Datasheet

LTC1096/LTC1096L
LTC1098/LTC1098L
22
10968fc
in and continues until the falling CLK edge after the MSBF
bit is received. On this falling edge, the S&H goes into hold
mode and the conversion begins.
Differential Inputs
With differential inputs, the ADC no longer converts just a
single voltage but rather the difference between two volt-
ages. In this case, the voltage on the selected “+” input
is still sampled and held and therefore may be rapidly
time varying just as in single-ended mode. However, the
voltage on the selected “–” input must remain constant
and be free of noise and ripple throughout the conver-
sion time. Otherwise, the differencing operation may not
be performed accurately. The conversion time is 8 CLK
cycles. Therefore, a change in the “–” input voltage during
this interval can cause conversion errors. For a sinusoidal
voltage on the “–” input this error would be:
V
ERROR (MAX)
= V
PEAK
• 2 • π • f(“–”) • 8/f
CLK
Where f(“–”) is the frequency of the “–” input voltage,
V
PEAK
is its peak amplitude and f
CLK
is the frequency of
the CLK. In most cases V
ERROR
will not be signifi cant. For
a 60Hz signal on the “–” input to generate a 1/4LSB error
(5mV) with the converter running at CLK = 500kHz, its
peak value would have to be 750mV.
ANALOG INPUTS
Because of the capacitive redistribution A/D conversion
techniques used, the analog inputs of the LTC1096(L)/
LTC1098(L )have capacitive switching input current spikes.
These current spikes settle quickly and do not cause a
problem. However, if large source resistances are used
or if slow settling op amps drive the inputs, care must be
taken to ensure that the transients caused by the current
spikes settle completely before the conversion begins.
“+” Input Settling
The input capacitor of the LTC1096(L) is switched onto
“+” input during the wake-up time (see Figure 1) and
samples the input signal within that time. However, the
input capacitor of the LTC1098(L) is switched onto “+”
input during the sample phase (t
SMPL
, see Figure 7). The
sample phase is 1.5 CLK cycles before conversion starts.
The voltage on the “+” input must settle completely within
t
WAKEUP
or t
SMPL
for the LTC1096(L) or the LTC1098(L)
respectively. Minimizing R
SOURCE
+
and C1 will improve the
input settling time. If a large “+” input source resistance
must be used, the sample time can be increased by using
a slower CLK frequency.
“–” Input Settling
At the end of the t
WAKEUP
or t
SMPL
, the input capacitor
switches to the “–” input and conversion starts (see
Figures 1 and 7). During the conversion the “+” input
voltage is effectively “held” by the sample-and-hold and
will not affect the conversion result. However, it is criti-
cal that the “–” input voltage settles completely during
the fi rst CLK cycle of the conversion time and be free of
noise. Minimizing R
SOURCE
–
and C2 will improve settling
time. If a large “–” input source resistance must be used,
the time allowed for settling can be extended by using a
slower CLK frequency.
Input Op Amps
When driving the analog inputs with an op amp it is im-
portant that the op amp settle within the allowed time (see
Figure 7). Again, the “+” and “–” input sampling times can
be extended as described above to accommodate slower
op amps. Most op amps, including the LT1006 and LT1413
single supply op amps, can be made to settle well even
with the minimum settling windows of 3μs (“+” input)
which occur at the maximum clock rate of 500kHz.
Source Resistance
The analog inputs of the LTC1096/LTC1098 look like a 25pF
capacitor (C
IN
) in series with a 500Ω resistor (R
ON
) as
shown in Figure 8. C
IN
gets switched between the selected
“+” and “–” inputs once during each conversion cycle.
Figure 8. Analog Input Equivalent Circuit
R
ON
= 500Ω
C
IN
= 25pF
LTC1096
LTC1098
“+”
INPUT
R
SOURCE
+
V
IN
+
C1
“–”
INPUT
R
SOURCE
–
V
IN
–
C2
10968 F08
APPLICATIONS INFORMATION