AD7854 Analog Devices, AD7854 Datasheet - Page 15

• Manufacturer Part Number: AD7854
• Description: 3 V to 5 V Single Supply, 200 kSPS, 12-Bit, Parallel Sampling ADC
• Manufacturer: Analog Devices

Specifications of AD7854

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 200kSPS
• Interface: Byte,Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (Vref) p-p
• Adc Architecture: SAR
• Pkg Type: DIP,SOIC,SOP

REV. B
Input Ranges
The analog input range for the AD7854/AD7854L is 0 V to
V
The only difference between the unipolar range and the bipolar
range is that in the bipolar range the AIN(–) should be biased
up to at least +V
ment (see Table V and Figures 14 and 15).
Analog Input
Range
0 V to V
± V
NOTES
1
2
Note that the AIN(–) pin on the AD7854/AD7854L can be
biased up above AGND in the unipolar mode, or above V
in bipolar mode if required. The advantage of biasing the lower
end of the analog input range away from AGND is that the analog
input does not have to swing all the way down to AGND. Thus,
in single supply applications 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
erence is the supply, AV
in unipolar mode or to AV
Range is ±V
Output code format is straight binary.
Figure 13. ± V
V
REF
IN
V
REF
IN
= 0 TO V
in both the unipolar and bipolar ranges.
Figure 12. 0 to V
= 0 TO V
/2
2
V
REF
REF
REF
REF
1
REF
/2
/2 biased about V
Table V. Analog Input Connections
REF
REF
/2 about V
AIN(+)
AIN(–)
/2 and the output coding is twos comple-
AIN(+)
AIN(–)
Input Connections
AIN(+)
V
V
DD
REF
IN
IN
, the AIN(–) should be tied to AGND
DD
DD
AD7854/AD7854L
REF
Unipolar Input Configuration
AD7854/AD7854L
TRACK AND HOLD
/2 in bipolar mode.
REF
/2. Output code format is twos complement.
TRACK AND HOLD
supply. In the case where the ref-
AMPLIFIER
/2 Bipolar Input Configuration
AMPLIFIER
AIN(–)
AGND
V
REF
. . .
/2
. . .
DB11
DB0
DB11
DB0
Connection
Diagram
Figure 12
Figure 13
2’S
COMPLEMENT
FORMAT
STRAIGHT
BINARY
FORMAT
REF
/2
–15–
Transfer Functions
For the unipolar range the designed code transitions occur
midway 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/4096 =
3.3 V/4096 = 0.8 mV when V
output transfer characteristic for the unipolar range is shown in
Figure 14.
Figure 13 shows the AD7854/AD7854L’s ± V
log input configuration. AIN(+) cannot go below 0 V, so for the
full bipolar range, AIN(–) should be biased to at least +V
Once again the designed code transitions occur midway between
successive integer LSB values. The output coding is twos
complement with 1 LSB = 4096 = 3.3 V/4096 = 0.8 mV. The
ideal input/output transfer characteristic is shown in Figure 15.
Figure 15. AD7854/AD7854L Bipolar Transfer Characteristic
Figure 14. AD7854/AD7854L Unipolar Transfer
Characteristic
OUTPUT
111...111
111...110
111...101
111...100
000...011
000...010
000...001
000...000
OUTPUT
011...111
011...110
000...001
000...000
111...111
000...010
000...001
000...000
CODE
CODE
0V 1LSB
0V
V
IN
V
IN
= (AIN(+) – AIN(–)), INPUT VOLTAGE
= (AIN(+) – AIN(–)), INPUT VOLTAGE
(V
REF
/2) – 1LSB
REF
AD7854/AD7854L
(V
V
REF
REF
= 3.3 V. The ideal input/
/2
/2) + 1 LSB
1LSB =
FS = V
1LSB =
4096
+FS –1LSB
FS
REF
+ FS – 1LSB
4096
REF
FS
V
/2 bipolar ana-
REF
/2.