Datasheet

19
LTC1293/LTC1294/LTC1296
129346fs
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Source Resistance
The analog inputs of the LTC1293/4/6 look like a 100pF
capacitor (C
IN
) in series with a 500Ω resistor (R
ON
). C
IN
gets switched between (+) and (–) inputs once during each
conversion cycle. Large external source resistors and
capacitances will slow the settling of the inputs. It is
important that the overall RC time constant is short
enough to allow the analog inputs to settle completely
within the allowed time.
“+” Input Settling
The input capacitor is switched onto the “+” input during
the sample phase (t
SMPL
, see Figure 8). The sample period
2 1/2 CLK cycles before a conversion starts. The voltage on
the “+” input must settle completely within the sample
period. Minimizing R
SOURCE
+ and C1 will improve the
settling time. If large “+” input source resistance must be
used, the sample time can be increased by using a slower
CLK frequency. With the minimum possible sample time
of 2.5µs R
SOURCE
+ < 1.5k
ΩΩ
ΩΩ
Ω and C1 < 20pF will provide
adequate settling time.
“–” Input Settling
At the end of the sample phase the input capacitor switches
to the “-” input and the conversion starts (see Figure 8).
During the conversion, the “+” input voltage is effectively
“held” by the sample and hold and will not affect the
conversion result. It is critical that the “–” input voltage be
free of noise and settle completely during the first CLK
cycle of the conversion. Minimizing R
SOURCE
– and C2 will
improve settling time. If large “–” input source resistance
must be used the time can be extended by using a slower
CLK frequency. At the maximum CLK frequency of 1MHz,
R
SOURCE
– < 250
ΩΩ
ΩΩ
Ω and C2 < 20pF will provide adequate
settling.
Input Op Amps
When driving the analog inputs with an op amp it is
important that the op amp settles within the allowed time
(see Figure 8). Again the “+” and “–” input sampling times
can be extended as described above to accommodate
slower op amps. Most op amps including the LT1006 and
LT1013 single supply op amps can be made to settle
Figure 8. “+” and “–” Input Settling Windows
D
IN
CLK
START
HI-Z
LTC1293 F08
CS
1ST BIT TEST (–) INPUT MUST
SETTLE DURING THIS TIME
t
SMPL
(+) INPUT MUST SETTLE DURING THIS TIME
(+) INPUT
(–) INPUT
SGL/
DIFF
MSBF
PS
D
OUT
B11
SAMPLE
HOLD