AD7886
this amplifier typically by 20 MHz which is much greater than
the Nyquist limit of the ADC; as a result, it can be used for
undersampling applications. The track-and-hold amplifier ac-
quires the input signal to 12-bit accuracy in less than 333 ns.
The overall throughput time is equal to the conversion time
plus the track/ hold amplifier acquisition time, which is 1.333
µs
for the AD7886.
+V
+ 5V
AIN
VIN1
0 TO 5V
OR
0 TO 10V
V
DD
VIN2**
AGND
The operation of the track/hold amplifier is essentially transpar-
ent to the user. The track-to-hold transition occurs at the start
of conversion on the falling edge of
CONVST.
The conversion
procedure does not start until the rising edge of
CONVST.
The
width of the
CONVST
pulse low time determines the track-to
hold settling time. The track/hold reverts back to the track
mode at the end of conversion when
BUSY
has returned high.
0 TO 5V ANALOG INPUT RANGE
3.5k
10k
+V
IN
V
OUT
+ 5V
AD7886*
+ 5REF
AD586
GND
SUM
AD707
–
+
– 3.5V
V
REF
VIN1
VIN2
0 TO 5V
–
10k
V
SS
C1
10µF
C2
0.1µF
– 5V
+
TO
COMPARATORS
0 TO 10V ANALOG INPUT RANGE
3.5k
0 TO 10V
10k
*ADDITIONAL PINS OMITTED FOR CLARITY
**0 TO 5V RANGE: CONNECT VIN2 TO VIN1
0 TO 10V RANGE: CONNECT VIN2 TO AGND
Figure 4. Unipolar Operation
OUTPUT
CODE
VIN1
VIN2
–
10k
+
TO
COMPARATORS
11...111
11...110
±5V
ANALOG INPUT RANGE
3.5k
11...101
11...100
±5V
+5V
VIN1
VIN2
10k
–
10k
00...011
1LSB =
FS
4096
+
TO
COMPARATORS
00...010
00...001
Figure 3. Analog Input Range Configurations
00...000
ANALOG INPUT RANGES
1
2
3
FS
The AD7886 has three user selectable analog input ranges: 0 V
to 5 V, 0 V to 10 V and
±
5 V. Figure 3 shows how to configure
the two analog inputs (VIN1 and VIN2) for these ranges.
UNIPOLAR OPERATION
VIN, INPUT VOLTAGE (LSBS)
FS – 1LSB
Figure 5. Ideal Input/Output Transfer Characteristic for
Unipolar Operation
Figure 4 shows a typical unipolar circuit for the AD7886. The
ideal input/output characteristic is shown in Figure 5. The
designed code transitions occur on integer multiples of 1 LSB.
The output code is natural binary with 1 LSB = FS/4096. FS is
either +5 V or +10 V, depending on how the analog inputs are
configured.
–6–
REV. B
AD [ ANALOG DEVICES ]
