PDF《CREATING A BIPOLAR INPUT RANGE FOR》

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1999 Burr-Brown Corporation AB-150 Printed in U.

S.A. October,

1999 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'

s unipolar input range is a perfect match. Other sensors, however, produce bipolar currents―currents 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'

s 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'

s input range. Figure

1 shows the DDC112'

s output code versus signal level. Re- ferred to as unipolar with offset in the DDC112'

s data sheet, this range reads

4096 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 safety margin , helps prevent negative input offsets and/or leakage currents from clipping the DDC112'

s output. Suitable for use with unipolar sensors, this range probably won'

t 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 DDC112'

s 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), TINT, 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. FIGURE 1. DDC112 Output Code vs Input Signal. As the DDC112'

s 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'

t clip on the low side until the sum of the input and offset currents reaches C0.4% of positive full scale. (The R Current from Sensor V IN DDC112 FIGURE 2. Conceptual Circuit to Add Offset. +FULL CFULL ZERO INPUT RANGE TINT R SCALE SCALE SIGNAL DDC112 (pC) (?s) (M?) (pC) (pC) OUTPUT CODE

50 500

100 29.5 C20.7 434,012

50 500

50 9 C41.2 863,927

150 500

100 129.5 C21.1 147,403

150 500

50 109 C41.6 290,707

150 500

20 47.5 C103.1 720,622

150 2000

100 68 C82.6 577,317

250 500

100 229.5 C21.5 90,079

250 500

50 209 C42 176,062

250 500

20 147.5 C103.5 434,012

250 2000

100 168 C83 348,029

250 2000

50 86 C165 691,961

350 500

100 329.5 C21.9 65,513

350 500

50 309 C42.4 126,929

350 500

20 347.5 C103.9 311,179

350 2000