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USING DSP101 WITH MULTIPLEXED ANALOG INPUTS
The DSP101 and DSP102 sampling analog-to-digital con-
verters have all the interface logic to connect directly to
popular digital signal processor ICs from ADI, AT&T,
Motorola, and Texas Instruments. A unique 鈥渢ag鈥?input
allows additional serial data to be appended to the serial data
stream that is sent to the DSP processor. When the DSP101
(1)
is coupled with an analog multiplexer, this tag feature can be
used to construct a low cost multiple input A/D that will
send a channel identification number along with that channel鈥檚
conversion results. The channel ID can then be used by the
DSP processor to separate the different inputs.
The DSP101 uses an internal data pipeline architecture to
synchronize the data from the Successive Approximation
Register (SAR) analog-to-digital converter to the data clock
of the DSP processor IC. The block diagram of the DSP101
(Figure 1) shows how data moves through the part and how
the tag bits are appended. The serial data from the SAR is
clocked into a shift register and held by a latch. On the next
convert command, the data is then loaded into an output shift
register and clocked out to the DSP processor IC, synchro-
nous to the bit transfer clock. As the serial data is clocked
out to the DSP processor IC, serial data inputted to TAG is
clocked into the output shift register. Figure 2 shows how
the serial tag information is appended after the 18th bit of
data.
The schematic of Figure 3 shows a complete eight-channel
analog input system. A 74HC163 counter is used to provide
the scan sequence to a Burr-Brown HI-508A analog multi-
plexer. In order to allow the HI-508A enough time to switch
to the next channel and settle before the DSP101 begins its
conversion, a 74HC221 one shot is used to produce a 3ms
delay for the DSP101 convert command input.
To avoid introducing distortion to the signal, the input to the
DSP101 must be driven by a low impedance source. Due to
the high output impedance of the HI-508A, an OPA627 in a
unity gain configuration is used as a buffer for the DSP101
input.
Since the DSP101 has an internal data pipeline delay of one
sample, a 74HC574 D-type latch is used to delay the tag bits
by one sample also. This delay causes the channel identifi-
cation tag to be appended directly to that channel鈥檚 conver-
sion results. Since the channel scanning shown in the sche-
matic is sequential, this delay latch could be left out and the
DSP processor software modified to recognize an N-1 chan-
nel ID. However, for systems using non-sequential scan
lists, this delay latch would be essential to maintain the data
and channel ID integrity.
The 74HC166 synchronous loading shift register is used so
that the rising edge of the bit clock, in conjunction with the
SYNC output of the DSP101, loads the tag data into the shift
register in a predictable manner. The tag data is then clocked
into the DSP101 by the bit clock, while conversion data is
clocked out the other end of the shift register.
This circuit is constructed on Burr-Brown鈥檚 DEM-DSP102/
202 demonstration fixture. Software for recognizing channel
tags and sorting the data was written for Burr-Brown鈥檚
ZPB34 DSP board for the IBM PC/AT. This board is based
on the AT&T WE
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DSP32C and the software for this circuit
is available from the ????.
Analog
Input
S.A.R.
A/D
Converter
Data
Out
CLK
Out
D
18-bit Input
Shift Register
CLK
18
D
Latch
Q
18
Convert
TAG
XCLK
D
Q
18-bit Output
Shift Register
CLK
Load
SOUT
SYNC
Since the SYNC output of the DSP101 that is used to load
the latches in this circuit is active low for AT&T DSP
processors, the circuit must be modified for use with DSP
processors from Texas Instruments, Motorola, and ADI. For
these processors, tie the SSF pin of the DSP101 high, and
use a 74HC04 hex inverter to invert the SYNC line input to
the 74HC574 and 74HC166.
FIGURE 1. DSP101 Internal Block Diagram.
NOTE: (1) This discussion applies to both the DSP101 and the dual
DSP102, but for simplicity we will talk only about the DSP101.
漏
1994 Burr-Brown Corporation
AB-068
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Printed in U.S.A. January, 1994
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