AD7858 Analog Devices, AD7858 Datasheet - Page 17

• Manufacturer Part Number: AD7858
• Description: 3 V to 5 V Single Supply, 200 kSPS, 8-Channel, 12-Bit, Serial Sampling ADC
• Manufacturer: Analog Devices

Specifications of AD7858

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 200kSPS
• Interface: Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: (Vref) p-p,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: DIP,SOIC

Input Range
The analog input range for the AD7858/AD7858L is 0 V to
V
up above AGND, if required. The advantage of biasing the
lower end of the analog input range away from AGND is that
the user does not need 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 need 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.
Transfer Function
For the AD7858/AD7858L input range the designed code tran-
sitions 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/4096 = 3.3 V/
4096 = 0.8 mV when V
transfer characteristic is shown in Figure 15.
REF
V
IN
. The AIN(–) pin on the AD7858/AD7858L can be biased
= 0 TO V
OUTPUT
111...111
111...110
111...101
111...100
000...011
000...010
000...001
000...000
CODE
REF
0V 1LSB
V
AIN(+)
AIN(–)
IN
= (AIN(+) – AIN(–)), INPUT VOLTAGE
REF
DD
DD
= 3.3 V. The ideal input/output
, the AIN(–) must be tied to
supply. In the case where the
TRACK AND HOLD
AMPLIFIER
1LSB = FS
AD7858/
AD7858L
4096
+FS –1LSB
DOUT
STRAIGHT
BINARY
FORMAT
REFERENCE SECTION
For specified performance, it is recommended that when using
an external reference this reference should be between 2.3 V
and the analog supply AV
reference pins are shown in the typical connection diagrams. If
the internal reference is being used, the REF
should have a 100 nF capacitor connected to AGND very close
to the REF
Figure 16.
If the internal reference is required for use external to the ADC,
it should be buffered at the REF
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 µV p-p.
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
capacitor 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
DD
ANALOG SUPPLY
. When using AV
ANALOG SUPPLY
+3V TO +5V
IN
+3V TO +5V
/REF
OUT
10 F
pin. These connections are shown in
DD
IN
10 F
0.01 F
/REF
DD
0.1 F
0.1 F
as the reference source, the 100 nF
IN
. The connections for the relevant
0.01 F
0.1 F
0.1 F
/REF
OUT
DD
0.1 F
IN
AD7858/AD7858L
0.1 F
IN
/REF
IN
to keep this pin at the same
DD
/REF
pin to AGND should be as
OUT
C
C
REF
/REF
DD
REF1
REF2
supply. This will have the
IN
C
C
REF
OUT
pin, and also the C
it may be necessary to
OUT
/REF
REF1
REF2
AV
OUT
IN
DD
/REF
pin and a 100 nF
OUT
AV
AD7858/
AD7858L
pin be overdriven
IN
pin to the internal
DD
/REF
DV
OUT
AD7858/
AD7858L
DD
DD
0.1 F
DV
OUT
DD
supply.
0.1 F
pin
REF1