Datasheet

ManualsBrandsTEXAS INSTRUMENTS ManualsData collecting ICsData acquisition IC - AD converter (ADC) External , Internal LQFP 64

CONVST_A

CONVST_C

BUSY

(C20=C21=0)

DB[15:0]

CONVST_B

DB[15:0]

CONVST_A

CONVST_B

ADS8556

ADS8557

ADS8558

SBAS404B –OCTOBER 2006– REVISED JANUARY 2012

www.ti.com

Conversion Clock

next edge of the conversion clock. CONVST_x should

remain high during the entire conversion cycle; this is

The device uses either an internally-generated or an

while the BUSY signal remains active. A falling edge

external (XCLK) conversion clock signal (in software

during an ongoing conversion puts the related ADC

mode only). In default mode, the device generates an

pair into partial power-down mode (see the Reset and

internal clock. When the CLKSEL bit is set high (bit

Power-Down Modes section for more details).

C11 in the CR), an external conversion clock of up to

20MHz (max) can be applied on pin 27. In both

For simultaneous sampling, it is recommended to

cases, 18.5 clock cycles are required for a complete

connect all associated CONVST_x pins together. If

conversion including the pre-charging of the sample

the CONVST_x signals are not tied together, a

capacitors. The external clock can remain low

maximum skew of 4 ns must be ensured for all three

between conversions.

signals in any order. A CONVST_x signal issued

during an ongoing conversion on any channel is

The conversion clock duty cycle should be 50%.

blocked, except in sequential mode (see the

However, the ADS8556/7/8 function properly with a

Sequential Mode section for more details).

duty cycle between 45% and 55%.

If a parallel interface is used, the behavior of the

CONVST_x

output port depends on which CONVST_x signals

have been issued. Figure 36 shows examples of

The analog inputs of each channel pair (CH_x0/1) are

different scenarios.

held with the rising edge of the corresponding

CONVST_x signal. Only in software mode (except

sequential mode), CONVST_A is used for all six

ADCs. The conversion automatically starts with the

NOTE: Boxed areas indicate the minimum required frame to acquire all data.

Figure 36. Data Output versus CONVST_x