LTC6601-2 datasheet(21/40 Pages) LINER

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LTC6601-2

66012f

APPLICATIONS INFORMATION

INTERFACING THE LTC6601 TO ADC CONVERTERS

The LTC6601’s rail-to-rail differential output and adjustable

output common mode voltage make the LTC6601 ideal

for interfacing to low voltage, single supply, differential

input ADCs. The sampling process of ADCs creates a

sampling transient that is caused by the switching-in

of the ADC sampling capacitor. The switching-in of this

sampling capacitor momentarily “shorts” the output of the

ampliﬁer as charge is transferred between ampliﬁer and

sampling capacitor. The ampliﬁer must recover and settle

from this load transient before this acquisition period has

ended, for a valid representation of the input signal. The

LTC6601 will settle much more quickly from these peri-

odic load impulses than it does from a 2V input step, but

it is a good idea to add an RC network after the outputs

of the LTC6601 to decouple the sampling transient of the

ADC (See Figure 6). The capacitance of the decoupling

network serves to provide the bulk of the charge during

the sampling process, while the two resistors of the ﬁlter

network are used to dampen and attenuate any transient

induced by the ADC. The ADC’s sampling bandwidth will

often be much greater than that of the LTC6601, so hav-

ing this discrete RC ﬁlter will give the additional beneﬁt

of band limiting broadband output noise.

The selection of the RC time constant is trial and error

for a given ADC, but the following guidelines are recom-

mended. Choose an RC pole frequency greater than the

cutoff frequency of the LTC6601. 80MHz RC ﬁlters are

good for ﬁltering broadband noise. Lower frequency RC

ﬁlters improve SNR at the expense of settling time. The

resistors in the decoupling network should be at least 25Ω.

Too much resistance in the decoupling network leaves

insufﬁcient settling time and will create a voltage divider

between the dynamic input impedance of the ADC and the

decoupling resistors. Using insufﬁcient resistance might

prevent proper dampening of the load transient caused by

the sampling process, and prolong the time required for

settling. In 16-bit applications, this will typically require

a minimum of 11 RC time constants. It is recommended

that the capacitor is chosen with low dielectric absorption

(such as a C0G multilayer ceramic capacitor).

–

0.1μF

VOUT

–

VOUT

10nF

BIAS

1μF

VOCM

66012 F06

t = R • (C1 + 2 • C2)

2.2μF

1μF

3.3V

D15

•

VIN

–

AIN

–

VCM

GND

CONTROL

LTC6601-2

1μF

Figure 6. Interfacing the LTC6601 to A/D Converters

LTC6601-2 Datasheet(PDF) 21 Page - Linear Technology

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