LTC2449_15 Datasheet, PDF (6/28 Pages) Linear Technology – 24-Bit High Speed 8-/16-Channel ADCs with Selectable Speed/Resolution

LTC2444/LTC2445/
LTC2448/LTC2449
PI FU CTIO S
GND – 0.3V to V_CC + 0.3V. Within these limits, the two
selected inputs (IN⁺ and IN^–) provide a bipolar input range
(V_IN = IN⁺ – IN^–) from –0.5 • V_REF to 0.5 • V_REF. Outside this
input range, the converter produces unique over-range
and under-range output codes.
CH0 to CH15 (Pins 8-23): LTC2448/LTC2449 Analog
Inputs. May be programmed for single-ended or differen-
tial mode.
CH0 to CH7 (Pins 9, 10, 13, 14, 17, 18, 21, 22): LTC2444/
LTC2445 Analog Inputs. May be programmed for single-
ended or differential mode.
NC (Pins 8, 11, 12, 15, 16, 19, 20, 23): LTC2444/
LTC2445 No Connect/Channel Isolation Shield. May be
left floating or tied to any voltage 0 to V_CC in order to
provide isolation for pairs of differential input channels.
NC (Pins 24, 25, 26, 27): LTC2444/LTC2448 No Connect.
These pins can either be tied to ground or left floating.
MUXOUTP (Pin 24): LTC2445/LTC2449 Positive Multi-
plexer Output. Used to drive the input to an external buffer/
amplifier.
ADCINP (Pin 25): LTC2445/LTC2449 Positive ADC Input.
Tie to output of buffer/amplifier driven by MUXOUTP.
ADCINN (Pin 26): LTC2445/LTC2449 Negative ADC Input.
Tie to output of buffer/amplifier driven by MUXOUTN.
MUXOUTN (Pin 27): LTC2445/LTC2449 Negative Multi-
plexer Output. Used to drive the input to an external buffer/
amplifier.
V_CC (Pin 28): Positive Supply Voltage. Bypass to GND with
a 10µF tantalum capacitor in parallel with a 0.1µF ceramic
capacitor as close to the part as possible.
REF⁺ (Pin 29), REF^– (Pin 30): Differential Reference
Input. The voltage on these pins can have any value
between GND and V_CC as long as the reference positive
input, REF⁺, is maintained more positive than the negative
reference input, REF⁺, by at least 0.1V.
SDI (Pin 34): Serial Data Input. This pin is used to select
the speed, 1X or 2X mode, resolution, and input channel,
for the next conversion cycle. At initial power up, the
default mode of operation is CH0-CH1, OSR of 256, and 1X
mode. The serial data input contains an enable bit which
6
determines if a new channel/speed is selected. If this bit is
low the following conversion remains at the same speed
and selected channel. The serial data input is applied to the
device under control of the serial clock (SCK) during the
data output cycle. The first conversion following a new
channel/speed is valid.
F_O (Pin 35): Frequency Control Pin. Digital input that
controls the internal conversion clock. When F_O is con-
nected to V_CC or GND, the converter uses its internal
oscillator running at 9MHz. The conversion rate is deter-
mined by the selected OSR such that t_CONV (ms) = 40 •
OSR + 170/f_OSC (kHz). The first digital filter null is located
at 8/t_CONV, 7kHz at OSR = 256 and 55Hz (Simultaneous 50/
60Hz) at OSR = 32768. This pin may be driven with a
maximum external clock of 10.24MHz resulting in a maxi-
mum 8kHz output rate (OSR = 64, 2X Mode).
CS (Pin 36): Active Low Chip Select. A LOW on this pin
enables the SDO ditital output and wakes up the ADC.
Following each conversion the ADC automatically enters
the sleep mode and remains in this low power state as long
as CS is HIGH. A LOW-to-HIGH transition on CS during the
Data Output aborts the data transfer and starts a new
conversion.
SDO (Pin 37): Three-State Digital Output. During the data
output period, this pin is used as serial data output. When
the chip select CS is HIGH (CS = V_CC) the SDO pin is in a
high impedance state. During the conversion and sleep
periods, this pin is used as the conversion status output.
The conversion status can be observed by pulling CS
LOW. This signal is HIGH while the conversion is in
progress and goes LOW once the conversion is complete.
SCK (Pin 38): Bidirectional Digital Clock Pin. In internal
serial clock operation mode, SCK is used as a digital output
for the internal serial interface clock during the data output
period. In the external serial clock operation mode, SCK is
used as the digital input for the external serial interface
clock during the data output period. The serial clock
operation mode is determined by the logic level applied to
the EXT pin.
Exposed Pad (Pin 39): Ground. The exposed pad on the
bottom of the package must be soldered to the PCB ground.
For prototyping purposes, this pin may remain floating.
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