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SBAS308A− MAY 2004 − REVISED MARCH 2005
REFERENCE CIRCUIT
CM
CM
The ADS5542 has built-in internal reference
generation, requiring no external circuitry on the printed
circuit board (PCB). For optimum performance, it is best
to connect both REFP and REFM to ground with a 1µF
decoupling capacitor in series with a 1Ω resistor, as
shown in Figure 41. In addition, an external 56.2kΩ
resistor should be connected from IREF (pin 31) to
AGND to set the proper current for the operation of the
ADC, as shown in Figure 41. No capacitor should be
connected between pin 31 and ground; only the 56.2kΩ
resistor should be used.
Ω
Ω
5k
5k
CLKP
CLKM
6pF
3pF
3pF
Ω
1
REFP
REFM
29
30
Figure 42. Clock Inputs
µ
1 F
Ω
1
When driven with a single-ended CMOS clock input, it
is best to connect CLKM (pin 11) to ground with a
0.01µF capacitor, while CLKP is AC-coupled with a
0.01µF capacitor to the clock source, as shown in
Figure 43.
µ
1 F
31 IREF
56kΩ
µ
0.01 F
Square Wave
or Sine Wave
CLKP
ADS5542
CLKM
Figure 41. REFP, REFM, and IREF Connections
for Optimum Performance
(3VPP
)
CLOCK INPUT
µ
0.01 F
The ADS5542 clock input can be driven with either a
differential clock signal or a single-ended clock input,
with little or no difference in performance between both
configurations. The common-mode voltage of the clock
inputs is set internally to CM (pin 17) using internal 5kΩ
resistors that connect CLKP (pin 10) and CLKM (pin 11)
to CM (pin 17), as shown in Figure 42.
Figure 43. AC-Coupled, Single-Ended Clock Input
The ADS5542 clock input can also be driven
differentially, reducing susceptibility to common-mode
noise. In this case, it is best to connect both clock inputs
to the differential input clock signal with 0.01µF
capacitors, as shown in Figure 44.
24
BB [ BURR-BROWN CORPORATION ]
