LTC1402 Linear Technology, LTC1402 Datasheet - Page 12

• Manufacturer Part Number: LTC1402
• Description: Serial 12-Bit/ 2.2Msps Sampling ADC with Shutdown
• Manufacturer: Linear Technology

APPLICATIONS
LTC1402
INTERNAL REFERENCE
The LTC1402 has an on-chip, temperature compensated,
curvature corrected, bandgap reference that is factory
trimmed to 2.048V. It is connected internally to a reference
amplifier, see Figure 4. The reference amplifier amplifies
the voltage at the V
internal reference voltage of 4.096V. This provides buffer-
ing for the high speed capacitive DAC. The reference
amplifier output V
capacitor to ground. The reference amplifier is stable with
capacitors of 1 F or greater. For the best noise perfor-
mance, a 10 F ceramic or a 10 F tantalum in parallel with
a 0.1 F ceramic is recommended.
The V
shown in Figure 5a. The GAIN pin (Pin 7) is tied to the
positive supply to disable the internal reference buffer.
A DAC may also be used to drive V
Figure 6. This is useful in applications where the peak
input signal amplitude may vary. The input span of the
12
Figure 5a. Using the LT1019-2.5 as an External Reference
REF
4.096V
LT1019-2.5
pin can be driven with an external reference as
V
5V
10 F
Figure 4. LTC1402 Reference Circuit
IN
5
7
6
V
OUT
V
GAIN
AGND2
LTC1402
REF
REF
U
REF
, (Pin 5) must be bypassed with a
64k
ANALOG INPUT
pin by 2 to create the required
INFORMATION
REFERENCE
U
64k
5V
AMP
10 F
+
–
2.048V
W
3
4
5
6
7
A
A
V
AGND2
GAIN
REF
IN
IN
REF
REFERENCE
+
–
BANGAP
LTC1402
as shown in
1402 F04
U
1402 F04a
ADC can then be adjusted to match the peak input signal,
maximizing the signal-to-noise ratio. The filtering of the
internal LTC1402 reference amplifier will limit the band-
width and settling time of this circuit. A settling time of
5ms should be allowed after a reference adjustment.
DIFFERENTIAL INPUTS
The LTC1402 has a unique differential sample-and-hold
circuit that allows inputs from –2.5V to 5V. The ADC will
always convert the difference of A
of the common mode voltage. The common mode rejec-
tion holds up at extremely high frequencies, see Figure 7.
The only requirement is that both inputs not exceed
– 2.5V or 5V. Integral nonlinearity errors (INL) and differ-
ential nonlinearity 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. Figure 5b shows the
use of bipolar mode with single 5V supply.
LT1019-2.5
Figure 5b. Bipolar Mode with Single Supply
V
5V
IN
LTC1450
Figure 6. Driving V
2.5V 2.048V
ANALOG INPUT
10 F
5V
10 F
V
10 F
IN
REF
IN
with a DAC
3
4
5
6
7
+
3
4
5
6
7
A
A
V
AGND2
GAIN
– A
IN
IN
REF
A
A
V
AGND2
GAIN
+
–
IN
IN
REF
LTC1402
+
–
IN
LTC1402
–
independent
1402 F04a
V
BIP
SS
1402 F06
14