AD7851 Analog Devices, AD7851 Datasheet - Page 17

• Manufacturer Part Number: AD7851
• Description: 14-Bit, 333 kSPS, Serial Sampling A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7851

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 333kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: DIP,SOIC,SOP

Input Ranges
The analog input range for the AD7851 is 0 V to V
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 +V
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).
Analog Input
Range
0 V to V
± V
NOTES
1
2
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 AV
reference is the supply, AV
AGND in unipolar mode.
REV. B
Output code format is straight binary.
Range is ± V
Figure 15. ± V
V
IN
V
REF
INPUT RANGE
IN
= 0 TO V
Figure 14. 0 V to V
= 0 TO V
INPUT RANGE
REF
SELECTED
/2
UNIPOLAR
ANALOG
SELECTED
V
UNIPOLAR
2
REF
/2 and the output coding is twos complement (see
REF
ANALOG
REF
REF
/2
REF
1
/2 biased about V
DV
Table V. Analog Input Connections
DD
REF
Input Connections Connection
AIN(+)
V
V
IN
IN
/2 about V
AIN(+)
AIN(–)
AMODE
AIN(+)
AMODE
AIN(–)
REF
REF
DD
AIN(–)
AGND
V
Unipolar Input Configuration
/2. Output code format is twos complement.
REF
, the AIN(–) must be tied to
REF
DD
TRACK AND HOLD
TRACK AND HOLD
/2 Bipolar Input Configuration
/2
AMPLIFIER
supply. In the case where the
AMPLIFIER
AD7851
AD7851
Diagram
Figure 8
Figure 9
DOUT
DOUT
REF
TWOS
COMPLEMENT
FORMAT
STRAIGHT
BINARY
FORMAT
AMODE
DGND
DV
in both
DD
–17–
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 V
input/output transfer characteristic for the unipolar range is
shown in Figure 16.
Figure 15 shows the AD7851’s ±V
figuration (where AIN(+) cannot go below 0 V, so for the full bipo-
lar range the AIN(–) pin should be biased to +V
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.
Figure 16. AD7851 Unipolar Transfer Characteristic
Figure 17. AD7851 Bipolar Transfer Characteristic
011...110
000...001
000...000
111...111
100...010
100...001
100...000
OUTPUT
011...111
OUTPUT
111...111
111...110
111...100
000...011
000...010
000...001
000...000
111...101
CODE
CODE
0V
0V
1LSB
V
IN
V
IN
= (AIN(+) – AIN(–)), INPUT VOLTAGE
= (AIN(+) – AIN(–)), INPUT VOLTAGE
(V
REF
/2) – 1 LSB
V
(V
REF
REF
REF
REF
1LSB =
/2
/2 bipolar analog input con-
/2) + 1 LSB
1LSB = FS
FS = V
= 4.096 V. The ideal
16384
REF
FS
16384
+ FS – 1 LSB
+FS – 1LSB
V
REF
AD7851
/2). Once again