ADS7229IPWR datasheet(24/46 Pages) TI

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REFERENCE

CONVERTER OPERATION

OSC

Divider

1/2

= 1

= 0

Conversion Clock

(CCLK)

CFR_D10

SPI Serial

Clock (SCLK)

Manual Channel Select Mode

ADS7229

ADS7230

SBAS437 – MAY 2008........................................................................................................................................................................................................ www.ti.com

The ADS7229/30 must operate with an external reference with a range from 0.3V to 5V. A clean, low-noise,

well-decoupled reference voltage on this pin is required to ensure good converter performance. A low-noise

bandgap reference such as the REF5040 can be used to drive this pin. A 22

µF ceramic decoupling capacitor is

required between the REF+ and REF– pins of the converter. These capacitors should be placed as close as

possible to the device pins. REF– should be connected to its own via to the analog ground plane with the

shortest possible distance. A series resistor between the reference and the REF50xx is neither required

(because the REF50xx is capable of driving a 22

µF capacitor while maintaining stability) nor recommended (due

to additional nonlinearity); see also Figure 67.

The ADS7229/30 has an oscillator that is used as an internal clock that controls the conversion rate. The

frequency of this clock is 21MHz minimum. The oscillator is always on unless the device is in the deep

power-down state or the device is programmed for using SCLK as the conversion clock (CCLK). The minimum

acquisition (sampling) time takes 3 CCLKs (equivalent to 143ns at 21MHz) and the conversion time takes 18

conversion clocks (CCLK) or approximately 857ns at 21MHz to complete one conversion.

The conversion can also be programmed to run based on the external serial clock, SCLK. This option allows a

system designer to achieve system synchronization. The serial clock SCLK, is first reduced to 1/2 of its

frequency before it is used as the conversion clock (CCLK). For example, with a 42MHz SCLK, this reduction

provides a 21MHz clock for conversions. If it is desired to start a conversion at a specific rising edge of SCLK

when the external SCLK is programmed as the source of the conversion clock (and manual conversion start is

selected), the setup time between CONVST and that rising SCLK edge should be observed. This configuration

ensures that the conversion is complete in 18 CCLKs (or 36 SCLKs). The minimum setup time is 20ns to ensure

synchronization between CONVST and SCLK. In many cases, the conversion can start one SCLK period (or

CCLK) later, which results in a conversion of 19 CCLKs (or 37 SCLKs). The 20ns setup time is not required if the

synchronization is not critical in the application.

The duty cycle of SCLK is not critical as long as it meets the minimum high and low time requirements of 8ns.

The ADS7229/30 is designed for high-speed applications; therefore, a higher serial clock (SCLK) must be

supplied to be able to sustain the high throughput with the serial interface. As a result, the clock period of SCLK

must be at most 1

µs (when used as the conversion clock, CCLK). The minimum clock frequency is also

governed by the parasitic leakage of the capacitive digital-to-analog (CDAC) capacitors internal to the

ADS7229/30.

Figure 56. Converter Clock

The conversion cycle starts with selecting an acquisition channel by writing a channel number to the command

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