Datasheet
LTC6405
21
6405fb
For more information www.linear.com/6405
Interfacing the LTC6405 to A/D Converters
Rail-to-rail input and fast settling time make the LTC6405
ideal for interfacing to low voltage, single supply, differ-
ential input ADCs. The sampling process of ADCs create
a sampling glitch caused by switching in the sampling
capacitor on the ADC front end which momentarily “shorts”
the output of the amplifier as charge is transferred between
the amplifier and the sampling capacitor. The amplifier
must recover and settle from this load transient before
this acquisition period ends for a valid representation of
the input signal. In general, the LTC6405 will settle much
more quickly from these periodic load impulses than from
a 2V input step, but it is a good idea to place an R-C filter
network between the differential outputs of the LTC6405
and the input of the ADC to help absorb the charge injection
that comes out of the ADC from the sampling process.
The capacitance of the filter network serves as a charge
reservoir to provide high frequency charging during the
sampling process, while the resistors of the filter network
are used to dampen and attenuate any charge kickback
from the ADC. The selection of
the R-C time constant is
trial
and error for a given ADC, but the following guidelines
are recommended: Choosing too large of a resistor in the
decoupling network leaving insufficient settling time will
create a voltage divider between the dynamic input imped-
ance of the ADC and the decoupling resistors. Choosing
too small of a resistor will possibly prevent the resistor
from properly dampening the load transient caused by
the sampling process, prolonging the time required for
settling. In 16-bit applications, this will typically require
a minimum of 11 R-C time constants. It is recommended
that the capacitor chosen have a high quality dielectric
(such as C0G multilayer ceramic).
applications inForMation
Figure 12. Interfacing the LTC6405 to an ADC
0.1µF
–
+
1
SHDN
5 6
–IN
7
+OUT
8
+OUTF
16 15
+INNC
14
–OUT
13
–OUTF
+INA
–INA
200Ω
2
V
+
3
V
–
V
+
V
–
5V
V
OCM
V
OCM
12
V
–
11
V
+
10
V
+
9
V
–
V
–
V
–
6405 F12
LTC6405
LTC2208
V
IN
, 2V
P-P
SHDN
200Ω200Ω
100Ω
200Ω
0.1µF
20Ω
20Ω
5V
4
0.1µF
0.1µF
CONTROL
GND
V
DD
V
CM
D15
•
•
D0
0.1µF
4.7pF
4.7pF
4.7pF
3.3V
1µF
1µF
V
TIP
1.8pF
1.8pF
1.25pF
1.25pF
1.25pF
50Ω
50Ω
2.2µF