LTC1402
U
W U U
APPLICATIONS INFORMATION
INTERNAL REFERENCE
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.
The LTC1402 has an on-chip, temperature compensated,
curvature corrected, bandgap reference that is factory
trimmedto2.048V.Itisconnectedinternallytoareference
amplifier, see Figure 4. The reference amplifier amplifies
the voltage at the VREF pin by 2 to create the required
internal reference voltage of 4.096V. This provides buffer-
ing for the high speed capacitive DAC. The reference
amplifier output VREF, (Pin 5) must be bypassed with a
capacitor to ground. The reference amplifier is stable with
capacitors of 1µF or greater. For the best noise perfor-
mance, a 10µF ceramic or a 10µF tantalum in parallel with
a 0.1µF ceramic is recommended.
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 AIN+ – AIN independent
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.
The VREF pin can be driven with an external reference as
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 VREF as shown in
Figure 6. This is useful in applications where the peak
input signal amplitude may vary. The input span of the
5V
3
4
5
6
7
V
IN
2.5V ±2.048V
+
–
LTC1402
A
A
V
IN
2.048V
BANGAP
+
REFERENCE
V
IN
5
V
REF
4.096V
2.5V
REFERENCE
IN
AMP
LT1019-2.5
–
10µF
LTC1402
8
5V
BIP
REF
10µF
64k
10µF
64k
14
GAIN
7
6
AGND2
GAIN
V
SS
AGND2
1402 F04
1402 F04a
Figure 4. LTC1402 Reference Circuit
Figure 5b. Bipolar Mode with Single Supply
5V
3
+
3
+
A
A
IN
IN
ANALOG INPUT
ANALOG INPUT
V
IN
4
–
4
–
A
A
IN
IN
LT1019-2.5
V
LTC1402
LTC1402
5
5
6
7
V
LTC1451
V
OUT
REF
REF
10µF
10µF
6
7
AGND2
GAIN
AGND2
GAIN
5V
5V
1402 F04a
1402 F06
Figure 6. Driving VREF with a 12 Bit, VOUT DAC
Figure 5a. Using the LT1019-2.5 as an External Reference
12
Linear [ Linear ]
