LTC6405 Linear Technology, LTC6405 Datasheet - Page 21

• Manufacturer Part Number: LTC6405
• Description: Rail-to-Rail Input Differential Amplifier/Driver
• Manufacturer: Linear Technology

www.DataSheet4U.com
APPLICATIONS INFORMATION
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 amplifi er as charge is transferred between
the amplifi er and the sampling capacitor. The amplifi er
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 fi lter
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.
0.1μF
0.1μF
5V
V
IN
, 2V
0.1μF
1
2
3
4
SHDN
V
V
V
P-P
+
–
OCM
16
V
5
SHDN
–
200Ω
200Ω
NC
V
TIP
15
6
+IN
–IN
200Ω
200Ω
1.8pF
+
–
1.8pF
V
OCM
14
7
–OUT
+OUT
1.25pF
50Ω
50Ω
Figure 12. Interfacing the LTC6405 to an ADC
13
8
1.25pF
1.25pF
–OUTF
+OUTF
V
+
LTC6405
V
V
–
–
6405 F12
V
V
V
V
–
+
+
–
12
11
10
20Ω
9
20Ω
The capacitance of the fi lter network serves as a charge
reservoir to provide high frequency charging during the
sampling process, while the resistors of the fi lter 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 insuffi cient 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).
0.1μF
5V
0.1μF
4.7pF
4.7pF
4.7pF
2.2μF
–INA
+INA
V
CM
LTC2208
100Ω
GND V
CONTROL
DD
LTC6405
D15
D0
1μF
•
•
1μF
21
3.3V
6405fa