Datasheet

15
LTC1408
1408fa
APPLICATIO S I FOR ATIO
WUUU
Figure 4 shows the ideal input/output characteristics for
the LTC1408 in unipolar mode (BIP = Low). 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 LTC1408. The LTC1408 has 0.7 LSB RMS
of Gaussian white noise.
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
reference buffer output V
REF
(Pin 23) and the voltage at
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 ADC will always convert the difference of CH
+
minus
CH
–
, independent of the common mode voltage at any pair
of inputs. The common mode rejection holds up at high
frequencies (see Figure 3.) The only requirement is that
both inputs not go below ground or exceed V
DD
.
Figure 3. CMRR vs Frequency
1408 G20
FREQUENCY (Hz)
–100
CMRR (dB)
–60
–20
–40
–80
0
10k 100k 1M 10M 100M 1G
–120
100 1k
Integral nonlinearity errors (INL) and differential nonlin-
earity errors (DNL) are largely independent of the common
mode voltage. However, the offset error will vary. DC
CMRR is typically better than –90dB.
Figure 5 shows the ideal input/output characteristics for
the LTC1408 in bipolar mode (BIP = High). 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 LTC1408. The LTC1408 has 0.7 LSB RMS
of Gaussian white noise.
Figure 5. LTC1408 Transfer Characteristic
in Bipolar Mode (BIP = High)
INPUT VOLTAGE (V)
2'S COMPLEMENT OUTPUT CODE
1408 F05
011...111
011...110
011...101
100...000
100...001
100...010
FS – 1LSB–FS
Figure 4. LTC1408 Transfer Characteristic
in Unipolar Mode (BIP = Low)
INPUT VOLTAGE (V)
STRAIGHT BINARY OUTPUT CODE
1408 F04
111...111
111...110
111...101
000...000
000...001
000...010
FS – 1LSB0