LTC6405CUD-PBF datasheet(21/24 Pages) LINER

21 / 24 page

LTC6405

6405fa

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

–

SHDN

–IN

+OUT

+OUTF

+IN

–OUT

–OUTF

+INA

–INA

200Ω

V+

V–

V+

V–

VOCM

V–

V+

V–

6405 F12

LTC6405

LTC2208

VIN, 2VP-P

SHDN

200Ω

100Ω

200Ω

0.1μF

20Ω

0.1μF

CONTROL

GND VDD

VCM

D15

•

0.1μF

4.7pF

3.3V

1μF

VTIP

1.8pF

1.25pF

50Ω

2.2μF

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