LTC1407 LINER [Linear Technology], LTC1407 Datasheet - Page 13

• Manufacturer Part Number: LTC1407
• Description: Serial 12-Bit/14-Bit, 3Msps Simultaneous Sampling Simultaneous Sampling
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
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 noninvert-
ing 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
If the difference between the CH0
CH1
stay fixed at all ones, and if this difference goes below 0V,
the ouput code will stay fixed 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 amplifier output V
a capacitor to ground. The reference amplifier is stable with
capacitors of 1 F or greater. For the best noise perfor-
mance, a 10 F ceramic or a 10 F tantalum in parallel with
a 0.1 F ceramic is recommended. The V
+
and CH1
–
3V REF
inputs exceeds 2.5V, the output code will
U
REF
Figure 2
U
, (Pin 3) must be bypassed with
10 F
11
+
3
and CH0
W
V
GND
REF
LTC1407A
LTC1407/
REF
1407 F02
–
inputs or the
pin can be
U
DD
.
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
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
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 CH1
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
+
minus CH0
–100
–120
–20
–40
–60
–80
–
0
, independent of the common mode voltage at
100
Figure 3. CMRR vs Frequency
1k
–
DD
or the unipolar difference of CH1
LTC1407/LTC1407A
. An external reference at 2.55V will
10k
FREQUENCY (Hz)
REF
CH0
100k
(Pin 3) and the voltage at the
1M
CH1
10M
1407 G08
100M
13
1407f
DD
+
.