EVAL-AD7851CB View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD7851CB' PDF : 36 Pages View PDF
AD7851
Input Ranges
The analog input range for the AD7851 is 0 V to VREF in both
the unipolar and bipolar ranges.
The only difference between the unipolar range and the bipolar
range is that in the bipolar range the AIN(–) has to be biased up
to +VREF/2 and the output coding is twos complement (see
Table V and Figures 14 and 15). The unipolar or bipolar mode
is selected by the AMODE pin (0 for the unipolar range and 1
for the bipolar range).
Table V. Analog Input Connections
Analog Input Input Connections Connection
Range
AIN(+) AIN(–) Diagram AMODE
0 V to VREF1 VIN
± VREF/22
VIN
AGND Figure 8
VREF/2 Figure 9
DGND
DVDD
NOTES
1Output code format is straight binary.
2Range is ± VREF/2 biased about VREF/2. Output code format is twos complement.
Note that the AIN(–) pin on the AD7851 can be biased up above
AGND in the unipolar mode also, if required. The advantage of
biasing the lower end of the analog input range away from
AGND is that the user does not have to have the analog input
swing all the way down to AGND. This has the advantage in
true single-supply applications that the input amplifier does not
have to swing all the way down to AGND. The upper end of the
analog input range is shifted up by the same amount. Care must
be taken so that the bias applied does not shift the upper end of
the analog input above the AVDD supply. In the case where the
reference is the supply, AVDD, the AIN(–) must be tied to
AGND in unipolar mode.
VIN = 0 TO VREF
AIN(+)
AIN(–)
TRACK AND HOLD
AMPLIFIER
DOUT
STRAIGHT
BINARY
FORMAT
UNIPOLAR
ANALOG
INPUT RANGE
SELECTED
AMODE
AD7851
Figure 14. 0 V to VREF Unipolar Input Configuration
VIN = 0 TO VREF
VREF/2
DVDD
UNIPOLAR
ANALOG
INPUT RANGE
SELECTED
AIN(+)
AIN(–)
AMODE
TRACK AND HOLD
AMPLIFIER
DOUT
TWOS
COMPLEMENT
FORMAT
AD7851
Figure 15. ±VREF/2 about VREF/2 Bipolar Input Configuration
Transfer Functions
For the unipolar range, the designed code transitions occur mid-
way between successive integer LSB values (i.e., 1/2 LSB,
3/2 LSBs, 5/2 LSBs . . . FS –3/2 LSBs). The output coding is
straight binary for the unipolar range with 1 LSB = FS/16384 =
4.096 V/16384 = 0.25 mV when VREF = 4.096 V. The ideal
input/output transfer characteristic for the unipolar range is
shown in Figure 16.
OUTPUT
CODE
111...111
111...110
111...101
111...100
000...011
000...010
FS
1LSB = 16384
000...001
000...000
0V 1LSB
+FS – 1LSB
VIN = (AIN(+) – AIN(–)), INPUT VOLTAGE
Figure 16. AD7851 Unipolar Transfer Characteristic
Figure 15 shows the AD7851’s ±VREF/2 bipolar analog input con-
figuration (where AIN(+) cannot go below 0 V, so for the full bipo-
lar range the AIN(–) pin should be biased to +VREF/2). Once again
the designed code transitions occur midway between successive
integer LSB values. The output coding is twos complement with
1 LSB = 16384 = 4.096 V/16384 = 0.25 mV. The ideal input/
output transfer characteristic is shown in Figure 17.
OUTPUT
CODE
011...111
011...110
000...001
000...000
0V
111...111
100...010
100...001
100...000
(VREF/2) – 1 LSB
+ FS – 1 LSB
(VREF/2) + 1 LSB
FS = VREFV
1LSB
=
FS
16384
VREF/2
VIN = (AIN(+) – AIN(–)), INPUT VOLTAGE
Figure 17. AD7851 Bipolar Transfer Characteristic
REV. B
–17–
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