17
LTC1292/LTC1297
12927fb
RC Input Filtering
It is possible to filter the inputs with an RC network as
shown in Figure 15. 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 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 the LTC1292(LTC1297) at the minimum cycle
time of 16.5µs (20µs), the input current equals 30µA
(25µA) at V
IN
= 5V. Here a filter resistor of 4Ω (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.
Figure 15. RC Input Filtering
curve of S&H Acquisition Time vs Source Resistance). The
input voltage is sampled during the t
SMPL
time as shown
in Figure 11. The sampling interval begins at the rising
edge of CS for the LTC1292, and at the falling edge of CS
for the LTC1297, and continues until the falling edge of the
CLK before the conversion begins. 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 as in single-ended mode. 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:
VfV
f
ERROR MAX IN PEAK
CLK
( ) (– )
=
()
2
12
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
V
f
IN MAX
ERROR MAX
PEAK
CLK
(– )
()
=
π
2
12
For 0.25LSB error (300µV) the maximum input sinusoid
with a 5V peak amplitude that can be digitized is 0.8Hz.
Reference Input
The voltage on the reference input of the LTC1292/
LTC1297 determine the voltage span of the A/D con-
verter. The reference input has transient capacitive
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 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 LTC1292/LTC1297 provide 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 allows the LTC1292/LTC1297 to convert rapidly
varying signals (see typical performance characteristics
R
FILTER
C
FILTER
LTC1292/7 F15
LTC1292
LTC1297
“+”
“–”
I
DC
V
IN
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S
A
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PP
L
IC
AT
I
WU
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