LTC1865L datasheet(11/16 Pages) LINER

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LTC2453

2453f

ﬁrst to the decoupling capacitors described above, the

second to the ground return for the input signal source,

and the third to the ground return for the power supply

voltage source.

Driving REF+ and REF–

A simpliﬁed equivalent circuit for REF+ and REF– is shown

in Figure 8. Like all other A/D converters, the LTC2453 is

only as accurate as the reference it is using. Therefore, it

is important to keep the reference line quiet by careful low

and high frequency power supply decoupling.

The LT6660 reference is an ideal match for driving the

LTC2453’s REF+ pin. The LTC6660 is available in a 2mm

2mm DFN package with 2.5V, 3V, 3.3V and 5V options.

A 0.1μF, high quality, ceramic capacitor in parallel with

a 10μF ceramic capacitor should be connected between

the REF+/REF– and GND pins, as close as possible to the

RSW

15k

(TYP)

ILEAK

VCC

CEQ

0.35pF

(TYP)

IN+

IN–

REF–

REF+

2453 F08

RSW

15k

(TYP)

ILEAK

RSW

15k

(TYP)

ILEAK

RSW

15k

(TYP)

ILEAK

Figure 8. LTC2453 Analog Input/Reference Equivalent Circuit

ILEAK

RSW

15k

(TYP)

ICONV

CIN

IN+

VCC

SIG+

SIG–

CEQ

0.35pF

(TYP)

CPAR

–

2453 F09

ILEAK

RSW

15k

(TYP)

ICONV

CIN

IN–

VCC

CEQ

0.35pF

(TYP)

CPAR

–

Figure 9. LTC2453 Input Drive Equivalent Circuit

package. The 0.1μF capacitor should be placed closest

to the ADC.

Driving VIN+ and VIN–

The input drive requirements can best be analyzed using

the equivalent circuit of Figure 9. The input signal VSIG is

connected to the ADC input pins (IN+ and IN–) through an

equivalent source resistance RS.Thisresistorincludesboth

the actual generator source resistance and any additional

optional resistors connected to the input pins. Optional

input capacitors CIN are also connected to the ADC input

pins. This capacitor is placed in parallel with the ADC

input parasitic capacitance CPAR. Depending on the PCB

layout, CPAR has typical values between 2pF and 15pF. In

addition, the equivalent circuit of Figure 9 includes the

converter equivalent internal resistor RSW and sampling

capacitor CEQ.

There are some immediate trade-offs in RS and CIN without

needing a full circuit analysis. Increasing RS and CIN can

give the following beneﬁts:

1) Due to the LTC2453’s input sampling algorithm, the

input current drawn by either VIN+ or VIN– over a con-

version cycle is 50nA. A high RS • CIN attenuates the

high frequency components of the input current, and

RS values up to 1k result in <1LSB error.

APPLICATIONS INFORMATION

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