LTC2355-14 Linear Technology, LTC2355-14 Datasheet - Page 11

• Manufacturer Part Number: LTC2355-14
• Description: (LTC2355-12/-14) 3.5Msps Sampling ADCs
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
INPUT SPAN VERSUS REFERENCE VOLTAGE
The differential input range has a 0V to V
voltage span that equals the difference between the volt-
age at the reference buffer output V
voltage at the ground (Exposed Pad Ground). The differ-
ential 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 LTC2355-12/LTC2355-14 has a unique differential
sample-and-hold circuit that measures input voltages
from ground to V
unipolar difference of A
–100
–120
–20
–40
–60
–80
0
100
Figure 3. CMRR vs Frequency
DD
U
1k
. The ADC will always convert the
10k
FREQUENCY (Hz)
IN
U
+
100k
– A
IN
1M
–
, independent of the
W
REF
10M 100M
at Pin 3, and the
2355 F03
REF
U
unipolar
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
(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 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 4. LTC2355-12/LTC2355-14 Transfer Characteristic
111...111
111...110
111...101
000...010
000...001
000...000
LTC2355-12/LTC2355-14
0
INPUT VOLTAGE (V)
DD
. Integral nonlinearity errors
FS – 1LSB
2355 F05
11
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