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USING EXTERNAL INTEGRATION CAPACITORS ON THE DDC112
By Jim Todsen
This application bulletin discusses issues that arise when
using external integration capacitors on the DDC112. It
expands on the explanation given in the data sheet and
provides some new data to help you select and use the
capacitors. It does assume a basic understanding of the
DDC112鈥檚 operation. For a good introduction to the DDC112,
please see the DDC112鈥檚 data sheet.
Digital input pins RANGE0, RANGE1 and RANGE2 of the
DDC112 set the full-scale range. Table I lists the corre-
sponding range for each combination. Ranges 1 through 7
provide full-scale ranges starting at 50pC and increasing in
FULL-SCALE
RANGE
0.96 C
EXT
V
REF
50pC
100pC
150pC
200pC
250pC
300pC
350pC
steps of 50pC. These ranges use capacitors internal to the
DDC112. For applications requiring other ranges, Range0
allows the user to choose the full-scale range by disabling
the internal capacitors and using external ones. Figure 1
shows a simplified block diagram of the front-end integra-
tors using external integration capacitors. The integration
capacitors connect to the operational amplifiers via pins 3-
6 and 23-26. Notice how pins 3, 5, 24 and 26 internally
connect directly to the inputs 1 and 2. These pins are
extremely sensitive and must be treated very carefully. Table
II summarizes the connections. When external capacitors are
not being used, leave pins 3-6 and 23-26 disconnected. The
DDC112 ties them internally to analog ground.
PIN TO OP AMP鈥橲
NEGATIVE INPUT
(VERY SENSITIVE)
5
3
26
24
RANGE2
0
0
0
0
1
1
1
1
RANGE1
0
0
1
1
0
0
1
1
RANGE0
0
1
0
1
0
1
0
1
SIDE
1A
1B
2B
2A
PIN TO OP AMP鈥橲
OUTPUT
6
4
25
23
Table I. Full-Scale Range Selection.
C
2B
Input 2
28
27
26
Table II. External Capacitor Pin Connections.
C
2A
25
24
23
2B
2A
To
Voltage-Input
ADC
DDC112
1B
1A
1
Input 1
2
3
4
5
6
C
1B
C
1A
FIGURE 1. Simplified Block Diagram of Front-End Integrators Using External Integration Capacitors.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user鈥檚 own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
漏
1998 Burr-Brown Corporation
AB-138
Printed in U.S.A. September, 1998
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