Datasheet

LTC2486
26
2486fd
Digital Signal Levels
The LTC2486’s digital interface is easy to use. Its digital
inputs SDI, f
O
, CS, and SCK (in external serial clock mode)
accept standard CMOS logic levels. Internal hysteresis
circuits can tolerate edge transition times as slow as
100μs.
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
diffi cult when shared control lines are used and multiple
refl ections occur.
APPLICATIONS INFORMATION
Parallel termination near the input pin of the LTC2486 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/under shoot 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 fi lter 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 LTC2486 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 simplifi ed equivalent circuit is
shown in Figure 12.
Figure 12. LTC2486 Equivalent Analog Input Circuit
IN
+
IN
–
10k
INTERNAL
SWITCH
NETWORK
10k
C
EQ
12pF
10k
I
IN
–
REF
+
I
REF
+
I
IN
+
I
REF
–
2486 F12
SWITCHING FREQUENCY
f
SW
= 123kHz INTERNAL OSCILLATOR
f
SW
= 0.4 • f
EOSC
EXTERNAL OSCILLATOR
REF
–
10k
100Ω
INPUT
MULTIPLEXER
100Ω
IIN
+
()
AVG
= IIN
–
()
AVG
=
V
IN(CM)
 V
REF(CM)
0.5•R
EQ
IREF
+
()
AVG

1.5V
REF
+ V
REF(CM)
–V
IN(CM)
()
0.5•R
EQ
–
V
IN
2
V
REF
•R
EQ
where:
V
REF
=REF
+
REF

V
REF(CM)
=
REF
+
–REF

2








V
IN
=IN
+
IN

,WHEREIN
+
ANDIN

ARE THE SELECTEDINPUT CHANNELS
V
IN(CM)
=
IN
+
–IN

2








R
EQ
= 2.71M INTERNAL OSCILLATOR 60Hz MODE
R
EQ
= 2.98MINTERNAL OSCILLATOR 50Hz/60Hz MODE
R
EQ
= 0.833•10
12
()
/f
EOSC
EXTERNAL OSCILLATOR