LTC2289
18
2289fa
offset binary output format. Connecting MODE to
2/3VDD or VDD selects 2’s complement output format. An
external resistor divider can be used to set the 1/3VDD or
2/3VDD logic values. Table 2 shows the logic states for the
MODE pin.
APPLICATIO S I FOR ATIO
WU
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Sleep and Nap Modes
The converter may be placed in shutdown or nap modes
to conserve power. Connecting SHDN to GND results in
normal operation. Connecting SHDN to VDD and OE to VDD
results in sleep mode, which powers down all circuitry
including the reference and typically dissipates 1mW. When
exiting sleep mode it will take milliseconds for the output
data to become valid because the reference capacitors have
to recharge and stabilize. Connecting SHDN to VDD and OE
to GND results in nap mode, which typically dissipates
30mW. In nap mode, the on-chip reference circuit is kept
on, so that recovery from nap mode is faster than that from
sleep mode, typically taking 100 clock cycles. In both sleep
and nap modes, all digital outputs are disabled and enter
the Hi-Z state.
Channels A and B have independent SHDN pins (SHDNA,
SHDNB). Channel A is controlled by SHDNA and OEA, and
Channel B is controlled by SHDNB and OEB. The nap, sleep
and output enable modes of the two channels are completely
independent, so it is possible to have one channel operat-
ing while the other channel is in nap or sleep mode.
Digital Output Multiplexer
The digital outputs of the LTC2289 can be multiplexed onto
a single data bus. The MUX pin is a digital input that swaps
the two data busses. If MUX is High, Channel A comes out
on DA0-DA9, OFA; Channel B comes out on DB0-DB9, OFB.
If MUX is Low, the output busses are swapped and Chan-
nel A comes out on DB0-DB9, OFB; Channel B comes out
on DA0-DA9, OFA. To multiplex both channels onto a single
output bus, connect MUX, CLKA and CLKB together (see
the Timing Diagram for the multiplexed mode). The mul-
tiplexed data is available on either data bus—the unused
data bus can be disabled with its OE pin.
Grounding and Bypassing
The LTC2289 requires a printed circuit board with a clean,
unbroken ground plane. A multilayer board with an inter-
nal ground plane is recommended. Layout for the printed
circuit board should ensure that digital and analog signal
Table 2. MODE Pin Function
Clock Duty
MODE Pin
Output Format
Cycle Stabilizer
0
Offset Binary
Off
1/3VDD
Offset Binary
On
2/3VDD
2’s Complement
On
VDD
2’s Complement
Off
Overflow Bit
When OF outputs a logic high the converter is either
overranged or underranged.
Output Driver Power
Separate output power and ground pins allow the output
drivers to be isolated from the analog circuitry. The power
supply for the digital output buffers, OVDD, should be tied
to the same power supply as for the logic being driven. For
example, if the converter is driving a DSP powered by a 1.8V
supply, then OVDD should be tied to that same 1.8V supply.
OVDD can be powered with any voltage from 500mV up to
3.6V. OGND can be powered with any voltage from GND up
to 1V and must be less than OVDD. The logic outputs will
swing between OGND and OVDD.
Output Enable
The outputs may be disabled with the output enable pin, OE.
OE high disables all data outputs including OF. The data ac-
cess and bus relinquish times are too slow to allow the
outputs to be enabled and disabled during full speed op-
eration. The output Hi-Z state is intended for use during long
periods of inactivity. Channels A and B have independent
output enable pins (OEA, OEB).