Datasheet

LTC2355-12/LTC2355-14
12
2355fa
applications inFormation
INPUT SPAN VERSUS REFERENCE VOLTAGE
The differential input range has a 0V to V
REF
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.
D
IFFERENTIAL
INPUTS
The LTC2355-12/LTC2355-14 has a unique differential
sample-and-hold circuit that measures input voltages 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 requirement is that both inputs not go below ground
or exceed V
DD
. Integral nonlinearity errors (INL) and dif-
ferential 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 LTC2355-12/LTC2355-14. The code transitions occur
midway between successive integer LSB values (i.e.,
0.5LSB, 1.5LSB, 2.5LSB, FS – 1.5LSB). The output code
is straight binary with 1LSB = 2.5V/16384 = 153µV for
the LTC2355-14, and 1LSB = 2.5V/4096 = 610µV for the
LTC2355-12. The LTC2355-14 has 1LSB RMS of random
white noise.
Figure 3. CMRR vs Frequency Figure 4. LTC2355-12/LTC2355-14 Transfer Characteristic
FREQUENCY (Hz)
100
CMRR (dB)
0
–20
–40
–60
–80
–100
–120
1k
10k 100k 1M
2355 F03
10M 100M
INPUT VOLTAGE (V)
UNIPOLAR OUTPUT CODE
2355 F04
111...111
111...110
111...101
000...000
000...001
000...010
FS – 1LSB0