Datasheet

LTC1403-1/LTC1403A-1
13
14031fc
APPLICATIONS INFORMATION
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 unipolar voltage span
that equals the difference between the voltage at the ref-
erence 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 LTC1403-1/LTC1403A-1 have a unique differential
sample-and-hold circuit that allows inputs from ground
to V
DD
. The ADC will always convert the bipolar differ-
ence 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 differ-
ential 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 LTC1403-1/LTC1403A-1. 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 LTC1403A-1, and 1LSB = 2.5V/4096 = 610µV for the
LTC1403-1. The LTC1403A-1 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
FREQUENCY (Hz)
100
CMRR (dB)
0
–20
–40
–60
–80
–100
–120
1k
10k 100k 1M
14031 F04
10M
100M
Figure 5
LTC1403-1/LTC1403A-1 Transfer
Characteristic
INPUT VOLTAGE (V)
2’S COMPLEMENT OUTPUT CODE
14031 F05
011...111
011...110
011...101
100...000
100...001
100...010
FS – 1LSB–FS
A
IN
–
LTC1403A-1
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
1403A F06a
–
+
U1
1/2 LT1819
–
+
U2
1/2 LT1819
Figure 6a. The LT1819 Driving the LTC1403A-1 Differentially