廬
APPLICATION BULLETIN
ADS7809 TAG FEATURE
By Jerry Horn
Mailing Address: PO Box 11400, Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 鈥?Tel: (520) 746-1111
Telex: 066-6491 鈥?FAX (520) 889-1510 鈥?Product Info: (800) 548-6132 鈥?Internet: www.burr-brown.com/ 鈥?FAXLine: (800) 548-6133
1
The ADS7809 is part of a family of capacitive redistribution
SAR A/D converters that feature a serial output and a tag pin
for cascading multiple converters. Other members of this
family include the ADS7806, ADS7807, ADS7808,
ADS7824, and ADS7825. Note that the even numbered
converters are 12-bit converters and the odd numbered
converters are 16-bit converters.
This application bulletin deals only with the tag feature on
the ADS7809. While other members of the family are
slightly different, they are similar enough that this document
can be used as a guide. Some members of the family do not
include the SYNC pulse. In the following diagrams, this
pulse and its associated clock cycle should be ignored for the
ADS7806 and ADS7807. Likewise, the 12-bit converters
require only 12 clock cycles for the conversion data instead
of 16.
The preferred method of using the tag feature is covered
first鈥攁 gated clock which is active after the conversion is
complete. While this is the best method for the entire family,
it is particularly true for the 16-bit converters. If this method
is not acceptable, other ways of using the tag feature are
described at the end of this document. The first method
should be studied to determine if it can be used. If not, the
alternatives will then be more understandable as well as the
disadvantages that accompany them.
THE BASIC SETUP
Figure 1 shows the digital connections between two
ADS7809s that have been set up to use the tag feature. The
connections for the two converters are identical except that
the DATA pin of converter A is connected to the TAG pin
of converter B. Also, the TAG pin of converter A is grounded
(why the TAG pin is tied LOW will be explained later). Note
that while CS (chip select) is shown as being supplied by the
user, this document assumes this input will always be LOW
unless specifically stated otherwise.
The tag feature only works with an external clock. It is very
important to remember that while the serial shift registers
inside the converters are being clocked by the external clock,
the actual conversion is still being run by the internal clock.
For the entire family, the conversion time is set by an
internal, laser-trimmed clock and cannot be changed by any
external settings.
3
4
5
6
+5
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
Serial Data
(Output)
R1IN
AGND1
R2IN
R3IN
CAP
REF
AGND2
SB/BTC
EXT/INT
DGND
ADS7809 (A)
VDIG
VANA
PWRD
BUSY
CS
R/C
TAG
DATA
DATACLK
SYNC
R1IN
AGND1
R2IN
R3IN
CAP
REF
AGND2
SB/BTC
EXT/INT
DGND
ADS7809 (B)
VDIG
VANA
PWRD
BUSY
CS
R/C
TAG
DATA
DATACLK
SYNC
CS (Input)
R/C (Input)
External Clock (Input)
FIGURE 1. Two ADS7809s Utilizing the Tag Feature.
漏
1996 Burr-Brown Corporation
AB-109
Printed in U.S.A. January, 1997
1
2
3
4
5
6
