LTC2412 Linear Technology, LTC2412 Datasheet - Page 23

• Manufacturer Part Number: LTC2412
• Description: 2-Channel Differential Input 24-Bit No Latency DS ADC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Parallel termination near the LTC2412 pin will eliminate
this problem but will increase the driver power dissipation.
A series resistor between 27 and 56 placed near the
driver or near the LTC2412 pin will also eliminate this
problem without additional power dissipation. The actual
resistor value depends upon the trace impedance and
connection topology.
An alternate solution is to reduce the edge rate of the
control signals. It should be noted that using very slow
edges will increase the converter power supply current
during the transition time. The multiple ground pins used
in this package configuration, as well as the differential
input and reference architecture, reduce substantially the
converter’s sensitivity to ground currents.
Particular attention must be given to the connection of the
F
conversion clock. This clock is active during the conver-
sion time and the normal mode rejection provided by the
internal digital filter is not very high at this frequency. A
normal mode signal of this frequency at the converter
reference terminals may result into DC gain and INL
errors. A normal mode signal of this frequency at the
converter input terminals may result into a DC offset error.
O
signal when the LTC2412 is used with an external
SWITCHING FREQUENCY
f
f
SW
SW
V
V
I
V
V
I
REF
REF
I
I
REF
REF
IN
IN
IN
IN
= 76800Hz INTERNAL OSCILLATOR (F
= 0.5 • f
+
+
–
–
+
+
–
–
U
V
V
EOSC
CC
CC
V
V
U
EXTERNAL OSCILLATOR
I
I
I
I
LEAK
LEAK
LEAK
LEAK
CC
CC
I
I
I
I
LEAK
LEAK
LEAK
LEAK
R
R
R
R
W
SW
SW
SW
SW
Figure 11. LTC2412 Equivalent Analog Input Circuit
20k
20k
20k
20k
(TYP)
(TYP)
(TYP)
(TYP)
O
= LOW OR HIGH)
U
2412 F11
C
18pF
(TYP)
EQ
Such perturbations may occur due to asymmetric capaci-
tive coupling between the F
input and/or reference connection traces. An immediate
solution is to maintain maximum possible separation
between the F
nals. When the F
converter, substantial AC current is flowing in the loop
formed by the F
ground return path. Thus, perturbation signals may be
inductively coupled into the converter input and/or refer-
ence. In this situation, the user must reduce to a minimum
the loop area for the F
the differential input and reference connections.
Driving the Input and Reference
The input and reference pins of the LTC2412 converter are
directly connected to a network of sampling capacitors.
Depending upon the relation between the differential input
voltage and the differential reference voltage, these ca-
pacitors are switching between these four pins transfering
small amounts of charge in the process. A simplified
equivalent circuit is shown in Figure 11, where IN
refer to the selected differential channel and the unselected
channel is omitted for simplicity.
I IN
I IN
I REF
I REF
R
R
R
where
V
V
V
V
IN
INCM
REF
REFCM
EQ
EQ
EQ
IN
AVG
AVG
: :
3 61
4 32
REF
0 555 10
AVG
.
.
AVG
.
IN
M
M
REF
IN
V
IN
V
2
1 5
IN
REF
.
INTERNAL OSCILLATOR
1 5
IN
INTERNAL OSCILLATOR
.
2
12
V
0 5
INCM
0 5
V
REF
.
V
O
REF
INCM
.
V
/
O
REF
f
EOSC
signal trace and the input/reference sig-
R
0 5
connection trace, the termination and the
R
0 5
EQ
.
O
V
EQ
.
V
REFCM
INCM
V
EXTERNAL OSCILLATOR
V
R
signal is parallel terminated near the
REFCM
INCM
R
EQ
EQ
O
V
signal as well as the loop area for
REFCM
V
REFCM
O
60
50
signal trace and the converter
Hz Notch F
Hz Notch F
V
REF
V
REF
V
IN
2
V
R
IN
2
EQ
R
EQ
O
O
LOW
HIGH
LTC2412
+
23
and IN
2412f
–