12
LTC1419
1419fb
APPLICATIONS INFORMATION
WU
U
Differential Inputs
The LTC1419 has a unique differential sample-and-hold
circuit that allows rail-to-rail inputs. The ADC will always
convert the difference of + AIN – (–AIN) independent of the
common mode voltage (see Figure 11a). The common
mode rejection holds up to extremely high frequencies,
see Figure 10a. The only requirement is that both inputs
can not exceed the AVDD or AVSS power supply voltages.
Integral nonlinearity errors (INL) and differential nonlin-
earity errors (DNL) are independent of the common mode
voltage, however, the bipolar zero error (BZE) will vary.
The change in BZE is typically less than 0.1% of the
common mode voltage. Dynamic performance is also
affected by the common mode voltage. THD will degrade
as the inputs approach either power supply rail, from 86dB
with a common mode of 0V to 76dB with a common mode
of 2.5V or – 2.5V.
Figure 10b. Selectable 0V to 5V or
±2.5V Input Range
Differential inputs allow greater flexibility for accepting
different input ranges. Figure 10b shows a circuit that
converts a 0V to 5V analog input signal with only an
additional buffer that is not in the signal path.
Full-Scale and Offset Adjustment
Figure 11a shows the ideal input/output characteristics
for the LTC1419. The code transitions occur midway
between successive integer LSB values (i.e., – FS +
0.5LSB, – FS + 1.5LSB, – FS + 2.5LSB,... FS – 1.5LSB,
FS – 0.5LSB). The output is two’s complement binary with
1LSB = FS – (– FS)/16384 = 5V/16384 = 305.2
V.
In applications where absolute accuracy is important,
offset and full-scale errors can be adjusted to zero. Offset
error must be adjusted before full-scale error. Figure 11b
shows the extra components required for full-scale error
adjustment. Zero offset is achieved by adjusting the offset
ANALOG
INPUT
1419 F11b
1
2
3
R4
100
R7
50k
R3
24k
5V
R6
24k
R8
50k
R5
47k
4
5
0.1
F
10
F
+AIN
–AIN
VREF
REFCOMP
AGND
LTC1419
+
Figure 11b. Offset and Full-Scale Adjust Circuit
Figure 10a. CMRR vs Input Frequency
Figure 11a. LTC1419 Transfer Characteristics
1419 F11a
011...111
011...110
000...001
000...000
111...111
111...110
100...001
100...000
FS – 1LSB
– (FS – 1LSB)
INPUT VOLTAGE [+AIN – (–AIN)]
OUTPUT
CODE
INPUT FREQUENCY (Hz)
1
COMMON
MODE
REJECTION
(dB)
80
70
60
50
40
30
20
10
0
10
100
1419 G09
1000
10000
LTC1419
+AIN
ANALOG INPUT
–AIN
VREF
0V TO
5V
REFCOMP
AGND
1419 F10
1
2
3
±2.5V
4
5
10
F
–
+