Datasheet

LTC2356-12/LTC2356-14
13
2356fb
A
IN
–
LTC2356-14
A
IN
+
C1
47pF TO
1000pF
1
R1
51Ω
C3
1µF
C5
0.1µF
5V
–5V
C4
1µF
R5
1k
1.5V
CM
R3
499Ω
R4
499Ω
R6
1k
C2
47pF TO
1000pF
R2
51Ω
C6
0.1µF
V
IN
1.25V
P-P
MAX
2356 F06a
–
+
U1
1/2 LT1819
–
+
U2
1/2 LT1819
INPUT VOLTAGE (V)
2’S COMPLEMENT OUTPUT CODE
2356 F05
011...111
011...110
011...101
100...000
100...001
100...010
FS – 1LSB–FS
FREQUENCY (Hz)
100
CMRR (dB)
0
–20
–40
–60
–80
–100
–120
1k
10k 100k 1M
2356 F04
10M 100M
APPLICATIONS INFORMATION
Figure 3. The voltage of the external reference must be
higher than the 2.5V output of the internal reference. The
recommended range for an external reference 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 bipolar ± V
REF
/2
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 ±1.25V 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 LTC2356-12/LTC2356-14 have a unique differential
sample-and-hold circuit that measures input voltages from
ground to V
DD
. The ADC will always convert the bipolar
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 4. 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 5 shows the ideal input/output characteristics for
the LTC2356-12/LTC2356-14. 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 2’s complement with 1LSB = 2.5V/16384 = 153µV for
the LTC2356-14, and 1LSB = 2.5V/4096 = 610µV for the
LTC2356-12. The LTC2356-14 has 1LSB RMS of random
white noise. Figure 6a shows the LTC1819 converting a
single ended input signal to differential input signals for
optimum THD and SFDR performance as shown in the
FFT plot (Figure 6b).
Figure 4. CMRR vs Frequency
Figure 5. LTC2356-12/LTC2356-14 Transfer Characteristic
Figure 6a. The LT1819 Driving the LTC2356-14 Differentially