Datasheet

9
LTC1420
1420fa
V
CM
SENSE
V
REF
1420 F02
–
+
R1
5k
LOGIC
2.5V
REFERENCE
2.048V
1µF
1µF
1k
TO
ADC
R2
5k
V
OUT
V
IN
LT1019A-2.5
5V
1420 F03a
1µF
1µF
V
REF
SENSE5V
V
CM
LTC1420
1420 F03b
1µF
1µF
V
REF
LTC1420
SENSE
V
CM
2.048V
–
+
5k
5k
LTC1450
Figure 2. Reference Circuit
Figure 3a. Using the LT1019-2.5 As an
External Reference; Input Range = ±1.25V
Figure 3b. Driving V
REF
with a DAC
APPLICATIONS INFORMATION
WUU
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V
REF
and V
REF
can be driven by an external voltage. With
two additional resistors, V
REF
can be set to any voltage
between 2.048V and 4.5V.
An external reference or a DAC can be used to drive V
REF
over a 0V to 5V range (Figures 3a and 3b). The input
impedance of the V
REF
pin is 1k, so a buffer may be
required for high accuracy. Driving V
REF
with a DAC is
useful in applications where the peak input signal ampli-
tude may vary. The input span of the ADC can then be
adjusted to match the peak input signal, maximizing the
signal-to-noise ratio.
Both the V
CM
and V
REF
pins must be bypassed with
capacitors to ground. For best performance, 1µF or larger
ceramic capacitors are recommended. For the case of
external circuitry driving V
REF
, a smaller capacitor can be
used at V
REF
so the input range can be changed quickly. In
this case, a 0.05µF or larger ceramic capacitor is accept-
able.
The V
CM
pin is a low output impedance 2.5V reference that
can be used by external circuitry. For single 5V supply
applications it is convenient to connect – A
IN
directly to the
V
CM
pin.
Driving the Analog Inputs
The differential inputs of the LTC1420 are easy to drive.
The inputs may be driven differentially or single-ended
(i. e., the –A
IN
input is held at a fixed value). The –A
IN
and
+A
IN
inputs are simultaneously sampled and any com-
mon mode signal is reduced by the high common mode
rejection of the sample-and-hold circuit. Any common
mode input value is acceptable as long as the input pins
stay between V
DD
and V
SS
. During conversion, the analog
inputs are high impedance. At the end of conversion, the
inputs draw a small current spike while charging the
sample-and-hold.
For superior dynamic performance in dual supply mode,
the LTC1420 should be operated with the analog inputs
centered at ground, and in single supply mode the inputs
should be centered at 2.5V. If required, the analog inputs
can be driven differentially via a transformer. Refer to
Table 2 for a summary of the analog input and reference
configurations and their relative advantages.