trigger Mode (Three
Conversion Clock Cycles for Acquisition)
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): ADS8331 ADS8332
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