AD7904BRUZ-REEL7 View Datasheet(PDF) - Analog Devices
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AD7904BRUZ-REEL7 Datasheet PDF : 32 Pages
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Data Sheet
ADC TRANSFER FUNCTION
The output coding of the AD7904/AD7914/AD7924 is either
straight binary or twos complement, depending on the status of
the LSB in the control register. The designed code transitions
occur at successive LSB values (that is, 1 LSB, 2 LSBs, and so on).
For the 0 V to REFIN input range, the LSB size is REFIN/256 for
the AD7904, REFIN/1024 for the AD7914, and REFIN/4096 for
the AD7924. For the 0 V to 2 × REFIN input range, the LSB size
is 2 × REFIN/256 for the AD7904, 2 × REFIN/1024 for the AD7914,
and 2 × REFIN/4096 for the AD7924. The ideal transfer charac-
teristic for the AD7904/AD7914/AD7924 when straight binary
coding is selected is shown in Figure 16; the ideal transfer
characteristic for the AD7904/AD7914/AD7924 when twos
complement coding is selected is shown in Figure 17.
111…111
111…110
•
•
111…000
•
011…111
•
•
000…010
000…001
000…000
0V 1LSB
1LSB = VREF/256 AD7904
1LSB = VREF/1024 AD7914
1LSB = VREF/4096 AD7924
+VREF – 1LSB
ANALOG INPUT
NOTES
1. VREF IS EITHER REFIN OR 2 × REFIN.
Figure 16. Straight Binary Transfer Characteristic
AD7904/AD7914/AD7924
011…111
011…110
•
•
000…001
000…000
111…111
•
•
100…010
100…001
100…000
1LSB = 2 × VREF/256 AD7904
1LSB = 2 × VREF/1024 AD7914
1LSB = 2 × VREF/4096 AD7924
–VREF + 1LSB
+VREF – 1LSB
VREF – 1LSB
ANALOG INPUT
Figure 17. Twos Complement Transfer Characteristic
with 0 V to 2 × REFIN Input Range
Handling Bipolar Input Signals
Figure 18 shows how the combination of the 0 V to 2 × REFIN
input range and the twos complement output coding scheme is
particularly useful for handling bipolar input signals. If the
bipolar input signal is biased about REFIN and twos complement
output coding is selected, REFIN becomes the zero code point,
−REFIN is negative full scale, and +REFIN becomes positive full
scale, with a dynamic range of 2 × REFIN.
VREF
VDD
0.1µF
REFINAVDD
VDRIVE
VDD
AD7904/
DSP/
V
R4
AD7914/
AD7924
MICRO-
PROCESSOR
R3
TWOS
COMPLEMENT
R2
VIN0
DOUT
0V
V
VIN3
R1
+REFIN (= 2 × REFIN)
011…111
R1 = R2 = R3 = R4
REFIN
000…000
–REFIN
Figure 18. Handling Bipolar Signals
(= 0V)
100…000
Rev. C | Page 19 of 32
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