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CREATING A BIPOLAR INPUT RANGE FOR THE DDC112
By Jim Todsen
Many current-output sensors produce unipolar currents. Pho-
todiodes are one such sensor and for them, the DDC112鈥檚
unipolar input range is a perfect match. Other sensors,
however, produce bipolar currents鈥攃urrents that flow both
into and out of the sensor. In order to use the DDC112 with
these sensors, the input range of the DDC112 must somehow
be made bipolar. Fortunately, this is easily done. The follow-
ing sections of this application note review the DDC112鈥檚
input range, describe how to make it bipolar, show how to
experiment with bipolar ranges using the DDC112 Evalua-
tion Fixture and finally how to derive the noise contribution
that comes with making the range bipolar.
First, a quick review of the DDC112鈥檚 input range. Figure 1
shows the DDC112鈥檚 output code versus signal level. Re-
ferred to as 鈥渦nipolar with offset鈥?in the DDC112鈥檚 data
sheet, this range reads 鈥?096鈥?with a zero input and clips at
all zeros with a negative input signal equal in magnitude to
approximately 0.4% of the positive full-scale range. Having
this small offset, or 鈥渟afety margin鈥? helps prevent negative
input offsets and/or leakage currents from clipping the
DDC112鈥檚 output. Suitable for use with unipolar sensors,
this range probably won鈥檛 work for sensors more bipolar in
nature. For these, the negative and positive signal ranges of
the DDC112 need to be made closer in size by introducing
a larger offset.
CREATING A BIPOLAR RANGE
INPUT SIGNAL
+ Full Scale
DDC112 OUTPUT CODE
FFFFFh = 1,048,575
DDC112
Current
from Sensor
R
IN
V
FIGURE 2. Conceptual Circuit to Add Offset.
DDC112鈥檚 input range is actually in units of
charge,
but it
is sometimes more convenient to talk about the equivalent
current
input range.) In general, the current offset can be
any value and should be chosen using the expected maxi-
mum positive and negative input signals. Of course, as the
value of the offset changes, the output code for a zero-input
signal will also change. Table I shows various combinations
of Range (set by DDC112 pins GAIN0, GAIN1, and GAIN2),
T
INT
, and the resistor (R) used to apply the offset versus the
resulting positive full scale, negative full scale and DDC112
output code with zero input signal. The resistor is assumed
to be connected to a voltage source equal to 4.1V.
+FULL
SCALE
(pC)
29.5
9
129.5
109
47.5
68
229.5
209
147.5
168
86
329.5
309
347.5
268
186
鈥揊ULL
SCALE
(pC)
鈥?0.7
鈥?1.2
鈥?1.1
鈥?1.6
鈥?03.1
鈥?2.6
鈥?1.5
鈥?2
鈥?03.5
鈥?3
鈥?65
鈥?1.9
鈥?2.4
鈥?03.9
鈥?3.4
鈥?65.4
ZERO INPUT
SIGNAL DDC112
OUTPUT CODE
434,012
863,927
147,403
290,707
720,622
577,317
90,079
176,062
434,012
348,029
691,961
65,513
126,929
311,179
249,762
495,428
RANGE
(pC)
50
50
150
150
150
150
250
250
250
250
250
350
350
350
350
350
T
INT
(
碌
s)
500
500
500
500
500
2000
500
500
500
2000
2000
500
500
500
2000
2000
R
(M鈩?
100
50
100
50
20
100
100
50
20
100
50
100
50
20
100
50
Zero
鈥?Full Scale
01000h = 4096
00000h
FIGURE 1. DDC112 Output Code vs Input Signal.
As the DDC112鈥檚 input naturally sums currents together,
adding an offset current at the input is easy to do. Figure 2
shows the circuit. For simplicity, only one of the DDC112's
two inputs is shown. All that is needed is a resistor and a
voltage source. The offset current is V/R and adds directly
to the signal current. With the added offset, the DDC112
doesn鈥檛 clip on the low side until the
sum
of the input and
offset currents reaches 鈥?.4% of positive full scale. (The
TABLE 1. Various Configurations and the Associated Full
Scale Ranges and Zero Input Signal DDC112
Output Codes.
AB-150
Printed in U.S.A. October, 1999
漏
1999 Burr-Brown Corporation
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