Datasheet

LTC1403/LTC1403A
11
1403fb
APPLICATIONS INFORMATION
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
at Pin 3, and the voltage at the ground
(Exposed Pad Ground). The differential input range of
the 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 LTC1403/LTC1403A has a unique differential sample-
and-hold circuit that allows inputs from ground to V
DD
.
The ADC will always convert the unipolar difference of
A
IN
+
– A
IN
–
, independent of the common mode voltage
at the inputs. The common mode rejection holds up at
extremely high frequencies, see Figure 3. The only require-
ment is that both inputs not go below ground or exceed
V
DD
. Integral nonlinearity errors (INL) and differential
nonlinearity errors (DNL) are largely independent of the
common mode voltage. However, the offset error will vary.
The change in offset error is typically less than 0.1% of
the common mode voltage.
Figure 4 shows the ideal input/output characteristics for
the LTC1403/LTC1403A. The code transitions occur mid-
way between successive integer LSB values (i.e., 0.5LSB,
1.5LSB, 2.5LSB, FS – 1.5LSB). The output code is natural
binary with 1LSB = 2.5V/16384 = 153μV for the LTC1403A,
and 1LSB = 2.5V/4096 = 610μV for the LTC1403. The
LTC1403A has 1LSB RMS of random white noise.
Figure 3 Figure 4
FREQUENCY (Hz)
100
CMRR (dB)
0
–20
–40
–60
–80
–100
–120
1k
10k 100k 1M
1403A F03
10M 100M
CMRR vs Frequency
INPUT VOLTAGE (V)
UNIPOLAR OUTPUT CODE
1403A F05
111...111
111...110
111...101
000...000
000...001
000...010
FS – 1LSB0
LTC1403/LTC1403A Transfer
Characteristic