17
LTC1291
1291fa
Figure 12. RC Input Filtering
Input Leakage Current
Input leakage currents also can create errors if the source
resistance gets too large. For example, the maximum input
leakage specification of 1µA (at 125°C) flowing through a
source resistance of 1k will cause a voltage drop of 1mV
or 0.8LSB. This error will be much reduced at lower
temperatures because leakage drops rapidly (see typical
performance characteristics curve Input Channel Leakage
Current vs Temperature).
SAMPLE-AND-HOLD
Single-Ended Input
The LTC1291 provides a built-in sample-and-hold (S/H)
function on the +IN input for signals acquired in the single-
ended mode (–IN pin grounded). The sample-and-hold
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
RC Input Filtering
It is possible to filter the inputs with an RC network as
shown in Figure 12. For large values of C
F
(e.g., 1µF) the
capacitive input switching currents are averaged into a net
DC current. A filter should be chosen with a small resistor
and a large capacitor to prevent DC drops across the
resistor. The magnitude of the DC current is approximately
I
DC
= 100pF • V
IN
/t
CYC
and is roughly proportional to V
IN
.
When running at the minimum cycle time of 18.5µs, the
input current equals 27µA at V
IN
= 5V. Here a filter resistor
of 4.5Ω will cause 0.1LSB of full-scale error. If a large filter
resistor must be used, errors can be reduced by increasing
the cycle time as shown in the Typical Performance
Characteristics curve Maximum Filter Resistor vs Cycle
Time.
R
FILTER
V
IN
–
C
FILTER
LTC1291 F12
LTC1291
“+”
“–”
I
DC
allows the LTC1291 to convert rapidly varying signals (see
typical performance characteristics curve of S/H Acquisition
Time vs Source Resistance). The input voltage is sampled
during the t
SMPL
time as shown in Figure 9. The sampling
interval begins as the bit preceding the MSBF bit is shifted
in and continues until the falling edge of the PS bit is
received. On this falling edge, the S/H goes into the hold
mode and the conversion begins.
Differential Input
With a differential input the A/D no longer converts a single
voltage but converts the difference between two voltages.
The voltage on the +IN pin is sampled and held and can be
rapidly time varying. The voltage on the –IN pin must
remain constant and be free of noise and ripple throughout
the conversion time. Otherwise the differencing operation
will not be done accurately. The conversion time is 12 CLK
cycles. Therefore a change in the –IN input voltage during
this interval can cause conversion errors. For a sinusoidal
voltage on the –IN input this error would be:
V2fV
12
f
ERROR MAX IN PEAK
CLK
() ()
=
()
−
π
Where f
(–IN)
is the frequency of the –IN input voltage,
V
PEAK
is its peak amplitude and f
CLK
is the frequency of the
CLK. Usually V
ERROR
will not be significant. For a 60Hz
signal on the –IN input to generate a 0.25LSB error
(300µV) with the converter running at CLK = 1MHz, its
peak value would have to be 66mV. Rearranging the above
equation, the maximum sinusoidal signal that can be
digitized to a given accuracy is given as:
f
V
2V
f
12
IN
ERROR MAX
PEAK
CLK
()
()
−
=
π
For 0.25LSB error (300µV), the maximum input sinusoid
with a 5V peak amplitude that can be digitized is 0.8Hz.
