LTC1402
U
W U U
APPLICATIONS INFORMATION
0
all the internal references. When AGND2 (Pin 6) is tied to
the external ground plane, it sources 2.7mA ±30% typi-
cally; approximately 2mA are sourced through an internal
equivalent 2k resistance tied to the VREF (Pin 5) at 4.096V
and the remaining 0.7mA supply the internal reference
ground. The VREF (Pin 5) equivalent input resistance is the
same 2k tied to AGND2 (Pin 6). When you bus a common
reference voltage to several LTC1402 ADCs, you need to
keep PC board track resistance low to avoid reference
voltage attenuation at each ADC. For example, 0.5Ω of
track resistance to Pins 5 or 6 causes 0.025% of reference
voltage and input range reduction. Figure 8 shows op-
tional buffer amplifiers at each ADC to eliminate resistive
voltagedropsfromthecommonexternalreferencetoeach
ADC. Figure 8 shows 10µF bypass capacitors tied to the
common analog ground plane, at VREF (Pin 5) and AGND2
(Pin 6), wired closely to each ADC to eliminate crosstalk of
internalADC glitchcurrentsfromoneADCtoanother. The
10µF bypass capacitors are recommended whether you
drive Pins 5 and 6 with amplifiers, or with copper traces
–10
–20
–30
–40
–50
–60
–70
0.1
1
10
100
1000
FREQUENCY (MHz)
1402 F07
Figure 7. CMRR vs Input Frequency
INPUT SPAN VERSUS REFERENCE VOLTAGE
The differential input range has a voltage span that equals
the difference between the voltage at the reference buffer
output VREF at Pin 5, and the voltage at the reference
ground AGND2 at Pin 6. The external reference voltage
may have any value between 2V and 5V. The internal ADC
is referenced to these two points. If you use an external
reference, tie the GAIN (Pin 7) to AVDD (Pin 1) to disable
the internal reference, and connect the external reference
between VREF (Pin 5) and AGND2 (Pin 6).
3
2
5
6
7
+
A
A
V
IN
ANALOG
INPUTS
1/2 LT1368CS8
5V
–
IN
6
8
If you cut the reference voltage in half by halving the gain
of the reference buffer with the GAIN (Pin 7) tied to VREF
(Pin 5), the input span also cuts in half. In bipolar mode,
the differential input range changes from ±2.048V to
±1.024V, when the reference is cut in half. In unipolar
mode, the differential input range changes from 0V-
4.096V to 0V-2.048V, for the same reference cut in half.
Note that in both unipolar and bipolar modes, the input
range pivots around 0V with changing reference voltage.
AGND2 (Pin 6) has no direct effect on the ADC offset
voltage, it only affects input voltage span. Any external
offsetting voltages must be applied through the AIN+ and
AIN– inputs, as shown in Figure 10b.
–
+
7
LTC1402
5
REF
10µF
1/2 LT1368CS8
2
AGND2
GAIN
–
+
1
3
4
10µF
–5V
5V
•
•
•
4.096V
3
2
5
6
7
+
A
A
V
IN
5V
ANALOG
INPUTS
1/2 LT1368CS8
5V
–
10k
1k
IN
6
8
–
+
7
LTC1402
5
0.1µF
10µF
REF
1/2 LT1368CS8
SEVERAL LTC1402 ADCs MAY SHARE ONE
EXTERNAL REFERENCE
2
AGND2
GAIN
–
+
1
3
LT1634AI-4.096
10µF
4
Figure 8 shows how several ADCs can share a single
common external reference. The VREF (Pin 5) and AGND2
(Pin 6) of several LTC1402 ADCs can be tied together to
share the same external reference in a data acquisition
system. Tie GAIN (Pin 7) to AVDD at each ADC to disable
–5V
1402 F08
5V
Figure 8. Several LTC1402 ADCs Can Share a Single
External Reference
13
Linear [ Linear ]
