AD7856 Analog Devices, AD7856 Datasheet - Page 17

• Manufacturer Part Number: AD7856
• Description: 5 V Single-Supply, 8-Channel, 14-Bit, 285 kSPS, Serial Sampling ADC
• Manufacturer: Analog Devices

Specifications of AD7856

• Resolution (bits): 14bit
• # Chan: 8
• Sample Rate: 285kSPS
• Interface: Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: (Vref) p-p,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: DIP,SOIC,SOP

Transfer Function
For the AD7856 input 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, with 1 LSB = FS/16384 = 4.096 V/16384 =
0.25 mV when V
characteristic is shown in Figure 15.
REFERENCE SECTION
For specified performance, it is recommended that when using
an external reference this reference should be between 4 V and
the analog supply AV
ence pins are shown in the typical connection diagrams. If the
internal reference is being used, the REF
have a 100 nF capacitor connected to AGND very close to the
REF
If the internal reference is required for use external to the ADC,
it should be buffered at the REF
capacitor connected from this pin to AGND. The typical noise
performance for the internal reference, with 5 V supplies is
150 nV/ Hz @ 1 kHz and dc noise is 100 V p-p.
REV. A
V
IN
IN
= 0 TO V
/REF
111...111
111...110
111...100
000...011
000...010
000...001
000...000
OUTPUT
111...101
Figure 14. 0 to V
REF
CODE
OUT
Figure 15. Transfer Characteristic
pin. These connections are shown in Figure 16.
0V
REF
1LSB
= 4.096 V. The ideal input/output transfer
DD
V
AIN(+)
AIN(–)
IN
. The connections for the relevant refer-
= (AIN(+) – AIN(–)), INPUT VOLTAGE
REF
IN
Input Configuration
TRACK AND HOLD
/REF
1LSB =
AMPLIFIER
AD7856
OUT
IN
16384
/REF
FS
pin and a 100 nF
+FS –1LSB
DOUT
OUT
pin should
STRAIGHT
BINARY
FORMAT
–17–
The other option is that the REF
by connecting it to an external reference. This is possible due to
the series resistance from the REF
reference. This external reference can have a range that includes
AV
externally applied reference approaches AV
pacitor from the REF
close as possible to the REF
pin should be connected to AV
level as the reference. The connections for this arrangement are
shown in Figure 17. When using AV
add a resistor in series with the AV
effect of filtering the noise associated with the AV
Figure 17. Relevant Connections When Using AV
Reference
ANALOG
Figure 16. Relevant Connections When Using Internal
Reference
SUPPLY
DD
+5V
. When using AV
ANALOG
SUPPLY
10 F
+5V
0.1 F
0.01 F
10
470nF
0.1 F
IN
10 F
DD
/REF
0.01 F
0.1 F
0.01 F
as the reference source or when an
IN
C
C
REF
OUT
REF1
REF2
/REF
0.01 F
DD
IN
/REF
IN
pin to AGND should be as
470nF
0.1 F
AV
IN
to keep this pin at the same
/REF
DD
OUT
/REF
DD
OUT
DD
supply. This will have the
10
pin, and also the C
it may be necessary to
C
AD7856
C
REF
OUT
REF1
REF2
OUT
AV
DD
IN
DD
/REF
pin be overdriven
0.1 F
pin to the internal
, the 100 nF ca-
10
AD7856
OUT
DV
AD7856
DD
DD
supply.
0.1 F
DD
DV
as the
REF1
DD