ADS8509-HT Datasheet, PDF (19/29 Pages) Texas Instruments – 16-BIT 250-KSPS SERIAL CMOS SAMPLING ANALOG-TO-DIGITAL CONVERTER

ADS8509-HT
www.ti.com
SLAS737A – DECEMBER 2012 – REVISED DECEMBER 2013
When EXT/INT is set high, the R/C and CS signals control the read state. When the read state is initiated, the
result from the previously completed conversion is shifted out the DATA pin synchronous to the external clock
that is connected to the DATACLK pin. Each bit is available on a falling and then a rising edge. The maximum
external clock speed of 28.5 MHz allows data to be shifted out quickly either at the beginning of conversion or
the beginning of sampling.
There are several modes of operation available when using an external clock. It is recommended that the
external clock run only while reading data. This is discontinuous clock mode. Since the external clock is not
synchronized to the internal clock that controls conversion slight changes in the external clock can cause
conflicts that can corrupt the conversion process. Specifications with a continuously running external clock
cannot be ensured. It is especially important that the external clock does not run during the second half of the
conversion cycle (approximately the time period specified by t_d11, see the TIMING REQUIREMENTS table).
In discontinuous clock mode data can be read during conversion or during sampling, with or without a SYNC
pulse. Data read during conversion must meet the t_d11 timing specification. Data read during sampling must be
complete before starting a conversion.
Whether reading during sampling or during conversion a SYNC pulse is generated whenever at least one rising
edge of the external clock occurs while the part is not in the read state. In the discontinuous external clock with
SYNC mode a SYNC pulse follows the first rising edge after the read command. The data is shifted out after the
SYNC pulse. The first rising clock edge after the read command generates a SYNC pulse. The SYNC pulse can
be detected on the next falling edge and then the next rising edge. Successively, each bit can be read first on the
falling edge and then on the next rising edge. Thus 17 clock pulses after the read command are required to read
on the falling edge. 18 Clock pulses are necessary to read on the rising edge.
Table 1. DATACLK Pulses
DESCRIPTION
Read on falling edge of DATACLK
Read on rising edge of DATACLK
DATACLK PULSES REQUIRED
WITH SYNC
WITHOUT SYNC
17
16
18
17
If the clock is entirely inactive when not in the read state a SYNC pulse is not generated. In this case the first
rising clock edge shifts out the MSB. The MSB can be read on the first falling edge or on the next rising edge. In
this discontinuous external clock mode with no SYNC, 16 clocks are necessary to read the data on the falling
edge and 17 clocks for reading on the rising edge. Data always represents the conversion already completed.
TAG FEATURE
The TAG feature allows the data from multiple ADS8509 converters to be read on a single serial line. The
converters are cascaded together using the DATA pins as outputs and the TAG pins as inputs as illustrated in
Figure 27. The DATA pin of the last converter drives the processor's serial data input. Data is then shifted
through each converter, synchronous to the externally supplied data clock, onto the serial data line. The internal
clock cannot be used for this configuration.
The preferred timing uses the discontinuous external data clock during the sampling period. Data must be read
during the sampling period because there is not sufficient time to read data from multiple converters during a
conversion period without violating the t_d11 constraint (see the EXTERNAL DATACLOCK section). The sampling
period must be sufficiently long to allow all data words to be read before starting a new conversion.
Note, in Figure 27, that a NULL bit separates the data word from each converter. The state of the DATA pin at
the end of a READ cycle reflects the state of the TAG pin at the start of the cycle. This is true in all READ
modes, including the internal clock mode. For example, when a single converter is used in internal clock mode,
the state of the TAG pin determines the state of the DATA pin after all 16 bits have shifted out. When multiple
converters are cascaded together, this state forms the NULL bit that separates the words. Thus, with the TAG
pin of the first converter grounded as shown in Figure 27 the NULL bit becomes a zero between each data word.
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