Datasheet

LTC1407/LTC1407A
13
1407fb
APPLICATIONS INFORMATION
INPUT RANGE
The analog inputs of the LTC1407/LTC1407A may be driven
fully differentially with a single supply. Either input may
swing up to 3V, provided the differential swing is no greater
than 2.5V. In the valid input range, the noninverting input
of each channel should always be more positive than the
inverting input of each channel. The 0V to 2.5V range is
also ideally suited for single-ended input use with single
supply applications. The common mode range of the
inputs extend from ground to the supply voltage V
DD
. If
the difference between the CH0
+
and CH0
–
inputs or the
CH1
+
and CH1
–
inputs exceeds 2.5V, the output code will
stay fi xed at all ones, and if this difference goes below 0V,
the ouput code will stay fi xed at all zeros.
INTERNAL REFERENCE
The LTC1407/LTC1407A have an on-chip, temperature
compensated, bandgap reference that is factory trimmed
near 2.5V to obtain a precise 2.5V input span. The refer-
ence amplifi er output V
REF
, (Pin 3) must be bypassed with
a capacitor to ground. The reference amplifi er is stable
with capacitors of 1µF or greater. For the best noise per-
formance, a 10µF ceramic or a 10µF tantalum in parallel
with a 0.1µF ceramic is recommended. The V
REF
pin can be
overdriven with an external reference as shown in Figure 2.
The voltage of the external reference must be higher than
the 2.5V of the open-drain P-channel output of the internal
reference. The recommended range for an external refer-
ence is 2.55V to V
DD
. An external reference at 2.55V will
see a DC quiescent load of 0.75mA and as much as 3mA
during conversion.
INPUT SPAN VERSUS REFERENCE VOLTAGE
The differential input range has a unipolar voltage span that
equals the difference between the voltage at the reference
buffer output V
REF
(Pin 3) and the voltage at the Exposed
Pad ground. The differential input range of ADC is 0V to
2.5V when using the internal reference. The internal ADC
is referenced to these two nodes. This relationship also
holds true with an external reference.
DIFFERENTIAL INPUTS
The ADC will always convert the unipolar difference of
CH0
+
minus CH0
–
or the unipolar difference of CH1
+
mi-
nus CH1
–
, independent of the common mode voltage at
either set of inputs. The common mode rejection holds up
at high frequencies (see Figure 3.) The only requirement
is that both inputs not go below ground or exceed V
DD
.
Figure 2
Figure 3. CMRR vs Frequency
LTC1407/
LTC1407A
V
REF
GND
1407 F02
3
11
10µF
3V REF
FREQUENCY (Hz)
–80
CMRR (dB)
–40
0
–100
–60
–20
100 1k
1407 G08
–120
10k 100k 1M 10M 100M
CH0 CH1