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.
5V
AIN
V
DD
VIN1
0 TO 5V
OR
0 TO 10V
VIN2**
AGND
+V
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.
AD7886*
+V
IN
+
5V
V
+
OUT
5REF
AD586
GND
SUM
AD707
0 TO 5V ANALOG INPUT RANGE
3.5k
–
3.5V
–
+
V
REF
10k
V
VIN1
SS
–
C1
10µF
C2
0.1µF
10k
–
5V
VIN2
TO
+
COMPARATORS
0 TO 5V
*ADDITIONAL PINS OMITTED FOR CLARITY
**0 TO 5V RANGE: CONNECT VIN2 TO VIN1
0 TO 10V RANGE: CONNECT VIN2 TO AGND
0 TO 10V ANALOG INPUT RANGE
3.5k
Figure 4. Unipolar Operation
10k
0 TO 10V
VIN1
VIN2
–
+
OUTPUT
CODE
11...111
10k
TO
COMPARATORS
11...110
11...101
11...100
±5V ANALOG INPUT RANGE
3.5k
10k
±5V
VIN1
VIN2
FS
4096
–
1LSB =
00...011
00...010
00...001
00...000
10k
+5V
TO
+
COMPARATORS
Figure 3. Analog Input Range Configurations
FS
1
2
3
ANALOG INPUT RANGES
VIN, INPUT VOLTAGE (LSBS)
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.
FS – 1LSB
Figure 5. Ideal Input/Output Transfer Characteristic for
Unipolar Operation
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.
REV. B
–6–
ADI [ ADI ]
