ADS5102

ADS5103

SLAS351B – OCTOBER 2001 – REVISED DECEMBER 2001

17

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PRINCIPLES OF OPERATION

100 k

Ω

BG

1

µF

AVDD

Figure 30. BG Reference Configuration

For systems that require more absolute accuracy or lower temperature coefficient drift than provided by the

internal VREF, an external voltage reference can be applied to the VREFB and VREFT inputs. To use external

reference, connect the PDREF pin to a logic high and this internally disconnects the VREF from the ADC. In

this mode it is also necessary to connect the BG and REFT pins together on the PWB. It is recommended to

use the input levels of VREFB = 0.75 V and VREFB = 1.25 V to achieve optimum ADC performance. It is also

recommended to apply a common-mode voltage to the input of 1 V.

clock input

V. Since both edges of the clock are used in the switch capacitor architecture, it is important to provide a clock

with (ideally) a 50% duty cycle. The performance variation with clock duty cycle can be examined from

Figures 25, 26, 27 and 28.

Clock jitter is also important for performance of the ADC to be maintained. Any clock jitter appears as noise when

sampling input frequencies. Clock Jitter reduces the signal to noise ratio (SNR) and is more severe as the input

frequency increases. The theoretical SNR limits based on clock jitter can be calculated as follows:

Theoretical SNR

(clock jitter)

(dB)

+ 20 log

1

2

p

F

I

CLK

(jitter)

Where:

Therefore for a Nyquist frequency input of 32.5 MHz and a design trying to achieve the most available

performance from the ADS5102/3, the clock jitter must be less than 3.98 ps rms. In under sampling applications,

the same equations apply and clock jitter becomes more critical and may be the limiting factor in system

performance. The aperture jitter of the SHA also contributes to overall jitter. For worst case designs, the jitter

of clock and aperture can be considered to add in quadrature, i.e.

The aperture jitter of the ADS5103 is 2 ps rms and at frequencies approaching Nyquist, the total jitter should

be accounted for.