CS5516 datasheet(24/41 Pages) CIRRUS

24 / 41 page

pling is shown on the system connection dia-

grams.

If absolute measurements are to be made by the

CS5516/20, then a precision reference should be

input into the VREF+ and VREF- terminals.

Clock Generator

The CS5516/20 includes a gate which can be

connected as a crystal oscillator to provide the

master clock to run the chip. Alternatively, an

external (CMOS compatible) clock can be input

into the XIN pin. Figure 10 illustrates a simple

model for the on-chip gate oscillator. The on-

chip oscillator is designed to typically operate

with crystal frequencies between 4.0 and 5.0

MHz without additional loading capacitors.

other crystal frequencies, or if ceramic resona-

tors are used, additional loading capacitance may

be necessary.

The XOUT pin can be used to drive one CMOS

gate for system clock requirements. Be sure to

include the gate’s input capacitance and stray ca-

pacitance as part of the loading capacitance for

the resonating element.

Digital Filter

The CS5516/20 is optimized to operate with clock

frequencies of 4.096 MHz or 4.9152 MHz. These

result in the filter having a 3dB bandwidth of 12

Hz or 15 Hz, with output word rates of 50 or

60 Sps. The rejection at 50Hz

± 3Hz is 70 dB mini-

mum with a 4.096 MHz clock. Similar rejection is

obtained at 60 Hz with a 4.9152 MHz clock.

The digital filter has a deep notch in its transfer

function at 50 Hz (XIN = 4.096 MHz) or 60 Hz

(XIN = 4.9152 MHz) but other XIN frequencies

can be used. The filter transfer function will

scale proportionally. Figure 11 shows the trans-

fer function of the filter when operated at three

different frequencies. With a 3.579 MHz XIN,

the filter offers greater than 90 dB rejection of

both 50 and 60 Hz.

The output word rate of the converter scales

with the XIN clock rate and is set by the ratio of

XIN/81,920; or 50 Sps for XIN = 4.096 MHz. If

very narrow signal bandwidths, such as 3 Hz,

are desired, averaging of the output words is rec-

ommended.

>1M

To Internal circuitry

400

XOUT

5pF

1pF

400

External XTAL

1pF

5pF

XIN

≅ 2000 umhos

Figure 10. On-Chip Gate Oscillator Model

Input Frequency (Hz)

-160

-140

-120

-100

-80

-60

-40

-20

(1) XIN = 3.579 MHz

(2) XIN = 4.096 MHz

(3) XIN = 4.915 MHz

21.8

43.7

87.3

100

120

131.0

150

180

174.7

200

240

218.5

250

300

Figure 11. Filter Magnitude Response

Input Frequency (Hz)

-180

-150

-120

-90

-60

-30

120

150

180

XIN = 4.096 MHz

Figure 12. Filter Phase Response.

CS5516, CS5520

DS74F1

DS74F2

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