AD1671
Table I is a list of grounding and decoupling rules that should
be reviewed before laying out a printed circuit board.
The gain trim is done by applying a signal 1 1/2 LSBs below the
nominal full scale (4.998 V for a 5 V range). Trim R2 to give
the last transition (1111 1111 1110 to 1111 1111 1111). This
circuit will give approximately ±0.5% FS of adjustment range.
Table I. Grounding and Decoupling Guidelines
Power Supply
V
IN
0 TO +5V
AIN1
AIN2
Decoupling
Comment
25Ω
5k
5k
SHA
+5V
R1
Capacitor Values
0.1 µF (Ceramic) and 1 µF
(Tantalum) Surface Mount Chip
Capacitors Recommended to
Reduce Lead Inductance
50k
OFFSET
10k
ADJ
–5V
R2
50Ω
SHA OUT
GAIN
ADJ
AD1671
Capacitor Locations
Directly at Positive and Negative
Supply Pins to Common Ground
Plane
BPO/UPO
REF IN
REF OUT
Reference (REF OUT)
Capacitor Value
Grounding
1µF
1 µF (Tantalum) to ACOM
Figure 9. Unipolar (0 V to +5 V) Calibration
Analog Ground
Ground Plane or Wide Ground
Return Connected to the Analog
Power Supply
BIPOLAR (؎5 V) CALIBRATION
The connections for the bipolar ±5 V input range is shown in
Figure 10.
Reference Ground
(REF COM)
Critical Common Connections
Should be Star Connected to REF
COM (as Shown in Figure 8)
V
IN
AIN1
25Ω
5k
5k
–5V TO +5V
SHA
+5V
R1
AIN2
Digital Ground
Ground Plane or Wide Ground
Return Connected to the Digital
Power Supply
50k
OFFSET
10k
ADJ
–5V
R2
50Ω
SHA OUT
GAIN
ADJ
AD1671
Analog and Digital Ground Connected Together Once at the
AD1671
BPO/UPO
REF IN
UNIPOLAR (0 V TO +5 V) CALIBRATION
REF OUT
The AD1671 is factory trimmed to minimize offset, gain and
linearity errors. In some applications the offset and gain errors
of the AD1671 need to be externally adjusted to zero. This is
accomplished by trimming the voltage at AIN2 (Pin 22). The
circuit in Figure 9 is recommended for calibrating offset and
gain errors of the AD1671 when configured in the 0 V to +5 V
input range. If the offset trim resistor R1 is used, it should be
trimmed as follows, although a different offset can be set for a
particular system requirement. This circuit will give approxi-
mately ±5 mV of offset trim range. Nominally the AD1671 is
intended to have a 1/2 LSB offset so that the exact analog input
for a given code will be in the middle of that code (halfway be-
tween the transitions to the codes above it and below it). Thus,
the first transition (from 0000 0000 0000 to 0000 0000 0001)
will occur for an input level of +1/2 LSB (0.61 mV for 5 V
range).
1µF
Figure 10. Bipolar (±5 V) Calibration
Bipolar calibration is similar to unipolar calibration. First, a sig-
nal 1/2 LSB above negative full scale (–4.9988 V) is applied and
R1 is trimmed to give the first transition (0000 0000 0000 to
0000 0000 0001). Then a signal 1 1/2 LSB below positive full
scale (+4.9963 V) is applied and R2 is trimmed to give the last
transition (1111 1111 1110 to 1111 1111 1111).
REV. B
–9–
ADI [ ADI ]
