AFE5804 datasheet(42/60 Pages) TI

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CLOCK, REFERENCE, AND DATA OUTPUT MODES

INPUT CLOCK

EXTERNAL REFERENCE

VREFT=1.5V+

VCM

1.5V

(1)

VREFB=1.5V -

VCM

1.5V

(2)

AFE5804

SBOS442A – JUNE 2008 – REVISED SEPTEMBER 2008 ................................................................................................................................................ www.ti.com

ADDRESS

IN HEX

D15

D14

D13

D12

D11

D10

NAME

DIFF_CLK

EN_DCC

EXT_REF_VCM

PHASE_DDR<1:0>

BTC_MODE

MSB_FIRST

EN_SDR

FALL_SDR

The AFE5804 is configured by default to operate with a single-ended input clock; CLKP is driven by a CMOS

clock and CLKM is tied to '0'. However, by programming DIFF_CLK to '1', the device can be made to work with a

differential input clock on CLKP and CLKM. Operating with a low-jitter differential clock generally provides better

SNR performance, especially at input frequencies greater than 30MHz.

In cases where the duty cycle of the input clock falls outside the 45% to 55% range, it is recommended to enable

an internal duty cycle correction circuit. Enable this circuit by setting the EN_DCC bit to '1'.

The AFE5804 can be made to operate in external reference mode by pulling the INT/EXT pin to '0'. In this mode,

the REFT and REFB pins should be driven with voltage levels of 2.5V and 0.5V, respectively, and must have

enough drive strength to drive the switched capacitance loading of the reference voltages by each ADC. The

advantage of using the external reference mode is that multiple AFE5804 units can be made to operate with the

same external reference, thereby improving parameters such as gain matching across devices. However, in

applications that do not have an available high drive, differential external reference, the AFE5804 can still be

driven with a single external reference voltage on the VCM pin. When EXT_REF_VCM is set as '1' (and the

INT/EXT pin is set to '0'), the VCM pin is configured as an input pin, and the voltages on REFT and REFB are

generated as shown in Equation 1 and Equation 2.

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