LTC2451
13
2451fg
It is also desirable to avoid any via in the circuit path,
starting from the converter VCCpin,passingthroughthese
two decoupling capacitors, and returning to the converter
GND pin. The area encompassed by this circuit path, as
well as the path length, should be minimized.
Very low impedance ground and power planes, and star
connections at both VCC and GND pins, are preferable. The
VCC pin should have three distinct connections: the 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 10. Like all other A/D converters, the LTC2451
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
LTC2451’s REF+ pin. The LTC6660 is available in a 2mm
× 2mm DFN package with 2.5V, 3V, 3.3V and 5V options.
Figure 10. LTC2451 Analog Input and
Reference Pins Equivalent Circuit
A 0.1F, high quality, ceramic capacitor in parallel with
a 10F ceramic capacitor should be connected between
the REF+/REF– and GND pins, as close as possible to the
package. The 0.1F capacitor should be placed closest
to the ADC.
Driving IN
The input drive requirements can best be analyzed using
the equivalent circuit of Figure 11. The input signal, VSIG,is
connected to the ADC input pin (IN) through an equivalent
sourceresistanceRS.Thisresistorincludesboththeactual
generator source resistance and any additional optional
resistors connected to the input pin. An optional input
capacitor, CIN, is also connected between the ADC input
pin and GND. This capacitor is placed in parallel with the
ADC input parasitic capacitance, CPAR. Depending on the
PCB layout, CPARhastypicalvaluesbetween2pFand15pF.
In addition, the equivalent circuit of Figure 11 includes the
converter equivalent internal resistor, RSW, and sampling
capacitor, CEQ.
APPLICATIONS INFORMATION
VCC
CEQ
0.35pF
(TYP)
REF+
IN
REF–
2451 F10
RSW
15k
(TYP)
ILEAK
RSW
15k
(TYP)
ILEAK
RSW
15k
(TYP)
ILEAK
Figure 11. LTC2451 Input Drive Equivalent Circuit
VSIG
2451 F11
ILEAK
RSW
15k
(TYP)
ICONV
CIN
IN
VCC
RS
CEQ
0.35pF
(TYP)
CPAR
+–