Datasheet

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