ADS5500IPAP datasheet(25/37 Pages) TI2

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REFERENCE CIRCUIT

CLOCK INPUT

ADS5500

SBAS303F – DECEMBER 2003 – REVISED FEBRUARY 2007

Using the serial interface PDN bit to power down the

device places the outputs in a high-impedance state

and only the internal reference remains on to reduce

the power-up time. The power-down mode reduces

power dissipation to approximately 180 mW.

The

ADS5500

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 1-

µF decoupling capacitor

(the 1-

Ω series resistor shown in Figure 45 is

optional). In addition, an external 56.2-k

Ω 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 45. No capacitor should be

Figure 46. Clock Inputs

connected between pin 31 and ground; only the

56.2-k

Ω resistor should be used.

When driven with a single-ended CMOS clock input,

it is best to connect CLKM (pin 11) to ground with a

0.01-

µF capacitor, while CLKP is ac-coupled with a

0.01-

µF capacitor to the clock source, as shown in

Figure 47.

Figure 45. REFP, REFM, and IREF Connections

Figure 47. AC-Coupled, Single-Ended Clock Input

for Optimum Performance

The ADS5500 clock input can also be driven

differentially,

reducing

susceptibility

common-mode noise. In this case, it is best to

The ADS5500 clock input can be driven with either a

connect both clock inputs to the differential input

differential clock signal or a single-ended clock input,

clock signal with 0.01-

µF capacitors, as shown in

with little or no difference in performance between

Figure 48.

both configurations. The common-mode voltage of

the clock inputs is set internally to CM (pin 17) using

internal 5-k

Ω resistors that connect CLKP (pin 10)

and CLKM (pin 11) to CM (pin 17), as shown in

Figure 46.

Figure 48. AC-Coupled, Differential Clock Input

For

high

input

frequency

sampling,

recommended to use a clock source with very low

jitter. Additionally, the internal ADC core uses both

edges of the clock for the conversion process. This

means that, ideally, a 50% duty cycle should be

provided. Figure 49 shows the performance variation

of the ADC versus clock duty cycle.

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