Datasheet

LTC2496
23
2496fb
applicaTions inForMaTion
The digital input signal range is 0.5V to V
CC
– 0.5V. During
transitions, the CMOS input circuits draw dynamic cur-
rent. For optimal performance, application of signals to
the serial data interface should be reserved for the sleep
and data output periods.
During the conversion period, overshoot and undershoot
of fast digital signals applied to both the serial digital
interface and the external oscillator pin (f
O
) may degrade
the converter performance. Undershoot and overshoot
occur due to impedance mismatch of the circuit board
trace at the converter pin when the transition time of an
external control signal is less than twice the propagation
delay from the driver to the input pin. For reference, on a
regular FR-4 board, the propagation delay is approximately
183ps/inch. In order to prevent overshoot, a driver with
a 1ns transition time must be connected to the converter
through a trace shorter than 2.5 inches. This becomes
difficult when shared control lines are used and multiple
reflections occur.
Parallel termination near the input pin of the LTC2496 will
eliminate this problem, but will increase the driver power
dissipation. A series resistor from 27Ω to 54Ω (depend-
ing on the trace impedance and connection) placed near
the driver will also eliminate over/undershoot without
additional driver power dissipation.
For many applications, the serial interface pins (SCK, SDI,
CS, f
O
) remain static during the conversion cycle and no
degradation occurs. On the other hand, if an external
oscillator is used (f
O
driven externally) it is active during
the conversion cycle. Moreover, the digital filter rejection
is minimal at the clock rate applied to f
O
. Care must be
taken to ensure external inputs and reference lines do not
cross this signal or run near it. These issues are avoided
when using the internal oscillator.
Driving the Input and Reference
The input and reference pins of the LTC2496 are connected
directly to a switched capacitor network. Depending on
the relationship between the differential input voltage and
the differential reference voltage, these capacitors are
switched between these four pins. Each time a capacitor
is switched between two of these pins, a small amount
of charge is transferred. A simplified equivalent circuit is
shown in Figure 10.
Figure 10. LTC2496 Equivalent Analog Input Circuit
I IN I IN
V V
R
AVG AVG
IN CM REF CM
EQ
+
( )
=
( )
=
−
•
–
( ) ( )
.0 5
II REF
V V V
R
AVG
REF REF CM IN CM
+
( )
≈
+
( )
1 5
0 5
. –
. •
( ) ( )
EEQ
IN
REF EQ
REF
REF CM
V
V R
where
V REF REF
V
–
•
:
(
2
= −
+ −
))
–
,
=








= −
+ −
+ − +
REF REF
V IN IN WHEREIN AN
IN
2
DDIN ARE THE SELECTEDINPUT CHANNELS
V
IN
IN CM
−
+
=
( )
––IN
−








= Ω
2
R 2.98M INTERNAL OSCILLATOR
R
EQ
EEQ
12
EOSC
0.833 10 /f EXTERNAL OSCILLATOR= •
( )
IN
+
IN
–
10k
INTERNAL
SWITCH
NETWORK
10k
C
EQ
12pF
10k
I
IN
–
REF
+
I
REF
+
I
IN
+
I
REF
–
2496 F10
SWITCHING FREQUENCY
f
SW
= 123kHz INTERNAL OSCILLATOR
f
SW
= 0.4 • f
EOSC
EXTERNAL OSCILLATOR
REF
–
10k
100Ω
INPUT
MULTIPLEXER
EXTERNAL
CONNECTION
100Ω
MUXOUTP ADCINP
EXTERNAL
CONNECTION
MUXOUTN ADCINN